1. Real World Scenario
Sometimes it's appropriate to have exactly one instance of a class:
window manager
print spooler
Filesystems
press Ctrl-F to display a Find dialog.
Typically, those types of objects—known as singletons—are accessed by disparate objects throughout a software system, and therefore require a global point of access.

2. Singleton pattern
Intent
Ensure a class only has one instance, and provide a global point of access to it
Motivation
It’s important for some classes to have exactly one instance.
Although there can be many printers in a system, there should be only one printer spooler.
There should be only one file system (or file system manager) and one window manager.

3. +getInstance(): Singleton

4. public class Singleton {
private static Singleton instance = null;
private Singleton() { }
public static Singleton getInstance() {
if(instance == null) {
instance = new Singleton();
return instance;
public class TestSingleton{
public static void main(String[] args){
Singleton s = Singleton.getInstance(); …

5. package singleton;
import javax.swing.*;
public class SingletonFrame extends JFrame {
private static SingletonFrame myInstance = null;
// the constructor
private SingletonFrame() {
this.setSize(400, 100);
this.setTitle("Singleton Frame. Timestamp:" + System.currentTimeMillis());
this.setDefaultCloseOperation(JFrame.HIDE_ON_CLOSE); }
public static SingletonFrame getInstance() {
if (myInstance == null)
myInstance = new SingletonFrame();
return myInstance;

6. package singleton;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class MyFrame extends JFrame {
JButton jButton1 = new JButton();
JButton jButton2 = new JButton();
public MyFrame() {
init(); }
public static void main(String[] args) {
MyFrame frame = new MyFrame();
frame.setSize(300, 250);
frame.setVisible(true);

7. public void actionPerformed(ActionEvent e) {
private void init() {
this.setDefaultCloseOperation(EXIT_ON_CLOSE);
jButton1.setText("Show Singleton Frame");
jButton1.setBounds(new Rectangle(12, 12, 220, 40));
jButton1.addActionListener(
new java.awt.event.ActionListener() {
public void actionPerformed(ActionEvent e) {
SingletonFrame singletonFrame = SingletonFrame.getInstance();
singletonFrame.setVisible(true); } );
jButton2.setText("Show the same Singleton Frame");
jButton2.setBounds(new Rectangle(12, 72, 220, 40));
jButton2.addActionListener(
this.getContentPane().setLayout(null);
this.getContentPane().add(jButton1, null);
this.getContentPane().add(jButton2, null);
} // end of class MyFrame