学号的单一

仿照课堂的身份证的例子，实现每个同学仅有一个学号这一问题。

Client：

package 实验7;

public class Client {
    public static void main(String a[]) {
        StudentID stu1,stu2;
        String id1,id2;
        System.out.println("第一次生成学生");
        stu1 = StudentID.getInstance();

        System.out.println("第二次生成学生");
        stu2 = StudentID.getInstance();

        System.out.println("两学生学号是否一致：" + (stu1==stu2));

        id1 = stu1.getStudentID();
        id2 = stu1.getStudentID();
        System.out.println("第一次产生的学号：" + id1);
        System.out.println("第二次产生的学号：" + id2);
        System.out.println("内容是否相等：" + id1.equalsIgnoreCase(id2));
        System.out.println("是否是相同对象：" + (id1==id2));
    }
}

StudentID：

package 实验7;

public class StudentID {
    private static StudentID instance = null;
    private String id;

    private StudentID(){}

    public static StudentID getInstance() {
        if (instance == null) {
            System.out.println("学生信息注册，分配学号！");
            instance = new StudentID();
            instance.setStudentID("20213961");
        } else {
            System.out.println("学生信息已经被注册，获取其已经存在的学号！");
        }
        return instance;
    }

    private void setStudentID(String id) {
        this.id = id;
    }

    public String getStudentID() {
        return this.id;
    }
}