Editing Java syntax (section)

====Functional interfaces and lambda expressions====

These features were introduced with the release of Java SE 8. An interface automatically becomes a functional interface if it defines only one method. In this case an implementation can be represented as a lambda expression instead of implementing it in a new class, thus greatly simplifying writing code in the [[functional programming|functional style]]. Functional interfaces can optionally be annotated with the <code>[[@FunctionalInterface]]</code> annotation, which will tell the compiler to check whether the interface actually conforms to a definition of a functional interface.

<syntaxhighlight lang="java">
// A functional interface
@FunctionalInterface
interface Calculation {
    int calculate(int someNumber, int someOtherNumber);
}

// A method which accepts this interface as a parameter
int runCalculation(Calculation calculation) {
    return calculation.calculate(1, 2);
}

// Using a lambda to call the method
runCalculation((number, otherNumber) -> number + otherNumber);

// Equivalent code which uses an anonymous class instead
runCalculation(new Calculation() {
    @Override
    public int calculate(int someNumber, int someOtherNumber) {
        return someNumber + someOtherNumber;
    }
})
</syntaxhighlight>

Lambda's parameters types do not have to be fully specified and can be inferred from the interface it implements. Lambda's body can be written without a body block and a <code>return</code> statement if it is only an expression. Also, for those interfaces which only have a single parameter in the method, round brackets can be omitted.<ref>{{Cite web|title=Lambda Expressions (The Java™ Tutorials > Learning the Java Language > Classes and Objects)|url=https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html|access-date=August 8, 2021|website=docs.oracle.com|archive-date=June 16, 2020|archive-url=https://web.archive.org/web/20200616055353/https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html|url-status=live}}</ref>

<syntaxhighlight lang="java">
// Same call as above, but with fully specified types and a body block
runCalculation((int number, int otherNumber) -> {
    return number + otherNumber;
});

// A functional interface with a method which has only a single parameter
interface StringExtender {
    String extendString(String input);
}

// Initializing a variable of this type by using a lambda
StringExtender extender = input -> input + " Extended";
</syntaxhighlight>