写一个SingleTon，（饿最终、懒同步）

1.饿汉式：

public class SingleTon {
    
    private SingleTon(){
        
    }
    
    private final static SingleTon instance = new SingleTon();
    
    public static SingleTon getInstance(){
        return instance;
    }
}

2.懒汉式：

public class SingleTon {
    
    private SingleTon(){
        
    }
    
    private static SingleTon instance = null;
    
    public static synchronized SingleTon getInstance(){
        if(instance == null)
            instance = new SingleTon();
        return instance;
    }
}

3.枚举：

public enum SingleTon {
    instance;
}