Generics in Java - Motivation

Motivation

The following block of Java code illustrates a problem that exists when not using generics. First, it declares an ArrayList of type Object. Then, it adds a String to the ArrayList. Finally, it attempts to retrieve the added String and cast it to an Integer.

List v = new ArrayList; v.add("test"); Integer i = (Integer)v.get(0); // Run time error

Although the code compiles without error, it throws a runtime exception (java.lang.ClassCastException) when executing the third line of code. This type of problem can be avoided by using generics and is the primary motivation for using generics.

Using generics, the above code fragment can be rewritten as follows:

List v = new ArrayList; v.add("test"); Integer i = v.get(0); // (type error) Compile time error

The type parameter String within the angle brackets declares the ArrayList to be constituted of String (a descendant of the ArrayList's generic Object constituents). With generics, it is no longer necessary to cast the third line to any particular type, because the result of v.get(0) is defined as String by the code generated by the compiler.

Compiling the third line of this fragment with J2SE 5.0 (or later) will yield a compile-time error because the compiler will detect that v.get(0) returns String instead of Integer. For a more elaborate example, see reference.

Here is a small excerpt from the definitions of the interfaces List and Iterator in package java.util:

public interface List { void add(E x); Iterator iterator; } public interface Iterator { E next; boolean hasNext; }

Read more about this topic:  Generics In Java

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