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I am reading up on multi-threaded programming in Java. In the java book it says separating the task from the thread is a preferred design. I have no idea what this means.
From what I understand it is better to use implements Runnable instead of extending Threads. I just don’t understand why this is.
Why is implementing better then extending? And when would you use extending then?
An important principle in software design is separation of concerns: Every module/object/class/method should have only one well-defined responsibility.
The Thread class mixes two concerns:
specifiying a sequence of steps to be executed (the Runnable)
interacting with the runtime system to manage how to implement execution of that sequence (in this case by means of a dedicated CPU thread)
An application programmer should not need to concern himself with that second part: She should just need to write down a code path that she wants executed, i.e. implement a Runnable.
That Runnable can then be executed in a number of ways, for example by starting a new Thread, but also by giving it to an ExecutionService. This flexibility goes away when you hard-code your runnable into a Thread.