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Why do many Collection classes in Java extend the Abstract class and also implement the interface (which is also implemented by the given abstract class)?
For example, class HashSet extends AbstractSet and also implements Set, but AbstractSet already implements Set.
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Remember, a Java class can only have 1 superclass, but it can implement multiple interfaces. Thus, if a class already has a different superclass, it can implement an interface, but it cannot extend another abstract class. Therefore interfaces are a more flexible mechanism for exposing a common interface.
If abstract class doesn't have any method implementation, its better to use interface because java doesn't support multiple class inheritance. The subclass of abstract class in java must implement all the abstract methods unless the subclass is also an abstract class.
An abstract class is used if you want to provide a common, implemented functionality among all the implementations of the component. Abstract classes will allow you to partially implement your class, whereas interfaces would have no implementation for any members whatsoever.
In Java, abstract classes give the ability to define both concrete and abstract methods whereas interfaces only give the ability to implement abstract methods.
It's a way to remember that this class really implements that interface.
It won't have any bad effect and it can help to understand the code without going through the complete hierarchy of the given class.
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