Question: Why Interface Has Default Method?

Can we have default method in functional interface?

You can have default methods in a functional interface but its contract requires you to provide one single abstract method (or SAM).

Conceptually, a functional interface has exactly one abstract method.

Since default methods have an implementation, they are not abstract..

Can we override default method of interface?

If you have default method in an interface, it is not mandatory to override (provide body) it in the classes that are already implementing this interface. In short, you can access the default methods of an interface using the objects of the implementing classes.

Can we define method in 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.

Can we override static method of interface?

You cannot override the static method of the interface; you can just access them using the name of the interface. If you try to override a static method of an interface by defining a similar method in the implementing interface, it will be considered as another (static) method of the class.

What is default method in interface?

Java 8 introduces the “Default Method” or (Defender methods) feature, which allows the developer to add new methods to the interfaces without breaking their existing implementation.

What is the use of default and static methods in interfaces?

Java 8 introduced default and static methods in interfaces. This feature enables us to add new functionality in the interfaces without breaking the existing contract of the implementing classes.

Can an interface be final?

If you make an interface final, you cannot implement its methods which defies the very purpose of the interfaces. Therefore, you cannot make an interface final in Java. Still if you try to do so, a compile time exception is generated saying “illegal combination of modifiers − interface and final”.

Can we have 2 default method in interface?

Multiple Defaults With default functions in interfaces, there is a possibility that a class is implementing two interfaces with same default methods. The following code explains how this ambiguity can be resolved. First solution is to create an own method that overrides the default implementation.

CAN interface have private methods?

As of Java 9, methods in an interface can be private. A private method can be static or an instance method, but it cannot be a default method since that can be overridden.

Why do we need default method in interface?

Default methods enable you to add new functionality to existing interfaces and ensure binary compatibility with code written for older versions of those interfaces. In particular, default methods enable you to add methods that accept lambda expressions as parameters to existing interfaces.

Can we override static method?

Can we override a static method? No, we cannot override static methods because method overriding is based on dynamic binding at runtime and the static methods are bonded using static binding at compile time. So, we cannot override static methods.

Can functional interface have more than one method?

A functional interface has only one abstract method but it can have multiple default methods. @FunctionalInterface annotation is used to ensure an interface can’t have more than one abstract method.

Can we declare constructor inside an interface?

No, you cannot have a constructor within an interface in Java. You can have only public, static, final variables and, public, abstract, methods as of Java7.

What can an interface contain?

In the Java programming language, an interface is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

Why static methods are not allowed in interface?

Any method you write in an interface can’t rely on any implementation details. As an example in favor of static methods in interfaces, consider utility classes like Collections in the Java API. That class only exists because those static methods can’t be declared in their respective interfaces.

CAN interface have main method?

Answer : Yes, we can provide different implementation of main() declared in an interface and classes implementing that interface by overriding method and can overload static main method if defined in an interface. Some more information regarding interface changes in Java 8.

Why we Cannot override static method?

Static methods cannot be overridden because they are not dispatched on the object instance at runtime. The compiler decides which method gets called. Static methods can be overloaded (meaning that you can have the same method name for several methods as long as they have different parameter types).

Can an interface contain constants?

Java programmers commonly define constants inside their interfaces, if it makes design sense. You can do so using variables in an interface because the values will be present instantly at runtime and their values shared among all classes implementing your interface, because they are static and final.