Chapter 78: Predicate in Java

Predicate<T> is a built-in functional interface for boolean test(T value). It answers yes/no questions about a single input: “is blank?”, “score high enough?”, “file name ends with .log?”

Lambdas (76) and functional interfaces (77) are prerequisites. Function and Consumer follow in Chapters 79–80.

1. Topic title

Predicate: a reusable yes/no probe you can chain with and/or/negate

2. What it means

Core method: boolean test(T t).

Combinators:

• and — both must pass.
• or — either passes.
• negate — flips the result.

Predicate.isEqual(target) builds a predicate that compares Objects.equals to target—handy for null-safe equality tests.

3. Why it is used

Validation pipelines, Stream.filter, configurable rules passed into services—anywhere you want named boolean logic without subclass explosion.

4. Mental picture

A Predicate is a bouncer checklist clipped on a clipboard: each patron (T) walks past; combined rules and two stamps, or allows VIP exceptions, negate flips “deny list” to “allow list.”

5. Java code example

import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
 
public class PredicateDemo {
    public static void main(String[] args) {
        Predicate<String> notBlank = s -> s != null && !s.isBlank();
        Predicate<String> shortWord = s -> s.length() <= 4;
        Predicate<String> rule = notBlank.and(shortWord);
 
        List<String> words = List.of("hi", " ", "java", "go");
        List<String> kept = words.stream().filter(rule).collect(Collectors.toList());
        System.out.println(kept);
    }
}

stream() / filter preview the Stream API (Chapter 82)—here focus on Predicate composition.

6. Explanation of code

notBlank.and(shortWord) returns a new predicate; originals stay reusable elsewhere.

7. Common mistakes

Side effects inside test—keep predicates pure when possible; surprises multiply when reused in parallel streams later.

Boxing costs with Predicate<Integer> on hot paths—consider IntPredicate for primitive int pipelines.

8. Best practices

Name predicates is… / has… / can… when storing them in final fields—call sites read like English.

Extract complex and chains to well-named predicate variables instead of one mega-lambda.

9. Small practice task

Write Predicate<Integer> inBand = … true for 30 ≤ x ≤ 50 using and. Test List.of(10, 40, 60) with stream().filter(inBand) and print.

Beginner tip

If filter confuses you, read it as “keep only items where the predicate cheers yes.” Everything else is dropped from the stream for that pipeline step.