java singleton

1public class SingletonClass {
2    private static final SingletonClass SINGLE_INSTANCE = new SingletonClass();
3    private SingletonClass() {}
4  public static SingletonClass getInstance() {
5      return SINGLE_INSTANCE;
6    }
7}

1- A private constructor
2- A static field containing its only instance
3- A static factory method for obtaining the instance
4
5public final class ClassSingleton 
6{
7
8    private static ClassSingleton INSTANCE;
9    private String info = "Initial info class";
10    
11    private ClassSingleton() 
12	{        
13    }
14    
15    public static ClassSingleton getInstance() 
16	{
17        if(INSTANCE == null) 
18        {
19            INSTANCE = new ClassSingleton();
20        }
21  
22        return INSTANCE;
23    }
24
25    // getters and setters
26}
27
28                      =============Usage============
29                        
30
31To use our ClassSingleton, we simply need to get the instance statically:
32
33ClassSingleton classSingleton1 = ClassSingleton.getInstance();
34
35System.out.println(classSingleton1.getInfo()); //Initial class info
36
37ClassSingleton classSingleton2 = ClassSingleton.getInstance();
38classSingleton2.setInfo("New class info");
39
40System.out.println(classSingleton1.getInfo()); //New class info
41System.out.println(classSingleton2.getInfo()); //New class info
42

1public class SingletonOnDemand {
2	
3	private SingletonOnDemand () {}
4	private static class Singleton {
5		private static final SingletonOnDemand instance = new SingletonOnDemand();
6	}
7	
8	public static SingletonOnDemand getInstance () {
9		System.out.println("create instance");
10		return Singleton.instance;
11	}
12}

1public final class SomeSingleton {
2   public static final SomeSingleton INSTANCE;
3
4   private SomeSingleton() {
5      INSTANCE = (SomeSingleton)this;
6      System.out.println("init complete");
7   }
8
9   static {
10      new SomeSingleton();
11   }
12}

1public final class ClassSingleton {
2
3    private static ClassSingleton INSTANCE;
4    private String info = "Initial info class";
5    
6    private ClassSingleton() {        
7    }
8    
9    public static ClassSingleton getInstance() {
10        if(INSTANCE == null) {
11            INSTANCE = new ClassSingleton();
12        }
13        
14        return INSTANCE;
15    }
16
17    // getters and setters
18}