super 关键字

super 关键字

class FatherClass {
    public int value;
    public void f(){
        value = 100;
        System.out.println("FatherClass.value="+value);
    }
}

class ChildClass extends FatherClass {
    public int value;
    public void f(){
        super.f();
        value = 200;
        System.out.println("ChildClass.value="+value);
        System.out.println(value);
        System.out.println(super.value);
    }
}

public class Child {
    public static void main(String[] args) {
        ChildClass childClass = new ChildClass();
        childClass.f();
    }
}

FatherClass.value=100
ChildClass.value=200
200
100

class Person {
    private String name;
    private String location;

    Person(String name) {
        this.name = name;
        location = "beijing";
    }

    Person(String name, String location) {
        this.name = name;
        this.location = location;
    }

    public String info() {
        return "name: " + name + " location: " + location;
    }
}

class Student extends Person {
    private String school;

    Student(String name, String school) {
        this(name, "beijing", school);
    }

    Student(String n, String l, String school) {
        super(n, l);
        this.school = school;
    }

    @Override
    public String info() {
        return super.info() + " school: " + school;
    }
}

public class Test {
    public static void main(String[] args) {
        Person p1 = new Person("A");
        Person p2 = new Person("B", "shanghai");
        Student s1 = new Student("C", "S1");
        Student s2 = new Student("C", "shanghai", "S2");
        System.out.println(p1.info());
        System.out.println(p2.info());
        System.out.println(s1.info());
        System.out.println(s2.info());
    }
}

name: A location: beijing
name: B location: shanghai
name: C location: beijing school: S1
name: C location: shanghai school: S2

1. 使用 super. 访问父类的变量

class Vehicle {
    int maxSpeed = 120;
}

class Car extends Vehicle {
    int maxSpeed = 180;

    void display() {
        System.out.println(maxSpeed);
        System.out.println("Maximum Speed: " + super.maxSpeed);
    }
}

public class Test {
    public static void main(String[] args) {
        Car small = new Car();
        small.display();
    }
}

180
Maximum Speed: 120

2. 使用 super. 访问父类的方法

class Person {
    void message() {
        System.out.println("This is person class");
    }
}

class Student extends Person {
    @Override
    void message() {
        System.out.println("This is student class");
    }

    void display() {
        message();
        super.message();
    }
}

public class Test {
    public static void main(String args[]) {
        Student s = new Student();
        s.display();
    }
}

This is student class
This is person class

3. 使用 super 访问父类构造函数

class Person {
    Person() {
        System.out.println("Person class Constructor");
    }
}

class Student extends Person {
    Student() {
        super();
        System.out.println("Student class Constructor");
    }
}

public class Test {
    public static void main(String[] args) {
        Student s = new Student();
    }
}

Person class Constructor
Student class Constructor

注意:

在子类构造方法中通过 super() 调用父类构造方法的时候必须放在构造方法的第一行。 如果子类没有显示的调用父类的构造方法，则 Java 编译器会自动在子类构造方法中调用父类默认无参的构造方法。如果父类没有提供无参的构造方法，则会出现编译错误。 Java 程序中如何没有写构造方法，则JVM会自动生成一个无参的构造方法，如果写了构造方法的话，就不会提供了。 super（）应该是任何构造函数中的第一个语句。它只能在构造函数内部使用，而不能在其他任何地方使用。super（）用于仅引用父类（超类）的构造函数。

* [https://www.geeksforgeeks.org/super-keyword/](https://www.geeksforgeeks.org/super-keyword/)

class Grandparent {
    public void Print() {
        System.out.println("Grandparent's Print()");
    }
}

class Parent extends Grandparent {
    public void Print() {
        super.Print();
        System.out.println("Parent's Print()");
    }
}

class Child extends Parent {
    public void Print() {
        super.Print();
        System.out.println("Child's Print()");
    }
}

public class Main {
    public static void main(String[] args) {
        Child c = new Child();
        c.Print();
    }
}

Grandparent's Print()
Parent's Print()
Child's Print()

不能这样使用 super.super.Print(); 会出现错误