Scenario of extending Thread class and implementin

问题:

This question already has an answer here:

• “implements Runnable” vs “extends Thread” in Java 41 answers
• The difference between the Runnable and Callable interfaces in Java 11 answers
• subclass of thread implementing Runnable interface 2 answers

I am new to thread programming in Java and hence this basic question. (I checked, but could not find this question previously asked)

I read that threads can be created either by inheriting the Thread class or by implementing the Runnable interface. I saw a code which had both to the same class.

public class ThreadExample extends Thread implements Runnable {
}

I was wondering what kind of situation would want this and if there is any advantage to this, what is it.

回答1:

extending Thread and implementing Runnable is useless (Thread already implements Runnable). You pretty much always want to implement Runnable (and not extend Thread). That gives you the flexibility of using a Thread directly (not recommended) or using one of the newer ThreadPool implementations in java.util.concurrent (recommended).

回答2:

No, There is no advantage of using this approach, because, Thread class implements Runnable interface. So, if your class extends Thread class. It means, it's also implementing Runnable interface.

回答3:

IN this specific situation it's not very useful, as other posters have explained it's fairly obvious that Thread already implements Runnable.

In some cases, "stating the obvious" can be useful though, just as a "reminder" to the user of your class: if you have a fairly large hierarchy of super-classes and interfaces, with several levels of inheritance (some of them in 3rd-party libraries), it could be useful as a helper to declare a class as implementing a specific interface, even though it implements it by definition because its superclass already implements it or implements one of the sub-class of that interface.

It's specially useful with marker interfaces (some people might object they should not be used at all, and they are bad practices - well sometimes you don't control the environment fully), i.e. interfaces with no actual implementation and just designed to mark your object eligible for a special function (e.g. Cloneable). In such a case, marking each of the allowed classes even though their parents are already eligible can be more explicit, so more useful.

回答4:

Runnable interface means assigning one job to many threads while Thread class,each of threads has a unique object associated with it.

回答5:

Implementing runnable interface would be preferable when you need needs more flexibility of extending other base classes.