单例模式

1.懒汉式（线程不安全）　　不建议使用　　

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

2.懒汉式（线程安全）　　缺点：使用了锁，效率较低 　　优点：需要使用时才创建对象，节约内存

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

3.饿汉式　　缺点：类加载时就创建对象，浪费内存　　优点：没有使用锁，效率高

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

4.还没学会