Java Tutorial - What is a generic type in Java

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The term generics means parameterized types. Using generics, it is possible to create a single class that works with different types of data. A class, interface, or method that operates on a parameterized type is called generic.

Syntax

Here is the syntax for declaring a generic class:

class className<type-param-list> {}

Here is the syntax for declaring a reference to a generic class:

Example

A Simple Generics Example

// 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
  Gen(T o) {//w ww  .  j a v a  2 s . co m
    ob = o;
  }
  // Return ob.
  T getob() {
    return ob;
  }
  // Show type of T.
  void showType() {
    System.out.println("Type of T is " + ob.getClass().getName());
  }
}

public class Main {
  public static void main(String args[]) {
    Gen<Integer> iOb = new Gen<Integer>(88);
    iOb.showType();
    int v = iOb.getob();
    System.out.println("value: " + v);
    Gen<String> strOb = new Gen<String>("Generics Test");
    strOb.showType();
    String str = strOb.getob();
    System.out.println("value: " + str);
  }
}

T is the name of a type parameter. T is used to declare an object. Generics work only with objects Generic types differ based on their type arguments

The code above generates the following result.

Example 2

You can declare more than one type parameter in a generic type.

// A simple generic class with two type parameters: T and V. 
class TwoGen<T, V> {
  T ob1;/* w ww.  j  a v a  2 s.  c  o m*/
  V ob2;
  TwoGen(T o1, V o2) {
    ob1 = o1;
    ob2 = o2;
  }
  void showTypes() {
    System.out.println("Type of T is " + ob1.getClass().getName());
    System.out.println("Type of V is " + ob2.getClass().getName());
  }

  T getob1() {
    return ob1;
  }

  V getob2() {
    return ob2;
  }
}

public class Main {
  public static void main(String args[]) {
    TwoGen<Integer, String> tgObj = new TwoGen<Integer, String>(88, "Generics");
    tgObj.showTypes();
    int v = tgObj.getob1();
    System.out.println("value: " + v);
    String str = tgObj.getob2();
    System.out.println("value: " + str);
  }
}

The code above generates the following result.

Example 3

The following code declares and uses a Queue<E> generic type.

class Queue<E> {
  private E[] elements;
  private int head=0, tail=0;
//from   www .j  ava2s  . c om
  Queue(int size) {
    elements = (E[]) new Object[size];
  }

  void insert(E element) throws QueueFullException {
    if (isFull())
      throw new QueueFullException();
    elements[tail] = element;
    tail = (tail + 1) % elements.length;
  }

  E remove() throws QueueEmptyException {
    if (isEmpty()){
      throw new QueueEmptyException();
    }
    E element = elements[head];
    head = (head + 1) % elements.length;
    return element;
  }

  boolean isEmpty() {
    return head == tail;
  }

  boolean isFull() {
    return (tail + 1) % elements.length == head;
  }
}
class QueueEmptyException extends Exception {
}

class QueueFullException extends Exception {
}
public class Main{
  public static void main(String[] args) 
      throws QueueFullException, QueueEmptyException {
    Queue<String> queue = new Queue<String>(6);
    System.out.println("Empty: " + queue.isEmpty());
    System.out.println("Full: " + queue.isFull());
    queue.insert("A");
    queue.insert("B");
    queue.insert("C");
    queue.insert("D");
    queue.insert("E");
    System.out.println("Empty: " + queue.isEmpty());
    System.out.println("Full: " + queue.isFull());
    System.out.println("Removing " + queue.remove());
    System.out.println("Empty: " + queue.isEmpty());
    System.out.println("Full: " + queue.isFull());
    System.out.println("Adding F");
    queue.insert("F");
    while (!queue.isEmpty()){
      System.out.println("Removing " + queue.remove());
    }
      
    System.out.println("Empty: " + queue.isEmpty());
    System.out.println("Full: " + queue.isFull());
  }
}

Output:

Deal with legacy code

To handle the transition to generics, Java allows a generic class to be used without any type arguments.

Here is an example that shows a raw type in action:

class MyClass<T> {
  T ob;/*from  w w  w.j  av  a  2s. c  o  m*/
  MyClass(T o) {
    ob = o;
  }
  T getob() {
    return ob;
  }
}
public class Main {
  public static void main(String args[]) {
    MyClass raw = new MyClass(new Double(98.6));
    double d = (Double) raw.getob();
    System.out.println("value: " + d);
  }
}

Output:

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