At its core, the term generics means parameterized types. Parameterized types are important because they enable you to create classes, interfaces, and methods in which the type of data upon which they operate is specified as a parameter.
Using generics, it is possible to create a single class, for example, that automatically works with different types of data. A class, interface, or method that operates on a parameterized type is called generic, as in generic class or generic method
Generics add the type safety that was lacking. They also streamline the process, because it is no longer necessary to explicitly employ casts to translate between Object and the type of data that is actually being operated upon. With generics, all casts are automatic and implicit
A Simple Generics Example in java
// A simple generic class.
// Here, T is a type parameter that
// will be replaced by a real type
// when an object of type Gen is created.
class Gen<T> {
T ob; // declare an object of type T
// Pass the constructor a reference to
// an object of type T.
Gen(T o) {
ob = o;
}
// Return ob.
T getob() {
return ob;
}
// Show type of T.
void showType() {
System.out.println("Type of T is " +
ob.getClass().getName());
}
}
Demonstrate the generic class
class GenDemo {
public static void main(String args[]) {
// Create a Gen reference for Integers.
Gen<Integer> iOb;
// Create a Gen<Integer> object and assign its
// reference to iOb. Notice the use of autoboxing
// to encapsulate the value 88 within an Integer object.
iOb = new Gen<Integer>(88);
// Show the type of data used by iOb.
iOb.showType();
// Get the value in iOb. Notice that
// no cast is needed.
int v = iOb.getob();
System.out.println("value: " + v);
System.out.println();
// Create a Gen object for Strings.
Gen<String> strOb = new Gen<String>("Generics Test");
// Show the type of data used by strOb.
strOb.showType();
// Get the value of strOb. Again, notice
// that no cast is needed.
String str = strOb.getob();
System.out.println("value: " + str);
}
}
Out of generic class
Type of T is java.lang.Integer
value: 88
Type of T is java.lang.String
value: Generics Test