java - 错误: Constructor A in class A cannot be applied to given types

Question

public class A{

    public A(String x){
        System.out.println("A constructor Called "+x);
    }
     public static void main(String []args){
        System.out.println("Hello World");
        A a= new B("b");
     }
}

class B extends A{
    public B(String x){
        System.out.println("B constructor Called "+x);
    }
}

这个非常简单的程序有什么问题，我无法找到它。编译时出现以下错误:

A.java:13: error: constructor A in class A cannot be applied to given types;                                                                                                    
    public B(String x){                                                                                                                                                         
                      ^                                                                                                                                                         
  required: String                                                                                                                                                              
  found: no arguments                                                                                                                                                           
  reason: actual and formal argument lists differ in length  

Answer 1

由于类 A 没有默认构造函数，因此您需要告诉类 B 如何构造其父类:

class B extends A{
    public B(String x){
        super(x);  // this constructs the parent class
        System.out.println("B constructor Called "+x);
    }
}

错误告诉您必须调用的构造函数需要一个字符串:

required: String

...但是您正在调用的构造函数(这是默认构造函数，因为您没有调用 super )没有参数:

found: no arguments