Is it OK to call abstract method from constructor in Java? [duplicate]

Question

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(); } 

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);   }  } 

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:

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

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!

Answer 1

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(); } 

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

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.