Three simple singleton pattern small examples

The first

public class SingtenDemo1 {
    private volatile static SingtenDemo1 singleton;  
    private SingtenDemo1 (){}  
    public static SingtenDemo1 getSingleton() {  
    if (singleton == null) {  
        synchronized (SingtenDemo1.class) {  
        if (singleton == null) {  
            singleton = new SingtenDemo1();  
        }  
        }  
    }  
    return singleton;  
    }  
}

the second

public class SingtenDemo2 {
    private static SingtenDemo2 instance = null;
    static{
        instance = new SingtenDemo2();
    }
    private SingtenDemo2(){}
    public static SingtenDemo2 getInstance(){
        return instance;
    }
}

the third

public enum SingtenDemo {
    INSTANCE;
}

The third singleton pattern supports serialization and deserialization by default.