Explain Predicate and Consumer in Java

Predicate and Consumer in Java

Predicate

• What: A functional interface in java.util.function representing a boolean-valued function of one argument.
• Signature: boolean test(T t)
• Common use: filtering, conditional checks (e.g., stream.filter).
• Default/composed methods:
  • default Predicate<T> and(Predicate<? super T> other)
  • default Predicate<T> or(Predicate<? super T> other)
  • default Predicate<T> negate()
  • static <T> Predicate<T> isEqual(Object targetRef)
• Example:

Predicate<String> isEmpty = String::isEmpty;
Predicate<String> startsWithA = s -> s.startsWith("A");
Predicate<String> combined = isEmpty.negate().and(startsWithA);
boolean ok = combined.test("Apple"); // true

Consumer

• What: A functional interface in java.util.function representing an operation that accepts a single input and returns no result.
• Signature: void accept(T t)
• Common use: performing actions (e.g., stream.forEach, side-effects, logging).
• Default/composed method: default Consumer andThen(Consumer<? super T> after)

Consumer<String> print = System.out::println;
Consumer<String> exclaim = s -> System.out.println(s + "!");
Consumer<String> combined = print.andThen(exclaim);
combined.accept("Hello"); // prints "Hello" then "Hello!"

List<String> names = List.of("Alice", "Bob", "Anna");
Predicate<String> startsWithA = s -> s.startsWith("A");
Consumer<String> printer = System.out::println;

names.stream()
     .filter(startsWithA)
     .forEach(printer); // prints "Alice" and "Anna"