Introduction
Table of Content
Functional Interface
FunctionalInterface.java
Following interface is a functional interface since it has only one abstract method
@FunctionalInterface
interface MyComputeInterface {
public int compute(int... a);
}Functional Interface with default method.
MyComputeInterfaceWithDefault.java
Following interface is a functional interface since it has only one abstract method. It can have default methods though.
@FunctionalInterface
interface MyComputeInterfaceWithDefault {
public int compute(int... a);
default boolean isEven(int x) {
return x / 2 == 0;
}
}
Invalid Functional Interface
ComputeInterface.java
Following interface is invalid since it contains more than one abstract methods. Also as we have the @FunctionalInterface annotation applied, the compiler will complain stating ComputeInterface is not a functional interface.
@FunctionalInterface
interface ComputeInterface {
public int add(int... a);
public int average(int... a);
}
Note the @FuncationalInterface annotation is a runtime informative annotation that will make the compiler to complain if the interface contains more than one abstract methods.
Summary
- 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. The use of this annotation is optional.
- instances of functional interfaces can be created with lambda expressions, method references, or constructor references
- Java provides out of the box Functional interfaces that we can use.
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