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Java Abstract class can implement interfaces without even providing the implementation of interface methods. Java Abstract class is used to provide common method implementation to all the subclasses or to provide default implementation. We can run abstract class in java like any other class if it has main() method.
Yes, it is mandatory to implement all the methods in a class that implements an interface until and unless that class is declared as an abstract class.
And yes, you can declare abstract class without defining an abstract method in it. Once you declare a class abstract it indicates that the class is incomplete and, you cannot instantiate it. Hence, if you want to prevent instantiation of a class directly you can declare it abstract.
An abstract class permits you to make functionality that subclasses can implement or override whereas an interface only permits you to state functionality but not to implement it. A class can extend only one abstract class while a class can implement multiple interfaces.
That's because if a class is abstract, then by definition you are required to create subclasses of it to instantiate. The subclasses will be required (by the compiler) to implement any interface methods that the abstract class left out.
Following your example code, try making a subclass of AbstractThing without implementing the m2 method and see what errors the compiler gives you. It will force you to implement this method.
Perfectly fine.
You can't instantiate abstract classes.. but abstract classes can be used to house common implementations for m1() and m3().
So if m2() implementation is different for each implementation but m1 and m3 are not. You could create different concrete IAnything implementations with just the different m2 implementation and derive from AbstractThing -- honoring the DRY principle. Validating if the interface is completely implemented for an abstract class is futile..
Update: Interestingly, I find that C# enforces this as a compile error. You are forced to copy the method signatures and prefix them with 'abstract public' in the abstract base class in this scenario.. (something new everyday:)
That's fine. To understand the above, you have to understand the nature of abstract classes first. They are similar to interfaces in that respect. This is what Oracle say about this here.
Abstract classes are similar to interfaces. You cannot instantiate them, and they may contain a mix of methods declared with or without an implementation.
So you have to think about what happens when an interface extends another interface. For example ...
//Filename: Sports.java
public interface Sports
{
public void setHomeTeam(String name);
public void setVisitingTeam(String name);
}
//Filename: Football.java
public interface Football extends Sports
{
public void homeTeamScored(int points);
public void visitingTeamScored(int points);
public void endOfQuarter(int quarter);
}
... as you can see, this also compiles perfectly fine. Simply because, just like an abstract class, an interface can NOT be instantiated. So, it is not required to explicitly mention the methods from its "parent". However, ALL the parent method signatures DO implicitly become a part of the extending interface or implementing abstract class. So, once a proper class (one that can be instantiated) extends the above, it WILL be required to ensure that every single abstract method is implemented.
Hope that helps... and Allahu 'alam !
Interface means a class that has no implementation of its method, but with just declaration.
Other hand, abstract class is a class that can have implementation of some method along with some method with just declaration, no implementation.
When we implement an interface to an abstract class, its means that the abstract class inherited all the methods of the interface. As, it is not important to implement all the method in abstract class however it comes to abstract class (by inheritance too), so the abstract class can left some of the method in interface without implementation here. But, when this abstract class will inherited by some concrete class, they must have to implements all those unimplemented method there in abstract class.
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