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Core java interview questions

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This document provides an overview of topics that are important to know for Java interviews, including core Java topics like platform independence, wrapper classes, strings, arrays, and enums. It also lists some key differences between Java and C++. The document recommends reviewing these topics in detail as well as watching related tutorial videos to prepare for Java interviews.

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Java  Interview  Questions  –  www.JavaInterview.in   1     Following  are  the  important  topics  that  are  important  from  interview  perspective  for  core  java.    We   cover  these  in  great  detail  in  this  guide.     CORE  JAVA  ...........................................................................................................................................................  2   WHY  IS  JAVA  SO  POPULAR?  .............................................................................................................................................  2   WHAT  IS  PLATFORM  INDEPENDENCE?  ...............................................................................................................................  2   WHAT  ARE  THE  IMPORTANT  DIFFERENCES  BETWEEN  C++  AND  JAVA?  .......................................................................................  3   WHAT  ARE  WRAPPER  CLASSES?  ........................................................................................................................................  3   WHAT  ARE  THE  DIFFERENT  UTILITY  METHODS  PRESENT  IN  WRAPPER  CLASSES?  ...........................................................................  4   WHAT  IS  AUTO  BOXING?  ................................................................................................................................................  5   ARE  ALL  STRING’S  IMMUTABLE?  .......................................................................................................................................  6   WHERE  ARE  STRING  LITERALS  STORED  IN  MEMORY?  ..............................................................................................................  6   CAN  YOU  GIVE  EXAMPLES  OF  DIFFERENT  UTILITY  METHODS  IN  STRING  CLASS?  ............................................................................  6   EXPLAIN  ABOUT  TOSTRING  METHOD    IN  JAVA?  ....................................................................................................................  7   WHAT  IS  THE  USE  OF  EQUALS  METHOD  IN  JAVA?  ..................................................................................................................  8   WHAT  ARE  THE  IMPORTANT  THINGS  TO  CONSIDER  WHEN  IMPLEMENTING  EQUALS  METHOD?  ........................................................  9   WHAT  IS  THE  HASHCODE  METHOD  USED  FOR  IN  JAVA?  .........................................................................................................  9   WHAT  IS  CASTING?  ......................................................................................................................................................  10   WHAT  IS  IMPLICIT  CASTING?  .........................................................................................................................................  10   WHAT  IS  EXPLICIT  CASTING?  .........................................................................................................................................  10   HOW  ARE  VARIABLES  INITIALIALIZED  IN  JAVA?  ...................................................................................................................  10   WHAT  IS  A  NESTED  IF  ELSE?  CAN  YOU  EXPLAIN  WITH  AN  EXAMPLE?  .......................................................................................  11   ARRAYS  ..............................................................................................................................................................  12   HOW  DO  YOU  DECLARE  AND  CREATE  AN  ARRAY?  ................................................................................................................  12   CAN  THE  SIZE  OF  AN  ARRAY  BE  CHANGED  DYNAMICALLY?  .....................................................................................................  12  
2   Java  Interview  Questions  –  www.JavaInterview.in       CAN  YOU  CREATE  AN  ARRAY  WITHOUT  DEFINING  SIZE  OF  AN  ARRAY?  ......................................................................................  12   WHAT  ARE  THE  DEFAULT  VALUES  IN  AN  ARRAY?  .................................................................................................................  12   HOW  DO  YOU  LOOP  AROUND  AN  ARRAY  USING  ENHANCED  FOR  LOOP?  ...................................................................................  13   HOW  DO  YOU  PRINT  THE  CONTENT  OF  AN  ARRAY?  ..............................................................................................................  13   HOW  DO  YOU  COMPARE  TWO  ARRAYS?  ...........................................................................................................................  13   HOW  DO  YOU  SORT  AN  ARRAY?  ......................................................................................................................................  14   ENUM  ................................................................................................................................................................  14   WHAT  IS  AN  ENUM?  ....................................................................................................................................................  14   HOW  DO  YOU  CREATE  AN  ENUM  FROM  A  STRING  VALUE?  ....................................................................................................  14   WHAT  IS  AN  ENUM  ORDINAL?  .......................................................................................................................................  14   HOW  DO  YOU  COMPARE  TWO  ENUMS?  ...........................................................................................................................  14   CAN  YOU  USE  A  SWITCH  STATEMENT  AROUND  AN  ENUM?  ...................................................................................................  14     Core  Java   Following  videos  cover  these  topics  in  great  detail.      In  addition  to  following  this  guide,  we  recommend   that  you  watch  the  videos  as  well.   Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:  https://www.youtube.com/watch?v=njZ48YVkei0.   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:  https://www.youtube.com/watch?v=xyXuo0y-xoU   Why  is  Java  so  Popular?   Two  main  reasons  for  popularity  of  Java  are   1. Platform  Independence   2. Object  Oriented  Language   We  will  look  at  these  in  detail  in  later  sections.   What  is  Platform  Independence?   This  video(https://www.youtube.com/watch?v=lLgcgvIHyAw)  explains  Platform  Independence   in  great  detail.  Refer  to  it  for  more  detailed  answer.   Platform  Independence  is  also  called  build  once,  run  anywhere.  Java  is  one  of  the  most  popular  platform   independent  languages.  Once  we  compile  a  java  program  and  build  a  jar,  we  can  run  the  jar  (compiled   java  program)  in  any  Operating  System  -­‐  where  a  JVM  is  installed.   Java  achieves  Platform  Independence  in  a  beautiful  way.  On  compiling  a  java  file  the  output  is  a  class  file   -­‐  which  contains  an  internal  java  representation  called  bytecode.  JVM  converts  bytecode  to  executable   instructions.   The   executable   instructions   are   different   in   different   operating   systems.   So,   there   are   different  JVM's  for  different  operating  systems.  A  JVM  for  windows  is  different  from  a  JVM  for  mac.   However,   both   the   JVM's   understand   the   bytecode   and   convert   it   to   the   executable   code   for   the   respective  operating  system.    
Java  Interview  Questions  –  www.JavaInterview.in   3     What  are  the  important  differences  between  C++  and  Java?   1. Java  is  platform  independent.  C++  is  not  platform  independent.   2. Java  is  a  pure  Object  Oriented  Language  (except  for  primitive  variables).  In  C++,  one  can  write   structural  programs  without  using  objects.   3. C++  has  pointers  (access  to  internal  memory).  Java  has  no  concept  called  pointers.   4. In  C++,  programmer  has  to  handle  memory  management.  A  programmer  has  to  write  code  to   remove   an   object   from   memory.   In   Java,   JVM   takes   care   of   removing   objects   from   memory   using  a  process  called  Garbage  Collection.   5. C++  supports  Multiple  Inheritance.  Java  does  not  support  Multiple  Inheritance.     Java  Interview  Questions  –  www.JavaInterview.in   At  http://www.JavaInterview.in,  we  want  you  to  clear  java  interview  with  ease.  So,  in  addition  to   focussing  on  Core  and  Advanced  Java  we  also  focus  on  topics  like  Code  Reviews,  Performance,    Design   Patterns,  Spring  and  Struts.   We  have  created  more  than  20  videos  to  help  you  understand  these  topics  and  become  an  expert  at   them.  Visit  our  website  http://www.JavaInterview.in  for  complete  list  of  videos.    Other  than  the  videos,   we  answer  the  top  200  frequently  asked  interview  questions  on  our  website.   With  more  900K  video  views  (Apr  2015),  we  are  the  most  popular  channel  on  Java  Interview  Questions   on  YouTube.   Register  here  for  more  updates  :     https://feedburner.google.com/fb/a/mailverify?uri=RithusTutorials   Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:     https://www.youtube.com/watch?v=njZ48YVkei0   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:     https://www.youtube.com/watch?v=xyXuo0y-xoU     What  are  wrapper  classes?   This video(https://www.youtube.com/watch?v=YQbZRw2yIBk) covers the topic in great detail. A brief description is provided below. A primitive wrapper class in the Java programming language is one of eight classes provided in the java.lang package to provide object methods for the eight primitive types. All of the primitive wrapper classes in Java are immutable.  
4   Java  Interview  Questions  –  www.JavaInterview.in       Wrapper: Boolean,Byte,Character,Double,Float,Integer,Long,Short Primitive: boolean,byte,char ,double, float, int , long,short Wrapper classes are final and immutable. Examples of creating wrapper classes are listed below. Integer  number  =  new  Integer(55);//int   Integer  number2  =  new  Integer("55");//String     Float  number3  =  new  Float(55.0);//double  argument   Float  number4  =  new  Float(55.0f);//float  argument   Float  number5  =  new  Float("55.0f");//String     Character  c1  =  new  Character('C');//Only  char  constructor   //Character  c2  =  new  Character(124);//COMPILER  ERROR     Boolean  b  =  new  Boolean(true);     //"true"  "True"  "tRUe"  -­‐  all  String  Values  give  True   //Anything  else  gives  false   Boolean  b1  =  new  Boolean("true");//value  stored  -­‐  true   Boolean  b2  =  new  Boolean("True");//value  stored  -­‐  true   Boolean  b3  =  new  Boolean("False");//value  stored  -­‐  false   Boolean  b4  =  new  Boolean("SomeString");//value  stored  -­‐  false     b  =  false;   What  are  the  different  utility  methods  present  in  wrapper  classes?   A  number  of  utility  methods  are  defined  in  wrapper  classes  to  create  and  convert  them  from  primitives.   valueOf    Methods   Provide another way of creating a Wrapper Object Integer  seven  =            Integer.valueOf("111",  2);//binary  111  is  converted  to  7     Integer  hundred  =            Integer.valueOf("100");//100  is  stored  in  variable   xxxValue  methods     xxxValue methods help in creating primitives   Integer  integer  =  Integer.valueOf(57);   int  primitive  =  seven.intValue();//57   float  primitiveFloat  =  seven.floatValue();//57.0f     Float  floatWrapper  =  Float.valueOf(57.0f);   int  floatToInt  =  floatWrapper.intValue();//57   float  floatToFloat  =  floatWrapper.floatValue();//57.0f   parseXxx  methods   parseXxx methods are similar to valueOf but they return primitive values
Java  Interview  Questions  –  www.JavaInterview.in   5     int  sevenPrimitive  =            Integer.parseInt("111",  2);//binary  111  is  converted  to  7     int  hundredPrimitive  =            Integer.parseInt("100");//100  is  stored  in  variable   static  toString  method   Look  at  the  example  of  the  toString  static  method  below.   Integer  wrapperEight  =  new  Integer(8);   System.out.println(Integer.                  toString(wrapperEight));//String  Output:  8     Overloaded  static  toString  method    2nd  parameter:  radix   System.out.println(Integer                  .toString(wrapperEight,  2));//String  Output:  1000   static  toXxxString  methods.     Xxx  can  be  Hex,Binary,Octal   System.out.println(Integer                  .toHexString(wrapperEight));//String  Output:8     System.out.println(Integer                  .toBinaryString(wrapperEight));//String  Output:1000   System.out.println(Integer                  .toOctalString(wrapperEight));//String  Output:10   What  is  Auto  Boxing?   Autoboxing  is  the  automatic  conversion  that  the  Java  compiler  makes  between  the  primitive  types  and   their  corresponding  object  wrapper  classes.  For  example,  converting  an  int  to  an  Integer,  a  double  to  a   Double,  and  so  on.  If  the  conversion  goes  the  other  way,  this  is  called  unboxing.   Integer  ten  =  new  Integer(10);   ten++;//allowed.  Java  does  had  work  behind  the  screen  for  us   Boxing  and  new  instances   Auto   Boxing   helps   in   saving   memory   by   reusing   already   created   Wrapper   objects.   However   wrapper   classes  created  using  new  are  not  reused.   Two  wrapper  objects  created  using  new  are  not  same  object.   Integer  nineA  =  new  Integer(9);   Integer  nineB  =  new  Integer(9);   System.out.println(nineA  ==  nineB);//false   System.out.println(nineA.equals(nineB));//true     Two  wrapper  objects  created  using  boxing  are  same  object.   Integer  nineC  =  9;  
6   Java  Interview  Questions  –  www.JavaInterview.in       Integer  nineD  =  9;   System.out.println(nineC  ==  nineD);//true   System.out.println(nineC.equals(nineD));//true   Are  all  String’s  immutable?   This  video  (https://www.youtube.com/watch?v=wh6L8zO_Hr4)  covers  all  the  topics  related  to   String’s  in  great  detail.  Refer  to  it  for  more  details.     Value  of  a  String  Object  once  created  cannot  be  modified.  Any  modification  on  a  String  object  creates  a   new  String  object.   String  str3  =  "value1";   str3.concat("value2");   System.out.println(str3);  //value1     Note  that  the  value  of  str3  is  not  modified  in  the  above  example.    The  result  should  be  assigned  to  a  new   reference  variable  (or  same  variable  can  be  reused).   String  concat  =  str3.concat("value2");   System.out.println(concat);  //value1value2   Where  are  string  literals  stored  in  memory?   All  strings  literals  are  stored  in  "String  constant  pool".  If  compiler  finds  a  String  literal,it  checks  if  it  exists   in  the  pool.  If  it  exists,  it  is  reused.   Following  statement  creates  1  string  object  (created  on  the  pool)  and  1  reference  variable.   String  str1  =  "value";       However,  if  new  operator  is  used  to  create  string  object,  the  new  object  is  created  on  the  heap.   Following  piece  of  code  create  2  objects.   //1.  String  Literal  "value"  -­‐  created  in  the  "String  constant  pool"   //2.  String  Object  -­‐  created  on  the  heap   String  str2  =  new  String("value");   Can  you  give  examples  of  different  utility  methods  in  String  class?   String  class  defines  a  number  of  methods  to  get  information  about  the  string  content.   String  str  =  "abcdefghijk";   Get  information  from  String   Following  methods  help  to  get  information  from  a  String.   //char  charAt(int  paramInt)   System.out.println(str.charAt(2));  //prints  a  char  -­‐  c   System.out.println("ABCDEFGH".length());//8   System.out.println("abcdefghij".toString());  //abcdefghij   System.out.println("ABC".equalsIgnoreCase("abc"));//true     //Get  All  characters  from  index  paramInt  
Java  Interview  Questions  –  www.JavaInterview.in   7     //String  substring(int  paramInt)   System.out.println("abcdefghij".substring(3));  //cdefghij     //All  characters  from  index  3  to  6   System.out.println("abcdefghij".substring(3,7));  //defg   Explain  about  toString  method    in  Java?   This video (https://www.youtube.com/watch?v=k02nM5ukV7w) covers toString in great detail. toString method is used to print the content of an Object. If the toString method is not overridden in a class, the default toString method from Object class is invoked. This would print some hashcode as shown in the example below. However, if toString method is overridden, the content returned by the toString method is printed. Consider the class given below: class  Animal  {            public  Animal(String  name,  String  type)  {                  this.name  =  name;                  this.type  =  type;          }            String  name;          String  type;     }   Run this piece of code: Animal  animal  =  new  Animal("Tommy",  "Dog");   System.out.println(animal);//com.rithus.Animal@f7e6a96     Output does NOT show the content of animal (what name? and what type?). To show the content of the animal object, we can override the default implementation of toString method provided by Object class. Adding  toString  to  Animal  class   class  Animal  {                    public  Animal(String  name,  String  type)  {                  this.name  =  name;                  this.type  =  type;          }            String  name;          String  type;            public  String  toString()  {                  return  "Animal  [name="  +  name  +  ",  type="  +  type                                  +  "]";          }     }  
8   Java  Interview  Questions  –  www.JavaInterview.in       Run this piece of code: Animal  animal  =  new  Animal("Tommy","Dog");   System.out.println(animal);//Animal  [name=Tommy,  type=Dog]   Output now shows the content of the animal object. What  is  the  use  of  equals  method  in  Java?   Equals method is used when we compare two objects. Default implementation of equals method is defined in Object class. The implementation is similar to == operator. Two object references are equal only if they are pointing to the same object. We need to override equals method, if we would want to compare the contents of an object. Consider the example Client class provided below. class  Client  {          private  int  id;            public  Client(int  id)  {                  this.id  =  id;          }   }   == comparison operator checks if the object references are pointing to the same object. It does NOT look at the content of the object.   Client  client1  =  new  Client(25);   Client  client2  =  new  Client(25);   Client  client3  =  client1;     //client1  and  client2  are  pointing  to  different  client  objects.   System.out.println(client1  ==  client2);//false     //client3  and  client1  refer  to  the  same  client  objects.   System.out.println(client1  ==  client3);//true     //similar  output  to  ==   System.out.println(client1.equals(client2));//false   System.out.println(client1.equals(client3));//true   We can override the equals method in the Client class to check the content of the objects. Consider the example below: The implementation of equals method checks if the id's of both objects are equal. If so, it returns true. Note that this is a basic implementation of equals and more needs to be done to make it fool- proof. class  Client  {          private  int  id;            public  Client(int  id)  {                  this.id  =  id;          }            @Override  
Java  Interview  Questions  –  www.JavaInterview.in   9            public  boolean  equals(Object  obj)  {                  Client  other  =  (Client)  obj;                  if  (id  !=  other.id)                          return  false;                  return  true;          }   }   Consider running the code below: Client  client1  =  new  Client(25);   Client  client2  =  new  Client(25);   Client  client3  =  client1;     //both  id's  are  25   System.out.println(client1.equals(client2));//true     //both  id's  are  25   System.out.println(client1.equals(client3));//true   Above code compares the values (id's) of the objects. What   are   the   important   things   to   consider   when   implementing   equals   method?     Any equals implementation should satisfy these properties: 1. Reflexive. For any reference value x, x.equals(x) returns true. 2. Symmetric. For any reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. 3. Transitive. For any reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) must return true. 4. Consistent. For any reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, if no information used in equals is modified. 5. For any non-null reference value x, x.equals(null) should return false. Let's now provide an implementation of equals which satisfy these properties: //Client class @Override   public  boolean  equals(Object  obj)  {          if  (this  ==  obj)                  return  true;          if  (obj  ==  null)                  return  false;          if  (getClass()  !=  obj.getClass())                  return  false;          Client  other  =  (Client)  obj;          if  (id  !=  other.id)                  return  false;          return  true;   }   What  is  the  hashCode  method  used  for  in  Java?   HashCode's are used in hashing to decide which group (or bucket) an object should be placed into. A group of object's might share the same hashcode.
10   Java  Interview  Questions  –  www.JavaInterview.in       The implementation of hash code decides effectiveness of Hashing. A good hashing function evenly distributes object's into different groups (or buckets). A good hashCode  method  should  have  the  following  properties   • If obj1.equals(obj2) is true, then obj1.hashCode() should be equal to obj2.hashCode() • obj.hashCode() should return the same value when run multiple times, if values of obj used in equals() have not changed. • If obj1.equals(obj2) is false, it is NOT required that obj1.hashCode() is not equal to obj2.hashCode(). Two unequal objects MIGHT have the same hashCode. A sample hashcode implementation of Client class which meets above constraints is given below: //Client class @Override   public  int  hashCode()  {          final  int  prime  =  31;          int  result  =  1;          result  =  prime  *  result  +  id;          return  result;   }   What  is  casting?   Casting is used when we want to convert on data type to another. There are two types of Casting • Implicit Casting • Explicit Casting What  is  Implicit  Casting?   Implicit Casting is done by the compiler. Good examples of implicit casting are all the automatic widening conversions i.e. storing smaller values in larger variable types. int  value  =  100;   long  number  =  value;  //Implicit  Casting   float  f  =  100;  //Implicit  Casting     What  is  Explicit  Casting?   Explicit Casting is done through code. Good examples of explicit casting are the narrowing conversions. Storing larger values into smaller variable types; long  number1  =  25678;   int  number2  =  (int)number1;//Explicit  Casting   //int  x  =  35.35;//COMPILER  ERROR   int  x  =  (int)35.35;//Explicit  Casting   Explicit casting would cause truncation of value if the value stored is greater than the size of the variable. int  bigValue  =  280;   byte  small  =  (byte)  bigValue;   System.out.println(small);//output  24.  Only  8  bits  remain.   How  are  variables  initialialized  in  Java?   Member  and  Static  variables  are  alway  initialized  with  default  values.  Default  values  for  numeric  types  is   0,  floating  point  types  is  0.0,  boolean  is  false,    char    is  'u0000'  and  object  reference  variable  is  null.   Local/block  variables  are  NOT  initialized  by  compiler.    
Java  Interview  Questions  –  www.JavaInterview.in   1 1     If  local  variables  are  used  before  initialization,  it  would  result  in  a  compilation  error.   package  com.rithus.variables;     public  class  VariableInitialization  {          public  static  void  main(String[]  args)  {                  Player  player  =  new  Player();                                    //score  is  an  int  member  variable  -­‐  default  0                  System.out.println(player.score);//0  -­‐  RULE1                                    //name  is  a  member  reference  variable  -­‐  default  null                  System.out.println(player.name);//null  -­‐  RULE1                                    int  local;  //not  initialized                  //System.out.println(local);//COMPILER  ERROR!  RULE3                    String  value1;//not  initialized                  //System.out.println(value1);//COMPILER  ERROR!  RULE3                                    String  value2  =  null;//initialized                  System.out.println(value2);//null  -­‐  NO  PROBLEM.          }   }   What  is  a  nested  if  else?  Can  you  explain  with  an  example?   Look  at  the  example  below.  The  code  in  first  if  condition  which  is  true  is  executed.  If  none  of  the  if   conditions  are  true,  then  code  in  else  is  executed.   int  z  =  15;   if(z==10){          System.out.println("Z  is  10");//NOT  executed   }  else  if(z==12){          System.out.println("Z  is  12");//NOT  executed   }  else  if(z==15){          System.out.println("Z  is  15");//executed.  Rest  of  the  if  else  are  skipped.   }  else  {          System.out.println("Z  is  Something  Else.");//NOT  executed   }     z  =  18;   if(z==10){          System.out.println("Z  is  10");//NOT  executed   }  else  if(z==12){          System.out.println("Z  is  12");//NOT  executed   }  else  if(z==15){          System.out.println("Z  is  15");//NOT  executed   }  else  {          System.out.println("Z  is  Something  Else.");//executed   }    
12   Java  Interview  Questions  –  www.JavaInterview.in       Arrays   Refer  to  this  video(https://www.youtube.com/watch?v=8bVysCXT-io)  for  exhaustive  coverage   of  all  the  interview  questions  about  arrays.   How  do  you  declare  and  create  an  array?   Let’s first discuss about how to declare an array. All   below   ways   are   legal.   However,   using   the   third   format  of  declaration  is  recommended.   int  marks[];  //Not  Readable   int[]  runs;  //Not  Readable   int[]  temperatures;//Recommended       Declaration  of  an  Array  should  not  include  size.     //int  values[5];//Compilation  Error!                     Declaring  2D  Array  Examples:   int[][]  matrix1;  //Recommended   int[]  matrix2[];  //Legal  but  not  readable.  Avoid.     Lets  now  look  at  how  to  create  an  array  (define  a  size  and  allocate  memory).   marks  =  new  int[5];  //  5  is  size  of  array     Declaring  and  creating  an  array  in  same  line.   int  marks2[]  =  new  int[5];   Can  the  size  of  an  array  be  changed  dynamically?   Once  An  Array  is  created,  its  size  cannot  be  changed.     Can  you  create  an  array  without  defining  size  of  an  array?   Size  of  an  array  is  mandatory  to  create  an  array.   //marks  =  new  int[];//COMPILER  ERROR   What  are  the  default  values  in  an  array?   New  Arrays  are  always  initialized  with  default  values.   int  marks2[]  =  new  int[5];   System.out.println(marks2[0]);//0     Default  Values   byte,short,int,long        0     float,double                0.0    
Java  Interview  Questions  –  www.JavaInterview.in   1 3     boolean                  false   object                        null   How  do  you  loop  around  an  array  using  enhanced  for  loop?   Name  of  the  variable  is  mark  and  the  array  we  want  to  loop  around  is  marks.   for  (int  mark:  marks)  {          System.out.println(mark);   }   How  do  you  print  the  content  of  an  array?   Let’s  look  at  different  methods  in  java  to  print  the  content  of  an  array.   Printing  a  1D  Array   int  marks5[]  =  {  25,  30,  50,  10,  5  };   System.out.println(marks5);  //[I@6db3f829   System.out.println(          Arrays.toString(marks5));//[25,  30,  50,  10,  5]   Printing  a  2D  Array   int[][]  matrix3  =  {  {  1,  2,  3  },  {  4,  5,  6  }  };   System.out.println(matrix3);  //[[I@1d5a0305   System.out.println(                  Arrays.toString(matrix3));     //[[I@6db3f829,  [I@42698403]   System.out.println(                  Arrays.deepToString(matrix3));     //[[1,  2,  3],  [4,  5,  6]]     matrix3[0]  is  a  1D  Array   System.out.println(matrix3[0]);//[I@86c347   System.out.println(                  Arrays.toString(matrix3[0]));//[1,  2,  3]   How  do  you  compare  two  arrays?   Arrays  can  be  compared  using  static  method  equals  defined  in  Arrays  class.  Two  arrays  are  equal  only  if   they  have  the  same  numbers  in  all  positions  and  have  the  same  size.   int[]  numbers1  =  {  1,  2,  3  };   int[]  numbers2  =  {  4,  5,  6  };     System.out.println(Arrays                  .equals(numbers1,  numbers2));  //false     int[]  numbers3  =  {  1,  2,  3  };   System.out.println(Arrays                  .equals(numbers1,  numbers3));  //true  
14   Java  Interview  Questions  –  www.JavaInterview.in       How  do  you  sort  an  array?   Array’s  can  be  sorted  using  static  utility  method  sort  defined  in  Arrays  class.   int  rollNos[]  =  {  12,  5,  7,  9  };   Arrays.sort(rollNos);   System.out.println(                  Arrays.toString(rollNos));//[5,  7,  9,  12]   Enum   Refer  to  this  video  (https://www.youtube.com/watch?v=64Y7EP8-Ark)  for  exhaustive   coverage  of  all  the  interview  questions  about  Enum.   What  is  an  Enum?   Enum allows specifying a list of values for a Type. Consider the example below. It declares an enum Season with 4 possible values.        enum  Season  {                  WINTER,  SPRING,  SUMMER,  FALL          };     How  do  you  create  an  enum  from  a  String  value?   Function valueOf(String) is used to convert a string to enum.                //Converting  String  to  Enum                  Season  season  =  Season.valueOf("FALL");   Function name() is used to find String value of an enum.                //Converting  Enum  to  String                  System.out.println(season.name());//FALL   What  is  an  Enum  Ordinal?   Java assigns default ordinals to an enum in order. However, it is not recommended to use ordinals to perform logic.                //Default  ordinals  of  enum                  //  By  default  java  assigns  ordinals  in  order                                  System.out.println(Season.WINTER.ordinal());//0                  System.out.println(Season.SPRING.ordinal());//1                  System.out.println(Season.SUMMER.ordinal());//2                  System.out.println(Season.FALL.ordinal());//3     How  do  you  compare  two  Enums? Values of Enum’s can be compared using == or the equals function.                //Comparing  two  Enums                  Season  season1  =  Season.FALL;                  Season  season2  =  Season.FALL;                  System.out.println(season1  ==  season2);//true                  System.out.println(season1.equals(season2));//true   Can  you  use  a  Switch  Statement  around  an  Enum?   Example  below  shows  how  we  can  use  a  switch  around  an  enum.  
Java  Interview  Questions  –  www.JavaInterview.in   1 5                    //Using  switch  statement  on  an  enum                  public  int  getExpectedMaxTemperature()  {                          switch  (this)  {                          case  WINTER:                                  return  5;                          case  SPRING:                          case  FALL:                                  return  10;                          case  SUMMER:                                  return  20;                          }                          return  -­‐1;//  Dummy  since  Java  does  not  recognize  this  is  possible                         }       Videos   We  have  created  more  than  20  videos  to  help  you  understand  these  topics  and  become  an  expert  at   them.    You  can  watch  these  videos  for  free  on  YouTube.  Visit  our  website  http://www.JavaInterview.in   for  complete  list  of  videos.  We  answer  the  top  200  frequently  asked  interview  questions  on  the  website.   Register  here  for  more  updates  :  https://feedburner.google.com/fb/a/mailverify?uri=RithusTutorials     Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:  https://www.youtube.com/watch?v=njZ48YVkei0   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:  https://www.youtube.com/watch?v=xyXuo0y-xoU   Java  Interview  :  A  Guide  for  Experienced:  https://www.youtube.com/watch?v=0xcgzUdTO5M Collections  Interview  Questions  1:  https://www.youtube.com/watch?v=GnR4hCvEIJQ Collections  Interview  Questions  2:  https://www.youtube.com/watch?v=6dKGpOKAQqs Collections  Interview  Questions  3:  https://www.youtube.com/watch?v=_JTIYhnLemA Collections  Interview  Questions  4:  https://www.youtube.com/watch?v=ZNhT_Z8_q9s Collections  Interview  Questions  5:  https://www.youtube.com/watch?v=W5c8uXi4qTw  

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Core java interview questions

  • 1. Java  Interview  Questions  –  www.JavaInterview.in   1     Following  are  the  important  topics  that  are  important  from  interview  perspective  for  core  java.    We   cover  these  in  great  detail  in  this  guide.     CORE  JAVA  ...........................................................................................................................................................  2   WHY  IS  JAVA  SO  POPULAR?  .............................................................................................................................................  2   WHAT  IS  PLATFORM  INDEPENDENCE?  ...............................................................................................................................  2   WHAT  ARE  THE  IMPORTANT  DIFFERENCES  BETWEEN  C++  AND  JAVA?  .......................................................................................  3   WHAT  ARE  WRAPPER  CLASSES?  ........................................................................................................................................  3   WHAT  ARE  THE  DIFFERENT  UTILITY  METHODS  PRESENT  IN  WRAPPER  CLASSES?  ...........................................................................  4   WHAT  IS  AUTO  BOXING?  ................................................................................................................................................  5   ARE  ALL  STRING’S  IMMUTABLE?  .......................................................................................................................................  6   WHERE  ARE  STRING  LITERALS  STORED  IN  MEMORY?  ..............................................................................................................  6   CAN  YOU  GIVE  EXAMPLES  OF  DIFFERENT  UTILITY  METHODS  IN  STRING  CLASS?  ............................................................................  6   EXPLAIN  ABOUT  TOSTRING  METHOD    IN  JAVA?  ....................................................................................................................  7   WHAT  IS  THE  USE  OF  EQUALS  METHOD  IN  JAVA?  ..................................................................................................................  8   WHAT  ARE  THE  IMPORTANT  THINGS  TO  CONSIDER  WHEN  IMPLEMENTING  EQUALS  METHOD?  ........................................................  9   WHAT  IS  THE  HASHCODE  METHOD  USED  FOR  IN  JAVA?  .........................................................................................................  9   WHAT  IS  CASTING?  ......................................................................................................................................................  10   WHAT  IS  IMPLICIT  CASTING?  .........................................................................................................................................  10   WHAT  IS  EXPLICIT  CASTING?  .........................................................................................................................................  10   HOW  ARE  VARIABLES  INITIALIALIZED  IN  JAVA?  ...................................................................................................................  10   WHAT  IS  A  NESTED  IF  ELSE?  CAN  YOU  EXPLAIN  WITH  AN  EXAMPLE?  .......................................................................................  11   ARRAYS  ..............................................................................................................................................................  12   HOW  DO  YOU  DECLARE  AND  CREATE  AN  ARRAY?  ................................................................................................................  12   CAN  THE  SIZE  OF  AN  ARRAY  BE  CHANGED  DYNAMICALLY?  .....................................................................................................  12  
  • 2. 2   Java  Interview  Questions  –  www.JavaInterview.in       CAN  YOU  CREATE  AN  ARRAY  WITHOUT  DEFINING  SIZE  OF  AN  ARRAY?  ......................................................................................  12   WHAT  ARE  THE  DEFAULT  VALUES  IN  AN  ARRAY?  .................................................................................................................  12   HOW  DO  YOU  LOOP  AROUND  AN  ARRAY  USING  ENHANCED  FOR  LOOP?  ...................................................................................  13   HOW  DO  YOU  PRINT  THE  CONTENT  OF  AN  ARRAY?  ..............................................................................................................  13   HOW  DO  YOU  COMPARE  TWO  ARRAYS?  ...........................................................................................................................  13   HOW  DO  YOU  SORT  AN  ARRAY?  ......................................................................................................................................  14   ENUM  ................................................................................................................................................................  14   WHAT  IS  AN  ENUM?  ....................................................................................................................................................  14   HOW  DO  YOU  CREATE  AN  ENUM  FROM  A  STRING  VALUE?  ....................................................................................................  14   WHAT  IS  AN  ENUM  ORDINAL?  .......................................................................................................................................  14   HOW  DO  YOU  COMPARE  TWO  ENUMS?  ...........................................................................................................................  14   CAN  YOU  USE  A  SWITCH  STATEMENT  AROUND  AN  ENUM?  ...................................................................................................  14     Core  Java   Following  videos  cover  these  topics  in  great  detail.      In  addition  to  following  this  guide,  we  recommend   that  you  watch  the  videos  as  well.   Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:  https://www.youtube.com/watch?v=njZ48YVkei0.   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:  https://www.youtube.com/watch?v=xyXuo0y-xoU   Why  is  Java  so  Popular?   Two  main  reasons  for  popularity  of  Java  are   1. Platform  Independence   2. Object  Oriented  Language   We  will  look  at  these  in  detail  in  later  sections.   What  is  Platform  Independence?   This  video(https://www.youtube.com/watch?v=lLgcgvIHyAw)  explains  Platform  Independence   in  great  detail.  Refer  to  it  for  more  detailed  answer.   Platform  Independence  is  also  called  build  once,  run  anywhere.  Java  is  one  of  the  most  popular  platform   independent  languages.  Once  we  compile  a  java  program  and  build  a  jar,  we  can  run  the  jar  (compiled   java  program)  in  any  Operating  System  -­‐  where  a  JVM  is  installed.   Java  achieves  Platform  Independence  in  a  beautiful  way.  On  compiling  a  java  file  the  output  is  a  class  file   -­‐  which  contains  an  internal  java  representation  called  bytecode.  JVM  converts  bytecode  to  executable   instructions.   The   executable   instructions   are   different   in   different   operating   systems.   So,   there   are   different  JVM's  for  different  operating  systems.  A  JVM  for  windows  is  different  from  a  JVM  for  mac.   However,   both   the   JVM's   understand   the   bytecode   and   convert   it   to   the   executable   code   for   the   respective  operating  system.    
  • 3. Java  Interview  Questions  –  www.JavaInterview.in   3     What  are  the  important  differences  between  C++  and  Java?   1. Java  is  platform  independent.  C++  is  not  platform  independent.   2. Java  is  a  pure  Object  Oriented  Language  (except  for  primitive  variables).  In  C++,  one  can  write   structural  programs  without  using  objects.   3. C++  has  pointers  (access  to  internal  memory).  Java  has  no  concept  called  pointers.   4. In  C++,  programmer  has  to  handle  memory  management.  A  programmer  has  to  write  code  to   remove   an   object   from   memory.   In   Java,   JVM   takes   care   of   removing   objects   from   memory   using  a  process  called  Garbage  Collection.   5. C++  supports  Multiple  Inheritance.  Java  does  not  support  Multiple  Inheritance.     Java  Interview  Questions  –  www.JavaInterview.in   At  http://www.JavaInterview.in,  we  want  you  to  clear  java  interview  with  ease.  So,  in  addition  to   focussing  on  Core  and  Advanced  Java  we  also  focus  on  topics  like  Code  Reviews,  Performance,    Design   Patterns,  Spring  and  Struts.   We  have  created  more  than  20  videos  to  help  you  understand  these  topics  and  become  an  expert  at   them.  Visit  our  website  http://www.JavaInterview.in  for  complete  list  of  videos.    Other  than  the  videos,   we  answer  the  top  200  frequently  asked  interview  questions  on  our  website.   With  more  900K  video  views  (Apr  2015),  we  are  the  most  popular  channel  on  Java  Interview  Questions   on  YouTube.   Register  here  for  more  updates  :     https://feedburner.google.com/fb/a/mailverify?uri=RithusTutorials   Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:     https://www.youtube.com/watch?v=njZ48YVkei0   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:     https://www.youtube.com/watch?v=xyXuo0y-xoU     What  are  wrapper  classes?   This video(https://www.youtube.com/watch?v=YQbZRw2yIBk) covers the topic in great detail. A brief description is provided below. A primitive wrapper class in the Java programming language is one of eight classes provided in the java.lang package to provide object methods for the eight primitive types. All of the primitive wrapper classes in Java are immutable.  
  • 4. 4   Java  Interview  Questions  –  www.JavaInterview.in       Wrapper: Boolean,Byte,Character,Double,Float,Integer,Long,Short Primitive: boolean,byte,char ,double, float, int , long,short Wrapper classes are final and immutable. Examples of creating wrapper classes are listed below. Integer  number  =  new  Integer(55);//int   Integer  number2  =  new  Integer("55");//String     Float  number3  =  new  Float(55.0);//double  argument   Float  number4  =  new  Float(55.0f);//float  argument   Float  number5  =  new  Float("55.0f");//String     Character  c1  =  new  Character('C');//Only  char  constructor   //Character  c2  =  new  Character(124);//COMPILER  ERROR     Boolean  b  =  new  Boolean(true);     //"true"  "True"  "tRUe"  -­‐  all  String  Values  give  True   //Anything  else  gives  false   Boolean  b1  =  new  Boolean("true");//value  stored  -­‐  true   Boolean  b2  =  new  Boolean("True");//value  stored  -­‐  true   Boolean  b3  =  new  Boolean("False");//value  stored  -­‐  false   Boolean  b4  =  new  Boolean("SomeString");//value  stored  -­‐  false     b  =  false;   What  are  the  different  utility  methods  present  in  wrapper  classes?   A  number  of  utility  methods  are  defined  in  wrapper  classes  to  create  and  convert  them  from  primitives.   valueOf    Methods   Provide another way of creating a Wrapper Object Integer  seven  =            Integer.valueOf("111",  2);//binary  111  is  converted  to  7     Integer  hundred  =            Integer.valueOf("100");//100  is  stored  in  variable   xxxValue  methods     xxxValue methods help in creating primitives   Integer  integer  =  Integer.valueOf(57);   int  primitive  =  seven.intValue();//57   float  primitiveFloat  =  seven.floatValue();//57.0f     Float  floatWrapper  =  Float.valueOf(57.0f);   int  floatToInt  =  floatWrapper.intValue();//57   float  floatToFloat  =  floatWrapper.floatValue();//57.0f   parseXxx  methods   parseXxx methods are similar to valueOf but they return primitive values
  • 5. Java  Interview  Questions  –  www.JavaInterview.in   5     int  sevenPrimitive  =            Integer.parseInt("111",  2);//binary  111  is  converted  to  7     int  hundredPrimitive  =            Integer.parseInt("100");//100  is  stored  in  variable   static  toString  method   Look  at  the  example  of  the  toString  static  method  below.   Integer  wrapperEight  =  new  Integer(8);   System.out.println(Integer.                  toString(wrapperEight));//String  Output:  8     Overloaded  static  toString  method    2nd  parameter:  radix   System.out.println(Integer                  .toString(wrapperEight,  2));//String  Output:  1000   static  toXxxString  methods.     Xxx  can  be  Hex,Binary,Octal   System.out.println(Integer                  .toHexString(wrapperEight));//String  Output:8     System.out.println(Integer                  .toBinaryString(wrapperEight));//String  Output:1000   System.out.println(Integer                  .toOctalString(wrapperEight));//String  Output:10   What  is  Auto  Boxing?   Autoboxing  is  the  automatic  conversion  that  the  Java  compiler  makes  between  the  primitive  types  and   their  corresponding  object  wrapper  classes.  For  example,  converting  an  int  to  an  Integer,  a  double  to  a   Double,  and  so  on.  If  the  conversion  goes  the  other  way,  this  is  called  unboxing.   Integer  ten  =  new  Integer(10);   ten++;//allowed.  Java  does  had  work  behind  the  screen  for  us   Boxing  and  new  instances   Auto   Boxing   helps   in   saving   memory   by   reusing   already   created   Wrapper   objects.   However   wrapper   classes  created  using  new  are  not  reused.   Two  wrapper  objects  created  using  new  are  not  same  object.   Integer  nineA  =  new  Integer(9);   Integer  nineB  =  new  Integer(9);   System.out.println(nineA  ==  nineB);//false   System.out.println(nineA.equals(nineB));//true     Two  wrapper  objects  created  using  boxing  are  same  object.   Integer  nineC  =  9;  
  • 6. 6   Java  Interview  Questions  –  www.JavaInterview.in       Integer  nineD  =  9;   System.out.println(nineC  ==  nineD);//true   System.out.println(nineC.equals(nineD));//true   Are  all  String’s  immutable?   This  video  (https://www.youtube.com/watch?v=wh6L8zO_Hr4)  covers  all  the  topics  related  to   String’s  in  great  detail.  Refer  to  it  for  more  details.     Value  of  a  String  Object  once  created  cannot  be  modified.  Any  modification  on  a  String  object  creates  a   new  String  object.   String  str3  =  "value1";   str3.concat("value2");   System.out.println(str3);  //value1     Note  that  the  value  of  str3  is  not  modified  in  the  above  example.    The  result  should  be  assigned  to  a  new   reference  variable  (or  same  variable  can  be  reused).   String  concat  =  str3.concat("value2");   System.out.println(concat);  //value1value2   Where  are  string  literals  stored  in  memory?   All  strings  literals  are  stored  in  "String  constant  pool".  If  compiler  finds  a  String  literal,it  checks  if  it  exists   in  the  pool.  If  it  exists,  it  is  reused.   Following  statement  creates  1  string  object  (created  on  the  pool)  and  1  reference  variable.   String  str1  =  "value";       However,  if  new  operator  is  used  to  create  string  object,  the  new  object  is  created  on  the  heap.   Following  piece  of  code  create  2  objects.   //1.  String  Literal  "value"  -­‐  created  in  the  "String  constant  pool"   //2.  String  Object  -­‐  created  on  the  heap   String  str2  =  new  String("value");   Can  you  give  examples  of  different  utility  methods  in  String  class?   String  class  defines  a  number  of  methods  to  get  information  about  the  string  content.   String  str  =  "abcdefghijk";   Get  information  from  String   Following  methods  help  to  get  information  from  a  String.   //char  charAt(int  paramInt)   System.out.println(str.charAt(2));  //prints  a  char  -­‐  c   System.out.println("ABCDEFGH".length());//8   System.out.println("abcdefghij".toString());  //abcdefghij   System.out.println("ABC".equalsIgnoreCase("abc"));//true     //Get  All  characters  from  index  paramInt  
  • 7. Java  Interview  Questions  –  www.JavaInterview.in   7     //String  substring(int  paramInt)   System.out.println("abcdefghij".substring(3));  //cdefghij     //All  characters  from  index  3  to  6   System.out.println("abcdefghij".substring(3,7));  //defg   Explain  about  toString  method    in  Java?   This video (https://www.youtube.com/watch?v=k02nM5ukV7w) covers toString in great detail. toString method is used to print the content of an Object. If the toString method is not overridden in a class, the default toString method from Object class is invoked. This would print some hashcode as shown in the example below. However, if toString method is overridden, the content returned by the toString method is printed. Consider the class given below: class  Animal  {            public  Animal(String  name,  String  type)  {                  this.name  =  name;                  this.type  =  type;          }            String  name;          String  type;     }   Run this piece of code: Animal  animal  =  new  Animal("Tommy",  "Dog");   System.out.println(animal);//com.rithus.Animal@f7e6a96     Output does NOT show the content of animal (what name? and what type?). To show the content of the animal object, we can override the default implementation of toString method provided by Object class. Adding  toString  to  Animal  class   class  Animal  {                    public  Animal(String  name,  String  type)  {                  this.name  =  name;                  this.type  =  type;          }            String  name;          String  type;            public  String  toString()  {                  return  "Animal  [name="  +  name  +  ",  type="  +  type                                  +  "]";          }     }  
  • 8. 8   Java  Interview  Questions  –  www.JavaInterview.in       Run this piece of code: Animal  animal  =  new  Animal("Tommy","Dog");   System.out.println(animal);//Animal  [name=Tommy,  type=Dog]   Output now shows the content of the animal object. What  is  the  use  of  equals  method  in  Java?   Equals method is used when we compare two objects. Default implementation of equals method is defined in Object class. The implementation is similar to == operator. Two object references are equal only if they are pointing to the same object. We need to override equals method, if we would want to compare the contents of an object. Consider the example Client class provided below. class  Client  {          private  int  id;            public  Client(int  id)  {                  this.id  =  id;          }   }   == comparison operator checks if the object references are pointing to the same object. It does NOT look at the content of the object.   Client  client1  =  new  Client(25);   Client  client2  =  new  Client(25);   Client  client3  =  client1;     //client1  and  client2  are  pointing  to  different  client  objects.   System.out.println(client1  ==  client2);//false     //client3  and  client1  refer  to  the  same  client  objects.   System.out.println(client1  ==  client3);//true     //similar  output  to  ==   System.out.println(client1.equals(client2));//false   System.out.println(client1.equals(client3));//true   We can override the equals method in the Client class to check the content of the objects. Consider the example below: The implementation of equals method checks if the id's of both objects are equal. If so, it returns true. Note that this is a basic implementation of equals and more needs to be done to make it fool- proof. class  Client  {          private  int  id;            public  Client(int  id)  {                  this.id  =  id;          }            @Override  
  • 9. Java  Interview  Questions  –  www.JavaInterview.in   9            public  boolean  equals(Object  obj)  {                  Client  other  =  (Client)  obj;                  if  (id  !=  other.id)                          return  false;                  return  true;          }   }   Consider running the code below: Client  client1  =  new  Client(25);   Client  client2  =  new  Client(25);   Client  client3  =  client1;     //both  id's  are  25   System.out.println(client1.equals(client2));//true     //both  id's  are  25   System.out.println(client1.equals(client3));//true   Above code compares the values (id's) of the objects. What   are   the   important   things   to   consider   when   implementing   equals   method?     Any equals implementation should satisfy these properties: 1. Reflexive. For any reference value x, x.equals(x) returns true. 2. Symmetric. For any reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. 3. Transitive. For any reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) must return true. 4. Consistent. For any reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, if no information used in equals is modified. 5. For any non-null reference value x, x.equals(null) should return false. Let's now provide an implementation of equals which satisfy these properties: //Client class @Override   public  boolean  equals(Object  obj)  {          if  (this  ==  obj)                  return  true;          if  (obj  ==  null)                  return  false;          if  (getClass()  !=  obj.getClass())                  return  false;          Client  other  =  (Client)  obj;          if  (id  !=  other.id)                  return  false;          return  true;   }   What  is  the  hashCode  method  used  for  in  Java?   HashCode's are used in hashing to decide which group (or bucket) an object should be placed into. A group of object's might share the same hashcode.
  • 10. 10   Java  Interview  Questions  –  www.JavaInterview.in       The implementation of hash code decides effectiveness of Hashing. A good hashing function evenly distributes object's into different groups (or buckets). A good hashCode  method  should  have  the  following  properties   • If obj1.equals(obj2) is true, then obj1.hashCode() should be equal to obj2.hashCode() • obj.hashCode() should return the same value when run multiple times, if values of obj used in equals() have not changed. • If obj1.equals(obj2) is false, it is NOT required that obj1.hashCode() is not equal to obj2.hashCode(). Two unequal objects MIGHT have the same hashCode. A sample hashcode implementation of Client class which meets above constraints is given below: //Client class @Override   public  int  hashCode()  {          final  int  prime  =  31;          int  result  =  1;          result  =  prime  *  result  +  id;          return  result;   }   What  is  casting?   Casting is used when we want to convert on data type to another. There are two types of Casting • Implicit Casting • Explicit Casting What  is  Implicit  Casting?   Implicit Casting is done by the compiler. Good examples of implicit casting are all the automatic widening conversions i.e. storing smaller values in larger variable types. int  value  =  100;   long  number  =  value;  //Implicit  Casting   float  f  =  100;  //Implicit  Casting     What  is  Explicit  Casting?   Explicit Casting is done through code. Good examples of explicit casting are the narrowing conversions. Storing larger values into smaller variable types; long  number1  =  25678;   int  number2  =  (int)number1;//Explicit  Casting   //int  x  =  35.35;//COMPILER  ERROR   int  x  =  (int)35.35;//Explicit  Casting   Explicit casting would cause truncation of value if the value stored is greater than the size of the variable. int  bigValue  =  280;   byte  small  =  (byte)  bigValue;   System.out.println(small);//output  24.  Only  8  bits  remain.   How  are  variables  initialialized  in  Java?   Member  and  Static  variables  are  alway  initialized  with  default  values.  Default  values  for  numeric  types  is   0,  floating  point  types  is  0.0,  boolean  is  false,    char    is  'u0000'  and  object  reference  variable  is  null.   Local/block  variables  are  NOT  initialized  by  compiler.    
  • 11. Java  Interview  Questions  –  www.JavaInterview.in   1 1     If  local  variables  are  used  before  initialization,  it  would  result  in  a  compilation  error.   package  com.rithus.variables;     public  class  VariableInitialization  {          public  static  void  main(String[]  args)  {                  Player  player  =  new  Player();                                    //score  is  an  int  member  variable  -­‐  default  0                  System.out.println(player.score);//0  -­‐  RULE1                                    //name  is  a  member  reference  variable  -­‐  default  null                  System.out.println(player.name);//null  -­‐  RULE1                                    int  local;  //not  initialized                  //System.out.println(local);//COMPILER  ERROR!  RULE3                    String  value1;//not  initialized                  //System.out.println(value1);//COMPILER  ERROR!  RULE3                                    String  value2  =  null;//initialized                  System.out.println(value2);//null  -­‐  NO  PROBLEM.          }   }   What  is  a  nested  if  else?  Can  you  explain  with  an  example?   Look  at  the  example  below.  The  code  in  first  if  condition  which  is  true  is  executed.  If  none  of  the  if   conditions  are  true,  then  code  in  else  is  executed.   int  z  =  15;   if(z==10){          System.out.println("Z  is  10");//NOT  executed   }  else  if(z==12){          System.out.println("Z  is  12");//NOT  executed   }  else  if(z==15){          System.out.println("Z  is  15");//executed.  Rest  of  the  if  else  are  skipped.   }  else  {          System.out.println("Z  is  Something  Else.");//NOT  executed   }     z  =  18;   if(z==10){          System.out.println("Z  is  10");//NOT  executed   }  else  if(z==12){          System.out.println("Z  is  12");//NOT  executed   }  else  if(z==15){          System.out.println("Z  is  15");//NOT  executed   }  else  {          System.out.println("Z  is  Something  Else.");//executed   }    
  • 12. 12   Java  Interview  Questions  –  www.JavaInterview.in       Arrays   Refer  to  this  video(https://www.youtube.com/watch?v=8bVysCXT-io)  for  exhaustive  coverage   of  all  the  interview  questions  about  arrays.   How  do  you  declare  and  create  an  array?   Let’s first discuss about how to declare an array. All   below   ways   are   legal.   However,   using   the   third   format  of  declaration  is  recommended.   int  marks[];  //Not  Readable   int[]  runs;  //Not  Readable   int[]  temperatures;//Recommended       Declaration  of  an  Array  should  not  include  size.     //int  values[5];//Compilation  Error!                     Declaring  2D  Array  Examples:   int[][]  matrix1;  //Recommended   int[]  matrix2[];  //Legal  but  not  readable.  Avoid.     Lets  now  look  at  how  to  create  an  array  (define  a  size  and  allocate  memory).   marks  =  new  int[5];  //  5  is  size  of  array     Declaring  and  creating  an  array  in  same  line.   int  marks2[]  =  new  int[5];   Can  the  size  of  an  array  be  changed  dynamically?   Once  An  Array  is  created,  its  size  cannot  be  changed.     Can  you  create  an  array  without  defining  size  of  an  array?   Size  of  an  array  is  mandatory  to  create  an  array.   //marks  =  new  int[];//COMPILER  ERROR   What  are  the  default  values  in  an  array?   New  Arrays  are  always  initialized  with  default  values.   int  marks2[]  =  new  int[5];   System.out.println(marks2[0]);//0     Default  Values   byte,short,int,long        0     float,double                0.0    
  • 13. Java  Interview  Questions  –  www.JavaInterview.in   1 3     boolean                  false   object                        null   How  do  you  loop  around  an  array  using  enhanced  for  loop?   Name  of  the  variable  is  mark  and  the  array  we  want  to  loop  around  is  marks.   for  (int  mark:  marks)  {          System.out.println(mark);   }   How  do  you  print  the  content  of  an  array?   Let’s  look  at  different  methods  in  java  to  print  the  content  of  an  array.   Printing  a  1D  Array   int  marks5[]  =  {  25,  30,  50,  10,  5  };   System.out.println(marks5);  //[I@6db3f829   System.out.println(          Arrays.toString(marks5));//[25,  30,  50,  10,  5]   Printing  a  2D  Array   int[][]  matrix3  =  {  {  1,  2,  3  },  {  4,  5,  6  }  };   System.out.println(matrix3);  //[[I@1d5a0305   System.out.println(                  Arrays.toString(matrix3));     //[[I@6db3f829,  [I@42698403]   System.out.println(                  Arrays.deepToString(matrix3));     //[[1,  2,  3],  [4,  5,  6]]     matrix3[0]  is  a  1D  Array   System.out.println(matrix3[0]);//[I@86c347   System.out.println(                  Arrays.toString(matrix3[0]));//[1,  2,  3]   How  do  you  compare  two  arrays?   Arrays  can  be  compared  using  static  method  equals  defined  in  Arrays  class.  Two  arrays  are  equal  only  if   they  have  the  same  numbers  in  all  positions  and  have  the  same  size.   int[]  numbers1  =  {  1,  2,  3  };   int[]  numbers2  =  {  4,  5,  6  };     System.out.println(Arrays                  .equals(numbers1,  numbers2));  //false     int[]  numbers3  =  {  1,  2,  3  };   System.out.println(Arrays                  .equals(numbers1,  numbers3));  //true  
  • 14. 14   Java  Interview  Questions  –  www.JavaInterview.in       How  do  you  sort  an  array?   Array’s  can  be  sorted  using  static  utility  method  sort  defined  in  Arrays  class.   int  rollNos[]  =  {  12,  5,  7,  9  };   Arrays.sort(rollNos);   System.out.println(                  Arrays.toString(rollNos));//[5,  7,  9,  12]   Enum   Refer  to  this  video  (https://www.youtube.com/watch?v=64Y7EP8-Ark)  for  exhaustive   coverage  of  all  the  interview  questions  about  Enum.   What  is  an  Enum?   Enum allows specifying a list of values for a Type. Consider the example below. It declares an enum Season with 4 possible values.        enum  Season  {                  WINTER,  SPRING,  SUMMER,  FALL          };     How  do  you  create  an  enum  from  a  String  value?   Function valueOf(String) is used to convert a string to enum.                //Converting  String  to  Enum                  Season  season  =  Season.valueOf("FALL");   Function name() is used to find String value of an enum.                //Converting  Enum  to  String                  System.out.println(season.name());//FALL   What  is  an  Enum  Ordinal?   Java assigns default ordinals to an enum in order. However, it is not recommended to use ordinals to perform logic.                //Default  ordinals  of  enum                  //  By  default  java  assigns  ordinals  in  order                                  System.out.println(Season.WINTER.ordinal());//0                  System.out.println(Season.SPRING.ordinal());//1                  System.out.println(Season.SUMMER.ordinal());//2                  System.out.println(Season.FALL.ordinal());//3     How  do  you  compare  two  Enums? Values of Enum’s can be compared using == or the equals function.                //Comparing  two  Enums                  Season  season1  =  Season.FALL;                  Season  season2  =  Season.FALL;                  System.out.println(season1  ==  season2);//true                  System.out.println(season1.equals(season2));//true   Can  you  use  a  Switch  Statement  around  an  Enum?   Example  below  shows  how  we  can  use  a  switch  around  an  enum.  
  • 15. Java  Interview  Questions  –  www.JavaInterview.in   1 5                    //Using  switch  statement  on  an  enum                  public  int  getExpectedMaxTemperature()  {                          switch  (this)  {                          case  WINTER:                                  return  5;                          case  SPRING:                          case  FALL:                                  return  10;                          case  SUMMER:                                  return  20;                          }                          return  -­‐1;//  Dummy  since  Java  does  not  recognize  this  is  possible                         }       Videos   We  have  created  more  than  20  videos  to  help  you  understand  these  topics  and  become  an  expert  at   them.    You  can  watch  these  videos  for  free  on  YouTube.  Visit  our  website  http://www.JavaInterview.in   for  complete  list  of  videos.  We  answer  the  top  200  frequently  asked  interview  questions  on  the  website.   Register  here  for  more  updates  :  https://feedburner.google.com/fb/a/mailverify?uri=RithusTutorials     Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:  https://www.youtube.com/watch?v=njZ48YVkei0   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:  https://www.youtube.com/watch?v=xyXuo0y-xoU   Java  Interview  :  A  Guide  for  Experienced:  https://www.youtube.com/watch?v=0xcgzUdTO5M Collections  Interview  Questions  1:  https://www.youtube.com/watch?v=GnR4hCvEIJQ Collections  Interview  Questions  2:  https://www.youtube.com/watch?v=6dKGpOKAQqs Collections  Interview  Questions  3:  https://www.youtube.com/watch?v=_JTIYhnLemA Collections  Interview  Questions  4:  https://www.youtube.com/watch?v=ZNhT_Z8_q9s Collections  Interview  Questions  5:  https://www.youtube.com/watch?v=W5c8uXi4qTw