懒汉单例and饿汉单例（案例）

package com;

public class Test {
    public static void main(String[] args) {
        //懒汉单例 一个类里面只存在一个对象
        Student student = Student.sun();
        Student student1 = Student.sun();
        System.out.println(student);
        System.out.println(student1);
        //饿汉单例 一个类里面只存在一个对象
        Student student2 = Student.student;
        Student student3 = Student.student;
        System.out.println(student3);
        System.out.println(student2);

    }
}

package com;

public class Student {

   //懒汉单例
    public static Student sum;

    //懒汉单例 提供方法
   public static Student sun(){
       if(sum==null){
           sum=new Student();
       }
       return sum;
   }
   //饿汉单例
    public static Student student=new Student();

    private Student(){

    }
}

运行结果
com.Student@4eec7777
com.Student@4eec7777

com.Student@3b07d329
com.Student@3b07d329