单例模式--Java实现

画类图

具体代码实现

//StudentNo.java
package org.example.design005;

public class StudentNo {
    private static StudentNo instance=null;

    private StudentNo(){}
    public static StudentNo getInstance() {
        if(instance==null){
            System.out.println("第一次注册，分配新的学号");
            instance=new StudentNo();
            instance.setNo("20214063");
        }else{
            System.out.println("重复获取，获取旧的学号");
        }
        return instance;
    }

    public static void setInstance(StudentNo instance) {
        StudentNo.instance = instance;
    }

    public String getNo() {
        return no;
    }

    public void setNo(String no) {
        this.no = no;
    }

    private String no;
}


//Main.java
package org.example.design005;

public class Main {
    public static void main(String[] args) {
        StudentNo x,y;
        x=StudentNo.getInstance();
        y=StudentNo.getInstance();

        System.out.println("查看学号是否一致："+(x==y));

        String a,b;
        a=StudentNo.getInstance().getNo();
        b=StudentNo.getInstance().getNo();

        System.out.println("第一次的学号："+a);
        System.out.println("第二次的学号："+b);

        System.out.println("内容是否一致："+(a.equals(b)));
        System.out.println("对象是否相同："+(a==b));

    }
}