Create Your Own Functional Interface in Java 8

What is a Functional Interface?

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Java 8 offers Lambda expressions to help us write better, and concise codes. Lambda expressions revolve around Functional Interfaces. So, what are Functional Interfaces?

The simplest definition is:

Functional Interfaces are Java Interfaces with only a single abstract method.

package lambda.example;

public interface DisplayOnly {
    public void print();
}

package lambda.example;

public interface DisplayOnly {

public void print();

}

If you are using some platform for continuous inspection of code quality like SonarQube, you may have noticed the following message:

Annotate the "DisplayOnly" interface with the @FunctionalInterface annotation (sonar.java.source not set. Assuming 8 or greater.)

1	Annotate the "DisplayOnly" interface with the @FunctionalInterface annotation (sonar.java.source not set. Assuming 8 or greater.)

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To fix this, simply annotate the interface with @FunctionalInterface.

For example,

package lambda.example;

@FunctionalInterface
public interface DisplayOnly {
    public void print();
}

package lambda.example;

@FunctionalInterface

public interface DisplayOnly {

public void print();

}

Sample Usage

The codes below uses your own Functional Interface

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package lambda.example;

public class DemoDisplayOnly {

    public static void main(String[] args) {
        DemoDisplayOnly demo = new DemoDisplayOnly();
        demo.processSomething(() -> System.out.println("Display message to console!"));
    }

    public void processSomething(DisplayOnly displayOnly) {
        // Do something here
        // ...
        displayOnly.print();
    }
}

package lambda.example;

public class DemoDisplayOnly {

public static void main(String[] args) {

DemoDisplayOnly demo = new DemoDisplayOnly();

demo.processSomething(() -> System.out.println("Display message to console!"));

}

public void processSomething(DisplayOnly displayOnly) {

// Do something here

// ...

displayOnly.print();

}

Use Java 8’s core Functional Interfaces

In most cases, you would need to create your own Functional Interfaces. Java provides several Functional Interfaces that we can use.

Predicate<T>
Consumer<T>
Function<T,R>
Supplier<T>
UnaryOperators<T>
BinaryOperator<T>

Which one can we replace our DisplayOnly interface with? Consumer<T>!

Here’s the updated DemoDisplayOnly.java

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package lambda.example;

import java.util.function.Consumer;

public class DemoDisplayOnly {

    public static void main(String[] args) {
        DemoDisplayOnly demo = new DemoDisplayOnly();
        demo.processSomething((x) -> System.out.println(x));
    }

    public void processSomething(Consumer<String> displayOnly) {
        // Do something here
        // ...
        displayOnly.accept("Display message to console!");
    }
}