Why interface methods are abstract?


Caleb Eyres   |   Member since 2008  |  10+ Answers Submitted  |  ✔ Verified

An interface is like a "purely" abstract class. The class and all of its methods are abstract. An abstract class can have implemented methods but the class itself cannot be instantiated (useful for inheritance and following DRY). If you implement the Interface then you must implement the methods in the interface .

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Eduardo Umney   |   Member since 2016  |  10+ Answers Submitted  |  ✔ Verified

Then, why abstract method is used in interface?

Abstract classes allow you to create blueprints for concrete classes. But the inheriting class should implement the abstract method. Abstract classes cannot be instantiated. Interfaces are used to achieve abstraction.

Similarly, why the methods of interface are public and abstract by default? All the methods of interfaces are public & abstract by default. Java 8 allows the interfaces to have default and static methods. The reason we have default methods in interfaces is to allow the developers to add new methods to the interfaces without affecting the classes that implements these interfaces.


Rufus Coll   |   Member since 2017  |  10+ Answers Submitted  |  ✔ Verified

In respect to this, can we write abstract methods in interface?

So the difference is an abstract class can have implemented methods whereas a interface cannot. All of the methods in an interface are implicitly abstract, so the abstract modifier is not used with interface methods (it could be—it's just not necessary). Abstract classes can have abstract methods.


Brad Armstrong   |   Member since 2019  |  10+ Answers Submitted  |  ✔ Verified

What is abstract and interface?

Main difference is methods of a Java interface are implicitly abstract and cannot have implementations. A Java abstract class can have instance methods that implements a default behavior. Variables declared in a Java interface is by default final. An abstract class may contain non-final variables.


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Related Answered Questions

Below is a list of answers to questions that have a similarity, or relationship to, the answers on "Why interface methods are abstract?". This list is displayed so that you can easily and quickly access the available answers, without having to search first.

Matt Gray   |   Member since 2009  |  ✔ Verified

How many abstract methods can an interface have?

These are also used to create blueprints for concrete classes but abstract classes may have implemented methods. Abstract classes can implement one or more interfaces and can extend one abstract class at most.


Juliette Vernon   |   Member since 2017  |  ✔ Verified

Can we extend abstract class?

In Java, abstract means that the class can still be extended by other classes but that it can never be instantiated (turned into an object). Abstract classes can include abstract methods. Any class that extends a class with an abstract method must implement that method.


Lucas Marshall   |   Member since 2010  |  ✔ Verified

How do you write an abstract method?

Abstract Methodsabstract keyword is used to declare the method as abstract. You have to place the abstract keyword before the method name in the method declaration. An abstract method contains a method signature, but no method body. Instead of curly braces, an abstract method will have a semoi colon (;) at the end.


Caleb Silva   |   Member since 2009  |  ✔ Verified

Can a final class have abstract methods?

Yes, there may be "final" methods in "abstract" class. But, any "abstract" method in the class can't be declared final. It will give "illegal combination of modifiers: abstract and final" error. Here is the working example of the implementation.


Jackeline Moran   |   Member since 2017  |  ✔ Verified

Why interface methods are not protected?

Protected methods are intended for sharing implementation with subclasses. Interfaces have nothing to offer as far as implementation sharing goes, because they have no implementation at all. Therefore all methods on interfaces must be public.


Thea Cadman   |   Member since 2019  |  ✔ Verified

How do you declare an interface?

An interface is declared by using the interface keyword. It provides total abstraction; means all the methods in an interface are declared with the empty body, and all the fields are public, static and final by default. A class that implements an interface must implement all the methods declared in the interface.


Emmanuelle Kelly   |   Member since 2008  |  ✔ Verified

Why interface methods are public?

Java interface methods aren't only public by default - they can only be public. The reason for this is because an interface method is a specification meant for consumption by the public (in Java terms - meaning, in any class). The interface method enforces that the implementing class method is public.


Victoria Sherry   |   Member since 2020  |  ✔ Verified

What is an interface?

An interface is a reference type in Java. It is similar to class. It is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface. Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types.


Benny Giles   |   Member since 2015  |  ✔ Verified

What are abstract methods?

An abstract method is a method that is declared, but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods. Let's look at an example of an abstract class, and an abstract method.


Lauren Wood   |   Member since 2011  |  ✔ Verified

When abstract methods are used?

Abstract Classes are a good fit if you want to provide implementation details to your children but don't want to allow an instance of your class to be directly instantiated (which allows you to partially define a class). If you want to simply define a contract for Objects to follow, then use an Interface.


Ema Holmes   |   Member since 2010  |  ✔ Verified

Which is better abstract class or interface?

You can only have one direct abstract superclass. Therefore, interfaces are useful if you need to expose two or more interfaces. An interface is better than a abstract class when you want multiple classes to implement that interface and when you don't have to inherit default behavior.


Matt White   |   Member since 2020  |  ✔ Verified

How do you declare an abstract method?

To declare an abstract method, use this general form: abstract type method-name(parameter-list); As you can see, no method body is present. Any concrete class(i.e. class without abstract keyword) that extends an abstract class must override all the abstract methods of the class.


Sara Connell   |   Member since 2005  |  ✔ Verified

Can you define methods in an interface?

Like a class, an interface can have methods and variables, but the methods declared in an interface are by default abstract (only method signature, no body). Interfaces specify what a class must do and not how. It is the blueprint of the class.


Denis Cann   |   Member since 2005  |  ✔ Verified

CAN interface have private methods?

As of Java 8, interfaces can have default methods, and as of Java 9, an interface is allowed to have a private methods which can only be accessed by default methods in the same interface. An interface is used for describing an API which is provided by any class implementing the interface.


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