Singleton class example in java

/**

 * In the Singleton class, when we first time call getInstance() method,

 *  it creates an object of the class and return it to the variable. 

 *  Since getInstance() is static, it is changed from null to some object.

 *   Next time, if we try to call getInstance() method,

 *    since single_instance is not null, it is returned to the variable,

 *     instead of instantiating the Singleton class again. 

 *     This part is done by if condition

 */

package com.belazy.algorithm;

/**

 * @author ravi shankar mullath

 * 

 */

public class SingletonClass {

private static SingletonClass singleton = null;

public String s = null;

/**

* I made default constructor as private

*/

private SingletonClass() {

s = "sachin tendulkar";

}

/**

* @param args

*/

public static SingletonClass getInstance() {

if (null == singleton)

singleton = new SingletonClass();

return singleton;

}

}

/**

 * 

 */

package com.belazy.algorithm;

/**

 * @author belazy

 *

 */

public class MainClass {

/**

* @param consumerfed kozhikode

*/

public static void main(String[] args) {

// TODO Auto-generated method stub

SingletonClass x = SingletonClass.getInstance();

SingletonClass y = SingletonClass.getInstance();

SingletonClass z = SingletonClass.getInstance();

x.s = x.s.toUpperCase();

System.out.println(y.s);

}

}

Output

SACHIN TENDULKAR

Explanation

Here we made the default constructor as private. so this constructor cannot be access from outside the class. So i restricted created objects out side from singleton class.

A new instance of the class can be created only through getInstance() method

SingletonClass x = SingletonClass.getInstance();

While creating an instance it will check whether the instance is already created or not.