Generic Types in Java

A generic type is a generic class or interface that is parameterized over types.

A generic class is defined with the following format:

class name { /* ... */ }

The most commonly used type parameter names are:

• E - Element (used extensively by the Java Collections Framework)
• K - Key
• N - Number
• T - Type
• V - Value
• S,U,V etc. - 2nd, 3rd, 4th types

The Diamond:

Replace the type arguments required to invoke the constructor of a generic class with an empty set of type arguments (<>) as long as the compiler can determine, or infer, the type arguments from the context. This pair of angle brackets, <>, is informally called the diamond. For example, you can create an instance of Box with the following statement:

Box integerBox = new Box<>();

Multiple Types Parameter:

public interface Pair {
    public K getKey();
    public V getValue();
}

public class OrderedPair implements Pair {

    private K key;
    private V value;

    public OrderedPair(K key, V value) {
 this.key = key;
 this.value = value;
    }

    public K getKey() { return key; }
    public V getValue() { return value; }
}



The following statements create two instantiations of the OrderedPair class:

Pair p1 = new OrderedPair("Even", 8);
Pair  p2 = new OrderedPair("hello", "world");



The code, new OrderedPair, instantiates K as a String and V as an Integer. Therefore, the parameter types of OrderedPair's constructor are String and Integer, respectively. Due to autoboxing, it is valid to pass a String and an int to the class.



As mentioned in The Diamond, because a Java compiler can infer the K and V types from the declaration OrderedPair, these statements can be shortened using diamond notation:

OrderedPair p1 = new OrderedPair<>("Even", 8);
OrderedPair  p2 = new OrderedPair<>("hello", "world");

Parameterized Types

You can also substitute a type parameter (i.e., K or V) with a parameterized type (i.e., List). For example, using the OrderedPair example:
OrderedPairBox

More details at:

http://docs.oracle.com/javase/tutorial/java/generics/types.html