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The abstract keyword signifies that the particular method will have no features in the class where it is declared and it is upto the child class to provide the functionality.

In case of an interface, the method is already abstract by default and has no code inside it. So there is no actual point in using the abstract keyword there.

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Q: Why don't use abstract keyword when we declare a method in interface?
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What if a final keyword is applied to a function?

The final keyword in JAVA means that the class can no longer be derived, i.e. it cannot be used as a base class for a new child class.If you declare a method as final, this method cannot be overridden in any of the child class that may extend this class.


Can you have a final abstract class?

No. The abstract keyword means that you cannot instantiate the class unless you extend it with a subclass. The final keyword means that you cannot create subclasses of that class.Combining them would lead to an unusable class, so the compiler will not let this happen.


What is an interface class and what is an abstract class?

The term interface class does not exist in C#. If this is a term describing a class being an interface to other component (human, subsystems, etc), it is very application specific. The designer of that application should know the abstraction.However, C# does have another type called interface. An interface is NOT a class. An interface defines the intention of some behaviors that classes may be extended it and provides the implementation. The intention, is nothing but method signatures, which defines the return data type, the method name, and any method arguments and associated data type. The implementation is the code of the method. Interface is used for separating the concern of design and implementation.Abstract class is a class with abstract keyword. It can be just like a class without that keyword (then, why it is an abstract class?). But it may have some methods or properties defined as abstract. These abstract methods, like the method signatures of an interface, defines the intention.The subclasses of such an abstract class would need to implement those abstract methods (providing the code).There are more common, differences between interfaces and abstract classes, please see answer(s) of those related questions in C# category.


Can you have an interface without any abstract method?

My answer to "Can an interface without any abstract method?" is YES.If abstract methods do not include the properties, then this interface should have at least 1 property (get or set) defined. It makes sense to have at least one of the abstract method or property/mutator/getter-setter.If the interface contains no properties, no methods, I cannot think of a reason to create such an interface, meaning - it is useless.


Abstract class vs interface?

Comparison between an Abstract Class and an Interface:While an abstract class can define both abstract and non-abstract methods, an interface can have only abstract methods. Another way interfaces differ from abstract classes is that interfaces have very little flexibility in how the methods and variables defined in the interface are declared. These rules are strict:

Related questions

What do mean by interface in java?

An interface in Java is like an abstract class, but there are no method bodies allowed in it and it has to be declared with the interface keyword. It is Java's way of getting around the Deadly Diamond of Death. Only abstract methods and constants are allowed in it.


What if a final keyword is applied to a function?

The final keyword in JAVA means that the class can no longer be derived, i.e. it cannot be used as a base class for a new child class.If you declare a method as final, this method cannot be overridden in any of the child class that may extend this class.


Can you have a final abstract class?

No. The abstract keyword means that you cannot instantiate the class unless you extend it with a subclass. The final keyword means that you cannot create subclasses of that class.Combining them would lead to an unusable class, so the compiler will not let this happen.


What is an interface class and what is an abstract class?

The term interface class does not exist in C#. If this is a term describing a class being an interface to other component (human, subsystems, etc), it is very application specific. The designer of that application should know the abstraction.However, C# does have another type called interface. An interface is NOT a class. An interface defines the intention of some behaviors that classes may be extended it and provides the implementation. The intention, is nothing but method signatures, which defines the return data type, the method name, and any method arguments and associated data type. The implementation is the code of the method. Interface is used for separating the concern of design and implementation.Abstract class is a class with abstract keyword. It can be just like a class without that keyword (then, why it is an abstract class?). But it may have some methods or properties defined as abstract. These abstract methods, like the method signatures of an interface, defines the intention.The subclasses of such an abstract class would need to implement those abstract methods (providing the code).There are more common, differences between interfaces and abstract classes, please see answer(s) of those related questions in C# category.


Can you have an interface without any abstract method?

My answer to "Can an interface without any abstract method?" is YES.If abstract methods do not include the properties, then this interface should have at least 1 property (get or set) defined. It makes sense to have at least one of the abstract method or property/mutator/getter-setter.If the interface contains no properties, no methods, I cannot think of a reason to create such an interface, meaning - it is useless.


Is Interface is a kind of abstract method?

No


Abstract class vs interface?

Comparison between an Abstract Class and an Interface:While an abstract class can define both abstract and non-abstract methods, an interface can have only abstract methods. Another way interfaces differ from abstract classes is that interfaces have very little flexibility in how the methods and variables defined in the interface are declared. These rules are strict:


What is difference between interface and abstract in java?

While an abstract class can define both abstract and non-abstract methods, an interface can have only abstract methods. Another way interfaces differ from abstract classes is that interfaces have very little flexibility in how the methods and variables defined in the interface are declared. These rules are strict: • All interface methods are implicitly public and abstract. In other words, you do not need to actually type the public or abstract modifiers in the method declaration, but the method is still always public and abstract. (You can use any kind of modifiers in the Abstract class) • All variables defined in an interface must be public, static, and final-in other words, interfaces can declare only constants, not instance variables. • Interface methods must not be static. • Because interface methods are abstract, they cannot be marked final, strictfp, or native. (More on these modifiers later.) • An interface can extend one or more other interfaces. • An interface cannot extend anything but another interface. • An interface cannot implement another interface or class. • An interface must be declared with the keyword interface. You must remember that all interface methods are public and abstract regardless of what you see in the interface definition.


When do you declare a method or class abstract in java?

when overriding of a class or a method is necessary, they can be declared as abstract


Interface in java?

include all abstract method


When an interface method is implemented in a class it must be declared as?

Abstract type


What is difference between interface and abstract class?

All the methods declared inside an Interface are abstract. Where as abstract class must have at least one abstract method and others may be concrete or abstract. In Interface we need not use the keyword abstract for the methods.