概念

继承的概念来源于----人工智能中的一个概念知识图谱
如果一个实体拥有(has)一个实体，我们就将被拥有的那个实体设置为一个字段。
student has (age,name,grade)
teacher has (age,name,salary)

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如果我们把两个实体中的共性抽象出来。

为了减少代码冗余，可以引入继承。

代码

首先我们新建一个类person.java

package ex02;

public class Person {
    private String name;
    private int age;

    public int getAge() {
        return age;
    }

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

    public String getName() {
        return name;
    }

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

然后新建一个类student继承类person，使用关键字extends
包名.extends

package ex02;

public class Student extends Person{
    private double grade;

    public double getGrade() {
        return grade;
    }

    public void setGrade(double grade) {
        this.grade = grade;
    }
}

继承冲突

如果person和son中都有name,student继承哪一个？
java是单继承的。

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protected

在student中做一个方法msg()
msg可以返回一个字符串（name+age+…）

这样定义这个方法可以吗？不可以因为在person类中，name是一个private类型，只能在类内方法访问。因此我们可以使用里面的public方法get()访问我们想使用的data.
拥有却不能直接访问。。
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可以这样编码。

    public String msg(){
        return this.getName() + "," + this.getAge() + "," + this.getGrade();
    }

如果我既不想让外人(student.age)访问，又可以让继承的类直接访问。
此时就可以使用新的修饰protected.

package ex02;

public class Person {
    protected String name;
    protected int age;

    public int getAge() {
        return age;
    }

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

    public String getName() {
        return name;
    }

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

package ex02;

public class Student extends Person{
    protected double grade;

    public double getGrade() {
        return grade;
    }

    public void setGrade(double grade) {
        this.grade = grade;
    }

    public String msg(){
        return name + "," + age + "," + grade;
    }


}

这样修饰就不会报错了。

super调用父类方法

package ex02;

public class Person {
    protected String name;
    protected int age;

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

    public int getAge() {
        return age;
    }

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

    public String getName() {
        return name;
    }

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

super调用

package ex02;

public class Student extends Person{
    protected double grade;

    public Student(String name, int age, double grade){
        super(name, age);
        this.grade = grade;

    }
    public double getGrade() {
        return grade;
    }

    public void setGrade(double grade) {
        this.grade = grade;
    }

    public String msg(){
        return name + "," + age + "," + grade;
    }
}

person可以引用student

package ex02;

public class Student extends Person{
    protected double grade;

    public Student(String name, int age, double grade){
        super(name, age);
        this.grade = grade;

    }
    public double getGrade() {
        return grade;
    }

    public void setGrade(double grade) {
        this.grade = grade;
    }

    public String msg(){
        return name + "," + age + "," + grade;
    }
}

类型不匹配

编译时候不会发现。

package ex02;

public class Main {
    public static void main(String[] args) {
        Person p1 = new Person();
        Person p2 = new Student();
//        不能直接setGrade
        ((Student)p2).setGrade(3.6);

        Student s1 = (Student) p1;
        Student s2 = (Student) p2;
        s1.setGrade(3.5);
    }
}

Exception in thread "main" java.lang.ClassCastException: class ex02.Person cannot be cast to class ex02.Student (ex02.Person and ex02.Student are in unnamed module of loader 'app')
	at ex02.Main.main(Main.java:10)