Java 入门24 设计模式 单例

 饿汉单例模式

SingleInstance

package com.ITheima._static_codeblock;

public class SingleInstance {

    /**
     * 1 把无参构造器私有藏起来
     */
    private SingleInstance(){

    }

    /**
     * 2 定义一个公开的静态的成员变量存储一个类的对象
     * 饿汉:在这里加载静态变量的时候就会创建对象了
     */
    public static SingleInstance instance=new SingleInstance();
}

Test类

package com.ITheima._static_codeblock;

public class Test {
    public static void main(String[] args) {
        SingleInstance s1=SingleInstance.instance;
        SingleInstance s2=SingleInstance.instance;
        SingleInstance s3=SingleInstance.instance;

        System.out.println(s1);
        System.out.println(s2);
        System.out.println(s3);

        System.out.println(s1 == s2 && s2 == s3);//true


    }
}

懒汉单例模式

 SingleInstance1类

package com.ITheima._static_codeblock;

public class SingleInstance1 {

    //1 私有化构造器
    private SingleInstance1() {

    }

    //2 创建一个静态的成员变量存储类的对象,注意此时不能初始化对象
    private static SingleInstance1 instance = null;

    //3定义一个方法 ,让其他地方可以来调用获取一个对象
    public static SingleInstance1 getInstance() {
        //第一次获取对象
        if (instance == null) {
            instance = new SingleInstance1();
        }
        return instance;
    }
}

Test1类

package com.ITheima._static_codeblock;

public class Test1 {
    public static void main(String[] args) {
        SingleInstance1 s1 = SingleInstance1.getInstance();
        SingleInstance1 s2 = SingleInstance1.getInstance();
        SingleInstance1 s3 = SingleInstance1.getInstance();


        System.out.println(s1 == s2 && s2 == s3);//true

    }
}