例如，我们编写两个类，一个是Pupil类（小学生），一个是Graduate（大学毕业生）
问题：两个类的属性和方法有很多是相同的，怎么办？

改进前

package student;
//小学生->模拟小学生考试情况
public class Pupil {
    public String name;
    public int age;
    private double score;//成绩
    public void setScore(double score) {
        this.score = score;
    }
    public void testing(){
        System.out.println("小学生 " + name + " 正在考小学数学..");
    }
    public void showInfo() {
        System.out.println("学生名 " + name + " 年龄 " + age + " 成绩 " + score);
    }
}

package student;
//大学生类-> 模拟大学生考试的简单情况
public class Graduate {
    public String name;
    public int age;
    private double score;//成绩
    public void setScore(double score) {
        this.score = score;
    }
    public void testing() {//和Pupil不一样
        System.out.println("大学生 " + name + " 正在考大学数学..");
    }
    public void showInfo() {
        System.out.println("学生名 " + name + " 年龄 " + age + " 成绩 " + score);
    }
}

package student;
public class Test {
    public static void main(String[] args) {
        Pupil pupil = new Pupil();
        pupil.name = "银角大王";
        pupil.age = 10;
        pupil.testing();
        pupil.setScore(60);
        pupil.showInfo();
        System.out.println("===========================");
        Graduate graduate = new Graduate();
        graduate.name = "金角大王";
        graduate.age = 22;
        graduate.testing();
        graduate.setScore(100);
        graduate.showInfo();
    }
}

解决方案：继承，提高代码复用性！

改进后

package student;
//父类,是Pupil 和 Graduate的父类
public class Stu {
    //共有属性
    public String name;
    public int age;
    private double score;//成绩
    //共有的方法
    public void setScore(double score) {
        this.score = score;
    }
    public void showInfo() {
        System.out.println("学生名 " + name + " 年龄 " + age + " 成绩 " + score);
    }
}

package student;
//小学生->模拟小学生考试情况
//让Pupil 继承 Student类
public class Pupil extends Stu {
    public void testing() {
        System.out.println("小学生 " + name + "  正在考小学数学..");
    }
}

package student;
//大学生类-> 模拟大学生考试的简单情况
public class Graduate extends Stu {
    public void testing() {//和Pupil不一样
        System.out.println("大学生 " + name + " 正在考大学数学..");
    }
}

package student;
public class Test {
    public static void main(String[] args) {
        Pupil pupil = new Pupil();
        pupil.name = "银角大王";
        pupil.age = 10;
        pupil.testing();
        pupil.setScore(60);
        pupil.showInfo();
        System.out.println("===========================");
        Graduate graduate = new Graduate();
        graduate.name = "金角大王";
        graduate.age = 22;
        graduate.testing();
        graduate.setScore(100);
        graduate.showInfo();
    }
}