@FunctionalInterface

Learn marking with @FunctionalInterface.

The @FunctionalInterface annotation

The @FunctionalInterface annotation declares that an interface has a single abstract method (SAM). This lets the compiler prevent a second abstract method from being added by accident.

Example: @FunctionalInterface interface StringOp { String apply(String s); }. Here apply is the single abstract method. If you add another abstract method to the interface the compiler errors out. The annotation is not required; but it is good practice to keep the single-abstract-method rule explicit.

The java.util.function package

Java's java.util.function package contains common functional interfaces. Besides Function, Predicate, Consumer and Supplier, there are specialized versions for primitive types.

For example IntFunction<R> takes an int and returns R; IntPredicate takes an int and returns boolean. These specialized interfaces reduce the boxing overhead. It is good practice to look for an existing one before defining a new interface.

Exercise Define and use a custom functional interface

Define a MathOp interface with @FunctionalInterface; it should have an int apply(int a, int b) method. In main, assign an addition lambda (a, b) -> a + b to a MathOp variable. Print the result of apply(3, 4) with System.out.println.

Show example solution

@FunctionalInterface
interface MathOp {
    int apply(int a, int b);
}

public class CustomFunc {
    public static void main(String[] args) {
        MathOp add = (a, b) -> a + b;
        System.out.println(add.apply(3, 4));
    }
}

Test yourself

Match the @FunctionalInterface rules

@FunctionalInterface

Single abstract method
Single Abstract Method

SAM type
default

Method with body
static

Class method
How many abstract methods can a @FunctionalInterface have at most?
- One
- Two
- Unlimited
- Three
Show answer

One — Exactly one abstract method is required.

Complete the annotation

@____
interface MathOp {
    int apply(int a, int b);
}

Show answer

@FunctionalInterface
interface MathOp {
    int apply(int a, int b);
}

Next lesson: Summary - Functional interface

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