Java 7 Pocket Guide, 2nd Edition

Chapter 16. Generics Framework

The Generics Framework, introduced in Java SE 5.0, provides support that allows for the parameterization of types.

The benefit of generics is the significant reduction in the amount of code that needs to be written when developing a library. Another benefit is the elimination of casting in many situations.

The classes of the Collections Framework, the class Class, and other Java libraries have been updated to include generics.

See Java Generics and Collections by Maurice Naftalin and Philip Wadler (O’Reilly, 2006) for comprehensive coverage of the Generics Framework.

Generic Classes and Interfaces

Generic classes and interfaces parameterize types by adding a type parameter within angular brackets (i.e., <T>). The type is instantiated at the place of the brackets.

Once instantiated, the generic parameter type is applied throughout the class for methods that have the same type specified. In the following example, the add() and get() methods use the parameterized type as their parameter argument and return types, respectively:

public interface List <E> extends Collection<E>{
  public boolean add(E e);
  E get(int index);
}

When a variable of a parameterized type is declared, a concrete type (i.e., <Integer>) is specified to be used in place of the type parameter (i.e., <E>).

Subsequently, the need to cast when retrieving elements from things such as collections would be eliminated:

// Collection List/ArrayList with Generics
List<Integer> iList = new ArrayList<Integer ...