Chapter 88: Method Reference in Java

A method reference is :: shorthand when a lambda would only call one existing method with parameters forwarded unchanged. It targets the same functional interface slot as a lambda (Chapters 76–80).

1. Topic title

Method reference: name the method directly when it already matches the SAM shape

2. What it means

Forms:

1. ClassName::staticMethod — s -> Integer.parseInt(s) becomes Integer::parseInt when types line up.
2. instance::method — x -> out.println(x) becomes out::println (bound receiver).
3. ClassName::instanceMethod — (a,b) -> a.compareToIgnoreCase(b) becomes String::compareToIgnoreCase (first parameter is receiver).
4. ClassName::new — () -> new ArrayList<>() with inferred type args sometimes collapses to ArrayList::new in context.

3. Why it is used

Shorter stream pipelines, grep-friendly names in stack traces sometimes read clearer than huge lambdas.

4. Mental sketch

If a lambda is a wrapper envelope around one stamp, a method reference is just the stamp—same ink, less paper.

5. Java code example

import java.util.List;
import java.util.stream.Collectors;
 
public class MethodRefDemo {
    public static void main(String[] args) {
        List<String> words = List.of("Java", "Go", "Rust");
 
        List<String> lower = words.stream().map(String::toLowerCase).collect(Collectors.toList());
        System.out.println(lower);
 
        List<Integer> nums = List.of(0, 1, 2);
        List<String> built = nums.stream().map(String::valueOf).collect(Collectors.toList());
        System.out.println(built);
    }
}

6. Explanation of code

String::toLowerCase uses the unbound instance form: each stream element becomes the receiver of toLowerCase. String::valueOf picks the valueOf(int) overload when the stream carries Integer elements.

7. Common mistakes

Wrong overload picked—when println ambiguity appears, fall back to a lambda with explicit casts.

Unreadable :: chains—if readers need six mental hops, keep the lambda.

8. Best practices

Use this::save for callbacks inside instance methods—clear receiver.

Pair constructor references with toCollection(ArrayList::new) when you care about list implementation.

9. Small practice task

Sort List<String> with list.sort(String::compareToIgnoreCase)—identify which method reference form it is.

Beginner tip

If :: autocompletion lists five overloads, the compiler picks one using target typing—when it fails, expand to a lambda once, then collapse again when you see the winning signature.