Java Generic Type
In this chapter you will learn:
- What is a generic type
- What is the syntax for declaring a generic class
- Example - Java Generic Type
- A Generic Class with Two Type Parameters
- Example - Generic Queue
Description
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) {/*from w w w .j a va 2 s. c om*/
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;/*from w w w . j ava 2 s . co 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 w w w. j a v a 2 s.co m
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:
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