单例设计模式

public class Student {
    private static Student s = new Student();

    private Student() {
    }

    public synchronized static Student getStudent() {
        return s;
    }
}

public class Teacher {
    private static Teacher t;

    private Teacher() {
    }

    public synchronized static Teacher getTeacher() {
        if (t == null) {
            t = new Teacher();
        }
        return t;
    }
}