Can a functional interface have more than one method?

A Functional interface is the interface, which contains only one abstract method, but it can have multiple concrete methods.

Java 8 uses functional interfaces while implementing lambda expressions.

Some of the examples of functional interfaces

java.util.function.Predicate

java.util.function.Function

java.lang.Runnable

I Would recommend you go through my below tutorials to understand lambda expressions.

Lambda Expressions in Java

Lambda Expressions Examples

java.util.function.Function interface

As you see the above image, ‘java.util.function.Function’ interface is defined like above.

1.   @FunctionalInterface annotation is used to notify jvm that it is a functional interface

2.   R apply(T t) : It is an abstract method

3.   Remaining all methods are concrete.

You can get more information about functional interface in my below post.

Function interface

You may like

Java Interview Questions

Programming Questions

Miscellaneous

How to bypass checked exception while creating object?

Is array a primitive type or object in java?

How to check an exception is checked or not?

How to check given object is an array or not?

How to get the system hash code of an object?