Java Multithreading and Inheritance

Question

class A implements Runnable  
class B extends A

Under these circumstances B IS A Runnable.

Is it valid to write:

class B extends A implements Runnable

If it is valid, will the run method in B override that of A? What could be the possible scenarios?
I'm uh confused...

Answer 1

Well, by extending A, you are already implementing Runnable, so you simply need to override the run() method and all will be well in Mordor.

If you don't, the superclasses run() will take effect.

Answer 2

It is valid as interfaces only define that a method exists. B.run will override that of A, same as if B did not implement Runnable directly or if A did not implement Runnable but did have a public void run() method.

Answer 3

Since "implements Runnable" doesn't introduce any executable code into a class but is basically just a promise to implement the necessary methods, repeating "implements Runnable" on a class that already extends another class that implements Runnable does effectively nothing.

There is a very slight difference that can be seen when using reflection, but other than that there is no difference.

Answer 4

Its enough to write

class B extends A

Class A already implements runnable so extending it makes this runnable too. To do something specific to class B override run() in class B.

Answer 5

class B extends A implements Runnable

is valid Java syntax, but doesn't really do anything. By extending A, B is Runnable.

When the run method is called on an A object, A's run method will be called. When run is called on a B object, A's run method will be called unless B overrides A's run method

Consider the following:

class A implements Runnable
{
    public void run()
    {
          System.out.println("A's run method");
    }
}

class B extends A implements Runnable
{
    public void run()
    {
          System.out.println("B's run method");
    }
}

class Test
{
    public static void main(String args[])
    {
       A obj1 = new A();
       A obj2 = new B();
       B obj3 = new B();
       Runnable obj4 = new A();
       Runnable obj5 = new B();

       obj1.run(); // prints "A's run method"
       obj2.run(); // prints "B's run method"
       obj3.run(); // prints "B's run method"
       obj4.run(); // prints "A's run method"
       obj5.run(); // prints "B's run method"

    }
}