subclass of thread implementing Runnable interface

问题

It confuses me why a subclass of thread that implements a runnable interface doesn't force me to override the run method. Basically, when i create simple class that implements Runnable it forces me to override the run method. But when i made the ordinary class a subclass of thread, it didn't force me to override the class anymore. What's the logic behind this?

回答1:

When a non-abstract class declares that it implements an interface, what that means is that the class must have a concrete implementation of every method that the interface defines, but those implementation methods don't have to be explicit in the class, they could be inherited from a superclass. In this specific example java.lang.Thread has its own public void run(), which subclasses inherit, so a subclass of Thread that implements Runnable doesn't need to add its own override to satisfy the compiler (though it probably does to actually be useful).

回答2:

This is because Thread itself implements run().

@Override
public void run() {
    if (target != null) {
        target.run();
    }
}

By default it does nothing. If we extend a Thread we implement this method so that it does something useful

来源：https://stackoverflow.com/questions/30543929/subclass-of-thread-implementing-runnable-interface