Java基础补全 之 继承

JAVA基础特性：多态，继承，封装

本文补充完善一下继承的相关知识

 

基础类间的继承

• 子类对父类函数的重写（Override）

有参无参构造函数的继承调用

 public class Heritage {
    class A{
        A(){
            System.out.println("this is construct function of class A");
        }
        A(String param){
            System.out.println("this is construct function with param of class A");
        }
    }
    class B extends A{
        B(){
            System.out.println("this is construct function of class B");
        }
        B(String param){
            System.out.println("this is construct function with param of class B");
        }
    }

    public void test(){
        B b = new B();
        B bb = new B("fuck");
    }

    public static void main(String[] args) {
        new Heritage().test();
    }
}

    this is construct function of class A
    this is construct function of class B
    this is construct function of class A
    this is construct function with param of class B

  实例化时，若不给定参数，编译器自动调用无参构造函数。此时创建子类对象，编译器先调用父类的无参构造函数，再调用子类的构造函数。

  给定参数时，若不在子类构造函数用super指定父类构造函数，则默认调用父类无参构造函数，若想要调用父类相应的有参构造函数，则需要手动用super()函数在子类构造函数内调用。

super

         B(String param){
            super(param);
            System.out.println("this is construct function with param of class B");
        }

this is construct function with param of class A
this is construct function with param of class B

• 子类父类构造函数间的调用关系

public class Heritage {
    class A{
        A(){
            System.out.println("this is construct function of class A");
        }
        A(String param){
            System.out.println("this is construct function with param of class A");
        }
        void display(){
            System.out.println("this is function of class A");
        }
    }
    class B extends A{
        B(){
            System.out.println("this is construct function of class B");
        }
        B(String param){
            super(param);
            System.out.println("this is construct function with param of class B");
        }
        @Override
        void display(){
            System.out.println("this is function of class B");
        }
    }

    public void test(){
//        B b = new B();
//        B b1 = new B("fuck");
        A b2 = new B();
        b2.display();
    }

    public static void main(String[] args) {
        new Heritage().test();
    }
}

this is construct function of class A
this is construct function of class B
this is function of class B

父类引用可指向子类对象，但此时调用的是子类函数