Java.Java8.What are the functional interfaces Predicate, DoublePredicate, IntPredicate, and LongPredicate?

✅ 1. Predicate<T>

Package: java.util.function

• A functional interface that takes one argument of type T and returns a boolean.

@FunctionalInterface
public interface Predicate<T> {
    boolean test(T t);
}

🧠 Example:

Predicate<String> isLongWord = str -> str.length() > 5;
System.out.println(isLongWord.test("hello"));     // false
System.out.println(isLongWord.test("predicate")); // true

🔧 Use Cases:

• Filtering elements in streams:

List<String> list = List.of("cat", "elephant", "bat");
list.stream().filter(isLongWord).forEach(System.out::println);

Combine with .and(), .or(), .negate():

Predicate<String> startsWithE = s -> s.startsWith("e");
Predicate<String> complex = isLongWord.and(startsWithE);

✅ 2. IntPredicate

Used for primitive int, avoids boxing overhead.

@FunctionalInterface
public interface IntPredicate {
    boolean test(int value);
}

✅ 3. LongPredicate

Same idea for long.

@FunctionalInterface
public interface LongPredicate {
    boolean test(long value);
}

✅ 4. DoublePredicate

For double primitives.

@FunctionalInterface
public interface DoublePredicate {
    boolean test(double value);
}

🧠 Example:

DoublePredicate isBetween0And1 = d -> d > 0 && d < 1;
System.out.println(isBetween0And1.test(0.5)); // true

🔄 Summary Table:

Interface	Input Type	Return Type	Use Case Example
Predicate<T>	T	boolean	str -> str.length() > 5
IntPredicate	int	boolean	x -> x % 2 == 0
LongPredicate	long	boolean	l -> l > 0
DoublePredicate	double	boolean	d -> d > 0 && d < 1