What Is a Method Reference in Java 8?

Direct Answer

A method reference is a target-typed expression such as System.out::println, String::trim, or ArrayList::new. It can replace a compatible lambda that simply invokes an existing method or constructor. Use it when the name and target type make the behavior clearer; parameter adaptation and overload resolution must still succeed.

Code Example

import java.util.List;
import java.util.function.Function;

public class MethodReferenceDemo {
    public static void main(String[] args) {
        List<String> names = List.of("alice", "bob", "charlie");

        names.stream()
             .map(name -> name.toUpperCase())
             .forEach(result -> System.out.println(result));

        names.stream()
             .map(String::toUpperCase)
             .forEach(System.out::println);

        Function<String, Integer> lambda = s -> s.length();
        Function<String, Integer> reference = String::length;
        System.out.println(lambda.apply("test"));
        System.out.println(reference.apply("test"));
    }
}

The paired forms have the same functional result in this example. Their bytecode and runtime cost are implementation details, so the code makes no performance claim.

Common Mistakes & Pitfalls

• Assuming identical bytecode or performance: a compatible lambda and method reference express the same functional-interface behavior here, but Java does not promise one translation or timing.
• Forcing method references everywhere: if (x) -> service.process(x, DEFAULT_MODE) is clearer than adding a wrapper only to reference it, keep the lambda.
• Mismatching the target function type: parameter adaptation, overload resolution, accessibility, and return compatibility all have to succeed; it is more precise than merely counting parameters.