单例模式

//1.懒汉模式

public class SingletonDemo {
    private static SingletonDemo instance;
    private SingletonDemo(){
    }
    public static SingletonDemo getInstance(){
        if(instance==null){
            instance=new SingletonDemo();
        }
        return instance;
    }
}
//2. 线程安全的懒汉模式

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

//3.饿汉

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