Predicate<T>

Learn condition functions with Predicate<T>.

The Predicate<T> interface

Predicate<T> is a condition function that takes a T value and returns a boolean. Its single abstract method is test. It is often passed to filter in streams.

Example: Predicate<String> longWord = s -> s.length() > 5;. Then longWord.test("notebook") returns true. When you want to pass conditions around like values instead of classes, Predicate is ideal.

Combining Predicates

Predicates can be combined with logical operators. The and, or and negate methods produce new Predicates; this lets you build composite conditions.

Example: longWord.and(startsWithA).or(isEmpty). This means 'long and starts with A, or empty'. It is handy for expressing complex business rules in one line. The same works in streams: stream.filter(longWord.and(startsWithA)).

Exercise Filter with a Predicate

Define a Predicate<String> that selects words longer than 5 characters. Take the list List.of("pen", "notebook", "case") into a stream, filter it, and print the count with System.out.println using count.

Show example solution

import java.util.List;
import java.util.function.Predicate;

public class PredicateExample {
    public static void main(String[] args) {
        Predicate<String> longWord = s -> s.length() > 5;
        long count = List.of("pen", "notebook", "case").stream()
            .filter(longWord)
            .count();
        System.out.println(count);
    }
}

Test yourself

Match the Predicate methods

Predicate<T>

Takes T returns boolean
test

Call method
and

And
or

Or
negate

Not
Which is the call method of Predicate<T>?
- apply
- test
- accept
- get
Show answer

test — test is Predicate's call method.

Complete the Predicate call

Predicate<String> longWord = s -> s.length() > 5;
boolean yes = longWord.____("defter");

Show answer

Predicate<String> longWord = s -> s.length() > 5;
boolean yes = longWord.test("defter");

Next lesson: Consumer<T>

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