java中如何将子类的输入提供给父类？

Question

前任

class A () {

  class A(int a, int b) {
  }

}

class B extends A {

   int m;
   int n;

   class B()
   {
      getInput(); // i wanna invoke this method first before calling super(). But it does not allow in Java. How to work around this ?
      super(m,n);
   }

   public void getInput() {
        Scanner scanner = new Scanner(System.in);
        m = scanner.nextInt();
        n = scanner.nextInt();
   }

   public static void main () {
      B b = new B();
   }
}

Answer 1

您可以强制超类在其构造函数的开头运行一个方法，然后在子类中重写该方法。许多框架都有一个“设置”类型的方法，您可以重写它来完成此类事情。

public class A {
   protected int a; // 'protected' so subclass can see it
   protected int b;

   public A() {
       setup(); // Runs whatever setup method is implemented, even in subclasses
   }

   protected void setup() { /* nothing */ }  // 'protected' to be overridden by subclass
}

public class B extends A {

   public B()
   {
      super();
   }

   /**
    *  When A's constructor calls setup(), this method will run.
    */
   @Override
   protected void setup() {
      Scanner scanner = new Scanner(System.in);
      a = scanner.nextInt(); // Stores value in A's protected variable.
      b = scanner.nextInt();
   }  
}

根据您所编写的类的具体情况，您可能有多个构造函数、用于设置值的公共或受保护方法等。这就是 Java 相当灵活的地方。正如下面的评论所示，这在构造函数中并不是一个很好的实践，但我需要更多的上下文来弄清楚如何完成您所要求的任务。

Answer 2

您可以将多个构造函数链接在一起，就像 jbrookover 以这种方式提到的那样。虽然有点复杂：

class A () {
  public A(int a, int b) {
  }
}

class B extends A {

   int m;
   int n;

   public B()
   {
      this(new Scanner(System.in));
   }

   private B(Scanner scanner) {
       this(scanner.nextInt(),scanner.nextInt())
   }

   private B(int m, int n) {
       super(m,n)
       this.m = m;
       this.n = n;
   }

   public static void main (String ... args) {
      B b = new B();
   }
}