Simple screenshot that explains the singleton invocation.

Here is the code:

 /*
 Some class,such as a config file,need to be only one.So we need to control the instance.
 1,private the constructor and create only one instance in the class itself.
 2,provide a method for the others to get the 'only one' instance.
 */
 package kju.obj;

 import static kju.print.Printer.*;
 public class SingletonDemo {
     public static void main(String[] args) {
         SingleConfig con01 = SingleConfig.getInstance();
         SingleConfig con02 = SingleConfig.getInstance();
         println("con01 color : " + con01.getColor());
         println("con02 set color : ");
         con02.setColor("Blue");
         println("con01 color : " + con01.getColor());
         /*
         con01 color : Orange
         con02 set color :
         con01 color : Blue
         */
     }
 }

 class SingleConfig {
     private String color = "Orange";
     private static SingleConfig s = new SingleConfig();

     private SingleConfig() {}
     public static SingleConfig getInstance() {
         return s;
     }

     public void setColor(String color) {
         this.color = color;
     }

     public String getColor() {
         return color;
     }
 }

And the corresponds the code above: