Object oriented programming2 Week 2.pptx

WEEK 2
Instructor:
Nazish Basir
OBJECT
ORIENTED
PROGRAMMIN
G

TOPICS
• T H E S C O P E A N D L I F E T I M E O F
VA R I A B L E S
• T Y P E C O N V E R S I O N I N J AVA
• W I D E N I N G O R AU TO M AT I C T Y P E
C O N V E R S I O N
• N A R R OW I N G O R E X P L I C I T C O N V E R S I O N
• T Y P E P R O M OT I O N I N E X P R E S S I O N S
• E X P L I C I T T Y P E C A S T I N G I N
E X P R E S S I O N S
• O N E D I M E N S I O N A L A R R AYS ( 1 D A R R AY )
• T WO – D I M E N S I O N A L A R R AY ( 2 D -

THE SCOPE AND LIFETIME OF VARIABLES
• Every time you start a new block, you are creating a new scope.
• A scope determines what objects are visible to other parts of
your program.
• It also determines the lifetime of those objects.
• Scopes can be nested.
• The objects declared in the outer scope will be visible to code
within the inner scope.
• Objects declared within the inner scope will not be visible
outside it.

TYPE CONVERSION IN JAVA
• When you assign value of one data type to another
if the data types are compatible, then Java will
perform the conversion automatically known as
AutomaticType Conversion.
• If not then they need to be casted or converted
explicitly.

AUTOMATIC TYPE CONVERSION
• Also known as ImplicitType Conversion OR
Widening Conversion
• This happens when:
–The two data types are compatible.
–When we assign value of a smaller data type to a bigger
data type.

VALID DATA TYPES FOR ASSIGNMENT
DataType Compatible DataTypes
byte byte
short byte, short
int byte, short, int, char
long byte, short, int, char, long
float byte, short, int, char, long, float
double byte, short, int, char, long, float, double
boolean boolean
char char

NARROWING OR EXPLICIT CONVERSION
• If we want to assign a value of larger data type to a
smaller data type we perform explicit type casting
or narrowing.
• To create a conversion between two incompatible
types, you must use a cast.
• A cast is simply an explicit type conversion. It has
this general form:
(target-type) value

TYPE PROMOTION IN EXPRESSIONS
• Some conditions for type promotion are:
–Java automatically promotes each byte, short, or char
operand to int when evaluating an expression.
–If one operand is a long, float or double the whole
expression is promoted to long, float or double
respectively.

EXPLICIT TYPE CASTING IN EXPRESSIONS
• If we store that result in any smaller data type it
generates compile time error, due to which we need to
type cast the result.
• For example:
byte b = 50;
b = b * 2; //Error! Can’t assign int to byte
• You should use an explicit cast, such as:
b = (byte)(b * 2);

EXERCISE 1:
Write down a program which has 4 int
variables a, b, c and d with different values.
Then store the result of equation
“ab + cd”
in a variable “ans” of float data type.
The result should be accurate.

ARRAYS
• Java provides a data structure, the array, which stores a fixed-
size sequential collection of elements of the same type.
• An array is used to store a collection of data, but it is often
more useful to think of an array as a collection of variables of
the same type.
• Instead of declaring individual variables, such as:
number0, number1, ..., and number99,
• You declare one array variable such as numbers to represent
individual variables:
numbers[0], numbers[1], and ..., numbers[99]

DECLARING ARRAY VARIABLES
• Here is the syntax for declaring an array variable:
dataType[] arrayRefVar;
or
dataType arrayRefVar[];
• You can declare multiple arrays of the same data type in
one statement by inserting a comma after each array
name, using this syntax:
dataType[] arrayName1, arrayName2;

CREATING ARRAYS
• You can create an array by using the new operator with the
following syntax:
arrayRefVar = new dataType[arraySize];
• It assigns the reference of the newly created array to the
variable arrayRefVar.
• Declaring an array variable, creating an array, and assigning the
reference of the array to the variable can be combined in one
statement, as shown below:
dataType[] arrayRefVar = new
dataType[arraySize];
• For Example: int[] arr = new int[10];

CREATING ARRAYS
• Alternatively you can create arrays as follows:
dataType[] arrayRefVar = {value0, value1, ...,
valuek};
• The number of elements in the array is determined by the number
of values in the initialization list.
• The values can be an expression, for example, nine and nine + 2.
int nine = 9;
int[] oddNumbers = {1 ,3 ,5 ,7 ,nine ,nine+2, 13,};

ACCESSING ARRAY ELEMENTS
• Elements of an array are accessed using index, within the array.
• The index of the first element in the array is always 0 and the
index of the last element is always 1 less than the number of
elements.
• Arrays have a read-only, integer instance variable, length, which
holds the number of elements in the array.
• To access the number of elements in an array named
arrayName, use this syntax:
arr.length
• Thus, to access the last element of an array, use this syntax:
arr [arr.length – 1 ]

EXERCISE 2:
Write down a program which has an array of
10 integer numbers then write the code to
find the largest and smallest number from that
array

TWO – DIMENSIONAL ARRAY (2D-ARRAY)
• A two – dimensional array can be seen as an array of one –
dimensional array for easier understanding.
• Syntax to Declare Multidimensional Array in Java:
dataType[][] arrayRefVar; (or)
dataType [][]arrayRefVar; (or)
dataType arrayRefVar[][]; (or)
dataType []arrayRefVar[];
• Example to instantiate Multidimensional Array in Java
int[][] arr=new int[3][3];//3 row and 3 column

TWO – DIMENSIONAL ARRAY (2D-ARRAY)
• When you allocate memory for a multidimensional array, you
need only specify the memory for the first (leftmost) dimension.
• You can allocate the remaining dimensions separately.
int twoD[][] = new int[3][];
twoD[0] = new int[5];
• You do not need to allocate the same number of elements for
each dimension.
• You can create a two-dimensional array in which the sizes of the
second dimension are unequal.

JAVA STRING
• String, is not a simple type.
• Nor is it simply an array of characters.
• Rather, String defines an object.
• The String type is used to declare string variables.
• You can also declare arrays of strings.
• String constant can be assigned to a String variable.
• A variable of type String can be assigned to another variable of type
String.
• You can use an object of type String as an argument to println( ).
String str = "this is a test";
System.out.println(str);

Object oriented programming2 Week 2.pptx

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