Java New Programming Features

Agenda GenericsCompile-time type safety for collections without casting Enhanced for loopAutomates use of Iterators to avoid errors Autoboxing/unboxingAvoids manual conversion between primitive types (such as int) and wrapper types (such as Integer) Typesafe enumsProvides all the well-known benefits of the Typesafe Enum pattern1

Generics - IntroTutorials : http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdfCompile-time type safety without casting.

Methods can be defined to operate on unknown types of Objects without using ‘Objects’ class. And no type casting is required.

Big advantage: collections are now type safe.2Example : ‘Generify ‘ your Java classes –public class Box<T> { private T t ; public void add(Tt ) { this.t = t; } public T get() { return t; } }How to use this :Box<Integer> integerBox = new Box<Integer>(); … integerBox.add(new Integer(25)); //typesafe only Integers can be passed Integer myInt = integerBox.get() //no typecasting requiredT is a type parameter that will be replaced by a real type.T is the name of a type parameter.This name is used as a placeholder for the actual type that will be passed to Gen when an object is created.

Generics – Example 23Another Example public class GenericFactory<E> { Class theClass = null; public GenericFactory(Class theClass) { this.theClass = theClass; } public E createInstance() throws …{ return (E) this.theClass.newInstance(); }Usage : Creates a Factory of Generic TypeGenericFactory<MyClass> factory = new GenericFactory<MyClass>(MyClass.class);MyClass myClassInstance = factory.createInstance();

Generics – Generics in Methods4static <T> void fromArrayToCollection(T[] a, Collection<T> c) { for (T o : a) { c.add(o); } } static <T> Collection<T> fromArrayToCollectionv2(T[] a) { Collection<T> c = new ArrayList<T>(); for (T o : a) { c.add(o); } return c ; }One more Interesting Example…public <T> T ifThenElse(boolean b, T first, T second) { return b ? first : second; }

Generics – With Collections5ListsList<String> list = new ArrayList<String>(); list.add(“A String”); String string2 = list.get(0); // no type casting.Maps (can define multiple typed parameters) Map<String, String> map = new HashMap<String, String>();map.put ("key1", "value1");map.put ("key2", "value2");String value1 = map.get("key1");Iterating a Generic List:List<String> list = new ArrayList<String>(); Iterator<String> iterator = list.iterator(); while(iterator.hasNext()){ String aString = iterator.next(); }Backward Compatibility List list = new ArrayList<String>(); Nested Generic Types List<List<String>> myListOfListsOfStrings;

Generics – Diving Deeper6Generics and SubtypingList<String> ls = new ArrayList<String>(); List<Object> lo = ls; //illegal won’t compile!!What’s the Problem ?lo.add(new Object()); String s = ls.get(0); // attempts to assign an Object to a String!In other words Collection<Object> is not a supertype of all the types of collectionsTo cope with this sort of situation use Wildcardsstatic void printCollection2(Collection<?> c) { // Collection<?> -“collection of unknown” for (Object e : c) { System.out.println(e); } }Limitation in using unbounded wildcardAny method that takes Generic type of argument cannot be invoked :Collection<?> c = new ArrayList<String>();c.add(new Object()); // compile time error

Generics – Subtyping continued…7Bounded WildcardsList<? extends Shape> is an example of a bounded wildcard.A wildcard with an upper bound looks like " ? extends Type “

A wildcard with a lower bound looks like " ? super Type " Example :public class Collections { public static <T> void copy ( List<? super T> dest, List<? extends T> src) { // uses bounded wildcards for (int i=0; i<src.size(); i++) dest.set(i,src.get(i)); } }List<Object> output = new ArrayList< Object >(); List<Long> input = new ArrayList< Long >(); ... Collections.copy(output,input); // fineList<String> output = new ArrayList< String >(); List<Long> input = new ArrayList< Long >(); ... Collections.copy(output,input); // error

Enhanced for-loop or for each loop8New syntax of for loop that works for Collections and Arrays - Collections :void cancelAll(Collection<TimerTask> c) { for (TimerTask t : c) { t.cancel(); } }Arrays :int sum(int[] a) { int result = 0; for (int i : a) { result += i; } return result; }Nested –for (Suit suit : suits) { for (Rank rank : ranks){ sortedDeck.add(new Card(suit, rank)); }} No need to use iterators. (The compiler does this for you behind your back)

The for-each construct gets rid of the clutter and the opportunity for error.Autoboxing9primitive types to be converted into respective wrapper objects and vice versa.(by the Compiler)int i; Integer j; j = 2; // int to Integer automatic conversioni = j; // Integer.intValue() callj = i; // int to Integer automatic conversionAdvantages : 1. Since Wrapper classes are immutable intObject = new Integer(10); Before Java 5 int intPrimitive = intObject.intValue(); intPrimitive++; intObject = new Integer(intPrimitive); Java 5 AutoBoxingintObject++;2. Easy to use wrapper classes within Collections ArrayList<Integer> list = new ArrayList<Integer>(); for(int i = 0; i < 10; i++) list.add(i); // Boxing

Enum - Introduction10Java Constants Before JDK 1.5 –public static final int SEASON_WINTER = 0; public static final int SEASON_SPRING = 1; ……Issues:Not Type-safe Since season is represented as integer one can pass any integer values, without any compiler warnings.No Name space one may need to refer to java-docs (if any) to check the possible values in case more than one type of constants are declared in a single interface/class. Compile time binding Print Values not self explanatory.JDK 1.5 gets linguistic support for enumerated types as follows :enum Season { WINTER, SPRING, SUMMER, FALL }** Don’t confuse enum with the Enumeration interface, an almost obsolete way of iterating over Collections, replaced by Iterator.

Enum - Example11How to define them ..public enum Flavor { CHOCOLATE(100), //declare all the possible values first. VANILLA(120), STRAWBERRY(80); private int fCalories; // can have state (member variables) int getCalories(){ // cusotm method return fCalories; } private Flavor(int aCalories){ // constructor fCalories = aCalories; } }How to use them …String getColor(Flavor flv){ // Used as normal java objects.. if(flv == Flavor.VANILLA) // “==“ can be used for value comparison return “White” ; ……. switch(flv) { case VANILLA : // You can use enums as case labels ……}String color = getColor(Flavor.VANILLA) // no constructor only constant values.

Java New Programming Features

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Java New Programming Features (20)

Recently uploaded (20)

Java New Programming Features