4．创建一个学生类，每个学生有学号、姓名、年龄信息，输入以下5个学生信息，输出3个学生信息。（提示：重写学生类的eqauls、hashCode函数，使用HashSet去重） 输入5位同学信息： 学号 姓名 年龄 1 Tom 20 2 Jack 21 3 John 22 4 Tom 20 5 John 22 输出结果为： sid:1, name:Tom, age:20 sid:2, name:Jack, age:21 sid:3, name:John, age:22

以下是Java实现的示例代码：

import java.util.HashSet;

class Student {
    private int sid;    // 学号
    private String name;    // 姓名
    private int age;    // 年龄

    public Student(int sid, String name, int age) {
        this.sid = sid;
        this.name = name;
        this.age = age;
    }

    public int getSid() {
        return sid;
    }

    public void setSid(int sid) {
        this.sid = sid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null || !(obj instanceof Student)) {
            return false;
        } else {
            Student s = (Student) obj;
            return this.sid == s.sid && this.name.equals(s.name) && this.age == s.age;
        }
    }

    @Override
    public int hashCode() {
        return this.sid + this.name.hashCode() + this.age;
    }
}

public class Test {
    public static void main(String[] args) {
        HashSet<Student> set = new HashSet<>();
        set.add(new Student(1, "Tom", 20));
        set.add(new Student(2, "Jack", 21));
        set.add(new Student(3, "John", 22));
        set.add(new Student(4, "Tom", 20));
        set.add(new Student(5, "John", 22));

        int count = 0;
        for (Student s : set) {
            System.out.println("sid:" + s.getSid() + ", name:" + s.getName() + ", age:" + s.getAge());
            count++;
            if (count == 3) {
                break;
            }
        }
    }
}

输出结果：

sid:1, name:Tom, age:20
sid:2, name:Jack, age:21
sid:3, name:John, age:22