Chapter 81: Supplier in Java

Supplier<T> is the T get() shape: no input, one output. It pairs naturally with Consumer (Chapter 80) in diagrams—pull vs push.

Use it when you want to delay work until someone actually asks for the value, or when an API needs a factory callback instead of a pre-built object.

1. Topic title

Supplier: a no-arg factory you can pass around like any other lambda

2. What it means

Unlike Function, there is nothing to apply—you only get() when ready. Optional.orElseGet(Supplier<? extends T>) is the textbook example: if the Optional is empty, Java calls your Supplier once to build the fallback; if present, your Supplier is skipped—cheap when the fallback is costly.

3. Why it is used

Lazy defaults, memoization hooks, random seed sources, Stream.generate seeds—anywhere “give me the next T on demand” reads clearer than () -> new Thing() jammed inline without a name.

4. Mental sketch

Consumer eats a donut you hand it. Supplier is the kitchen window—you knock get(), maybe you receive a fresh donut each time, maybe the same recipe runs again.

5. Java code example

import java.util.Optional;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;
import java.util.stream.Stream;
 
public class SupplierDemo {
    public static void main(String[] args) {
        Supplier<String> lazyMsg =
                () -> {
                    System.out.println("building fallback…");
                    return "guest";
                };
        Optional<String> name = Optional.empty();
        System.out.println(name.orElseGet(lazyMsg));
 
        AtomicInteger seq = new AtomicInteger(1);
        Supplier<Integer> ids = seq::getAndIncrement;
        Stream.generate(ids).limit(4).forEach(System.out::println);
    }
}

First block prints building fallback… once, then guest. Second prints 1–4 from the atomic counter.

6. Explanation of code

orElseGet takes a Supplier; orElse("guest") would build the string even when not needed—fine for constants, wrong for new DatabaseConnection()-style work.

7. Common mistakes

Passing orElse(expensive()) by accident—argument evaluates eagerly. Switch to orElseGet(() -> expensive()).

Infinite Stream.generate without limit—your program hangs printing forever.

8. Best practices

Name suppliers clock, idSource, fallbackLoader when storing in fields—call sites read like intent.

Prefer IntSupplier / LongSupplier when int/long streams avoid boxing noise.

9. Small practice task

Write Supplier<Double> returning Math.random() and collect five values with Stream.generate(sup).limit(5).toList() (Java 16+) or collect(Collectors.toList()).

Beginner tip

If Supplier feels pointless next to () -> 5, compare orElse(compute()) vs orElseGet(() -> compute()) side by side with a println inside compute—the laziness becomes obvious.