Method References

A method reference, written with the :: operator, is a compact shorthand for a lambda whose only job is to call one existing method — turning s -> obj.print(s) into obj::print .

Learn Method References in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

You should be comfortable with lambda expressions and the basics of the Stream API . Method references have been available since Java 8 .

💡 Analogy: A lambda like s -> System.out.println(s) is you describing, step by step, "take the thing and print it." A method reference is simply pointing at the tool that already does that: System.out::println — "use that printer." When a method to do the job already exists, you don't re-explain it; you just name it.

Method references are not a new capability — every one is exactly equivalent to a lambda. They're a readability feature: shorter to write and, when used well, clearer to read.

If a lambda's body is just one method call that forwards its arguments, swap it for a reference. n -> System.out.println(n) becomes System.out::println ; s -> Integer.parseInt(s) becomes Integer::parseInt . Same behaviour, less noise.

There are exactly four forms. The trick to telling them apart is asking where the receiver object comes from .

This is where references shine. .map(Person::name) , .map(String::toUpperCase) , Comparator.comparingInt(Person::age) , and Collectors.toCollection(ArrayList::new) all read as a clean list of operations rather than a pile of tiny lambdas.

Use a reference when the lambda is a pure pass-through to one method ( s -> Main.shout(s) → Main::shout ). Keep a lambda the moment you reorder arguments, add a constant, or call more than one method — a reference can't express that, and forcing it hurts clarity.

Predict the result before opening each answer.

Answer: Type::staticMethod. max is a static method of Math , so it's a static-method reference equivalent to (a, b) -> Math.max(a, b) .

Answer: No. It adds a constant prefix — that's logic beyond a single existing method call, so it must stay a lambda (or you'd write "X"::concat only if the prefix were fixed and concat fit, which it does here as a bound reference). Keep it as a lambda for clarity.

Answer: 5 . String::length is unbound: the argument "hello" becomes the receiver, so it returns "hello".length() .

🎯 Your Turn — Trim and upper-case

Chain two unbound ( Type::instanceMethod ) references in a stream — no lambdas.

🧩 Mini-Challenge — A title extractor

Build a Function<Book, String> from the unbound reference Book::title and use it.

You can now read and write all four kinds of method reference — Type::staticMethod , obj::instanceMethod , Type::instanceMethod , and Type::new — use them throughout stream pipelines, and judge when a lambda is the better choice.

Next up: Built-in Functional Interfaces — the Function , Predicate , Consumer , and Supplier types your references plug into.

Practice quiz

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