是否可以从Java中的构造函数调用抽象方法？ [英] Is it OK to call abstract method from constructor in Java?

问题描述

让我们假设我有一个抽象的Base类来实现Runnable接口。

Let's suppose I have an abstract Base class that implements Runnable interface.

public abstract class Base implements Runnable {

  protected int param;

  public Base(final int param) {
      System.out.println("Base constructor");
      this.param = param;
      // I'm using this param here
      new Thread(this).start();
      System.out.println("Derivative thread created with param " + param);
  }

  @Override
  abstract public void run();
}

这里是几个衍生类之一。

And here is one of a few derivative classes.

public class Derivative extends Base {

  public Derivative(final int param) {
      super(param);
  }

  @Override
  public void run() {
      System.out.println("Derivative is running with param " + param);
  }

  public static void main(String[] args) {
      Derivative thread = new Derivative(1);
  }

}

基类做一些一般的东西，而不是每次都复制它。
实际上，它运行正常，输出总是相同的：

The point is that I want my Base class do some general stuff instead of copying it every time. Actually, it's running fine, the output is always the same:

基本构造函数
使用param创建的派生线程1
派生使用param 1运行

Base constructor Derivative thread created with param 1 Derivative is running with param 1

但是在JAVA中启动一个线程调用构造函数中的抽象方法是否安全？因为，在C ++和C＃在大多数情况下是不安全的，就我所知。
谢谢！

But is it safe IN JAVA to start a thread calling the abstract method in constructor? Because, in C++ and C# it is unsafe in most cases, so far as I know. Thank you!

推荐答案

此代码演示了为什么您应该永远不要调用抽象方法，或任何其他可覆盖的方法，从构造函数：

This code demonstrates why you should never call an abstract method, or any other overridable method, from a constructor:

abstract class Super {
    Super() {
        doSubStuff();
    }
    abstract void doSubStuff();
}

class Sub extends Super {
    String s = "Hello world";

    void doSubStuff() {
        System.out.println(s);
    }
}

public static void main(String[] args) {
    new Sub();
}

运行时，输出 null 。这意味着在构造函数中唯一的安全方法是私有的和/或最终的。

When run, this prints null. This means the only "safe" methods to have in a constructor are private and/or final ones.

另一方面，你的代码实际上并不调用抽象方法从构造函数。相反，你将一个未初始化的对象传递给另一个线程进行处理，这是更糟糕的，因为你的开始的线程可能被赋予优先级并在你的 Base 完成其初始化之前执行。

On the other hand, your code doesn't actually call an abstract method from a constructor. Instead, you pass an uninitialized object to another thread for processing, which is worse, since the thread you're starting may be given priority and execute before your Base finishes its initialization.

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