java - 关于父类(super class)和子类构造函数的问题

Question

为什么代码(1)不会产生错误，而代码(2)(3)会产生错误？

我认为当子类调用构造函数时，它会先调用父类(super class)构造函数，但我不知道为什么代码(1)是正确的，而其他两个是错误的。

//(1)

public class Parent {
  public int a;

  public Parent() {
    this.a = 0;
  }
}

public class Child extends Parent {
  public Child() {}
}

//(2)

public class Parent {
  public int a;

  public Parent(int number) {
    this.a = number;
  }
}

public class Child extends Parent {
  public Child() {}
}

//(3)

public class Parent {
  public int a;

  public Parent(int number) {
    this.a = number;
  }
}

public class Child extends Parent {
  public Child(int numb) {      
  }
}

代码(1)是正确的，而其他两个是错误的。

Answer 1

Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.

因此，您的代码 (2)(3) 没有无参构造函数，而且您也没有显式调用有参数构造函数，因此出现了编译时错误。更多详情来自https://docs.oracle.com/javase/tutorial/java/IandI/super.html