Can we have 2 abstract method in functional interface?

A functional interface is an interface that contains only one abstract method. They can have only one functionality to exhibit.

What happens if we have more than one abstract method in functional interface?

The @FunctionalInterface Annotation

Able to be specified on an interface, @FunctionalInterface marks a normal interface as a functional interface. When this annotation is specified on an interface, a compilation error will occur if the interface has more than one abstract method.

CAN interface have multiple abstract methods?

Example 5:

So we can have methods that can have their own body inside the interface one of them being static methods. So from Java 8 onwards it's not 100 % correct to say that interface can only have abstract methods.

How is Comparator a functional interface if it has two abstract methods?

Methods From Object Class in Functional Interfaces

How is it a functional interface when it has two abstract methods? Because equals() method signature matches from Object, and the compare() method is the only remaining abstract method, hence the Comparator interface is a functional interface.

Can we have multiple abstract method in Java?

abstract classes can't be instantiated, only subclassed. other classes extend abstract classes. can have both abstract and concrete methods. similar to interfaces, but (1) can implement methods, (2) fields can have various access modifiers, and (3) subclasses can only extend one abstract class.

Abstract Classes and Methods - Learn Abstraction in Java

Why functional interface contains only one abstract method?

The functional interface also known as Single Abstract Method Interface was introduced to facilitate Lambda functions. Since a lambda function can only provide the implementation for 1 method it is mandatory for the functional interface to have ONLY one abstract method.

How many abstract methods can an abstract class have?

To create an abstract class, just use the abstract keyword before the class keyword, in the class declaration. You can observe that except abstract methods the Employee class is same as normal class in Java. The class is now abstract, but it still has three fields, seven methods, and one constructor.

Can we override object class methods in functional interface?

In other words, every interface implicitly defines each of Object 's methods, and you can therefore @Override those methods. The other methods aren't defined in Object , so you can't override them.

Can functional interface have concrete methods?

It is important to note that a functional interface can have multiple default methods (it can be said concrete methods which are default), but only one abstract method.

Can functional interface have static methods?

A functional interface can contain default and static methods which do have an implementation, in addition to the single unimplemented method.

How many abstract methods can an interface have?

Interface can have only abstract methods. Since Java 8, it can have default and static methods also. 2) Abstract class doesn't support multiple inheritance.

Can functional interface have non abstract methods?

A functional interface must have exactly one abstract method. A functional interface has any number of default methods because they are not abstract and implementation already provided by the same.

Can we extend functional interface?

A functional interface can extends another interface only when it does not have any abstract method.

Can an abstract class be a functional interface?

A functional interface is an interface that has only one abstract method. It can contain any number of default and static methods but the abstract method it contains is exactly one. Additionally, a functional interface can have declarations of object class methods.

Can we inherit functional interface?

You can't inherit any functional interface to another functional interface. Because it breaks the law of functional interface has exactly one abstract method.

Can lambda be used only with functional interface?

No, all the lambda expressions in this code implement the BiFunction<Integer, Integer, Integer> function interface. The body of the lambda expressions is allowed to call methods of the MathOperation class. It doesn't have to refer only to methods of a functional interface.

How many default methods can a functional interface have?

Function interface. It contains two default methods: compose and andThen .

Can we create non static variables in an interface?

No you cannot have non-static variables in an interface. By default, All the members (methods and fields) of an interface are public. All the methods in an interface are public and abstract (except static and default).

Can I inherit static method from interfaces?

Static methods in interfaces are never inherited.

Can we have more than 1 default 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 we override static and default method in interface?

Static method in interface are part of the interface class can't implement or override it whereas class can override the default method.

Can we overload default method?

you can override a default method of an interface from the implementing class.

Can we achieve abstraction without abstract class and interface?

In java, abstraction is achieved by interfaces and abstract classes. We can achieve 100% abstraction using interfaces.

Can we have abstract method without 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.

Can abstract functions exist in non-abstract classes?

Yes, we can declare an abstract class with no abstract methods in Java. An abstract class means that hiding the implementation and showing the function definition to the user. An abstract class having both abstract methods and non-abstract methods.