Violation of Software Principle [closed] - software-design

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I was reading about threads and it was mentioned that by implementing the Runnable interface, we are violating the principle that each object should have a single, clearly-defined responsibility. Can someone explain how is it that it violate the software principle ?

Answer 1

The principle comes from object-oriented modeling community. For example, in OO modeling, if class "A" extends lass "B", then the sentence "A is a kind of B" should make some kind of sense. Maybe the analog for class "A" implements interface "C" is "A is capable of doing C"?
Let's look at some examples:
public class Mammal extends Animal
This example reads well: A mammal is a kind of animal.
Now let's add some interfaces:
public class Mammal extends Animal implements Walking, Carnivorous
Everything is still good: A mammal is a kind of animal that is capable of walking and eating meat.
Now let's add Runnable:
public class Mammal extends Animal implements Walking, Carnivorous, Runnable
Read it: A mammal is an animal that is capable of walking, eating meat, and whose instances are intended to be executed by a thread.
That doesn't sound good. What does being a mammal have to do with the Java Runtime Environment and its threads? Should mammals know they are models running in a computer? The answer, from the object-oriented modeling perspective, is clear: mammals have nothing to do with JRE threads and should not implement the interface Runnable.