Generics

Quote

When you take an element out of a Collection, you must cast it to the type of element that is stored in the collection. Besides being inconvenient, this is unsafe. The compiler does not check that your cast is the same as the collection's type, so the cast can fail at run time.

Generics provides a way for you to communicate the type of a collection to the compiler, so that it can be checked. Once the compiler knows the element type of the collection, the compiler can check that you have used the collection consistently and can insert the correct casts on values being taken out of the collection.

Here is a simple example taken from the existing Collections tutorial:

Code

GeSHi (java):
    // Removes 4-letter words from c. Elements must be strings
    static void expurgate(Collection c) {
        for (Iterator i = c.iterator(); i.hasNext(); )
          if (((String) i.next()).length() == 4)
            i.remove();
    }
Created by GeSHI 1.0.7.20

Here is the same example modified to use generics:

Code

GeSHi (java):
    // Removes the 4-letter words from c
    static void expurgate(Collection<String> c) {
        for (Iterator<String> i = c.iterator(); i.hasNext(); )
          if (i.next().length() == 4)
            i.remove();
    }
 
Created by GeSHI 1.0.7.20

Generic implements:

Code

GeSHi (java):
public interface MyInterface {}
public class MyClass<T extends MyInterface> {
   //etc, using T as normal
}
Created by GeSHI 1.0.7.20

More info:
http://java.sun.com/j2se/...de/language/generics.html