How do I use the LongPredicate functional interface in Java?

In Java, the LongPredicate is a functional interface introduced in Java 8 as part of the java.util.function package. It represents a predicate (a boolean-valued function) that takes a single long value as its input argument.

The key method in LongPredicate is:

boolean test(long value);

Here’s how we can use the LongPredicate functional interface:

Example Usage of `LongPredicate`

1. Basic Example with Lambda Expression:

package org.kodejava.util.function;

import java.util.function.LongPredicate;

public class LongPredicateExample {
   public static void main(String[] args) {
      // Define a LongPredicate to check if a number is even
      LongPredicate isEven = value -> value % 2 == 0;

      long number = 10;

      // Use the LongPredicate
      if (isEven.test(number)) {
         System.out.println(number + " is even.");
      } else {
         System.out.println(number + " is odd.");
      }
   }
}

2. Combining `LongPredicate`s Using `and`, `or`, and `negate`:

package org.kodejava.util.function;

import java.util.function.LongPredicate;

public class LongPredicateExample2 {
   public static void main(String[] args) {
      // Define two LongPredicates: even and greater than 5
      LongPredicate isEven = value -> value % 2 == 0;
      LongPredicate isGreaterThanFive = value -> value > 5;

      long number = 10;

      // Combine using 'and'
      LongPredicate isEvenAndGreaterThanFive = isEven.and(isGreaterThanFive);
      System.out.println("Is " + number + " even and greater than 5? " +
                         isEvenAndGreaterThanFive.test(number)); // true

      // Combine using 'or'
      LongPredicate isEvenOrGreaterThanFive = isEven.or(isGreaterThanFive);
      System.out.println("Is " + number + " even or greater than 5? " +
                         isEvenOrGreaterThanFive.test(number)); // true

      // Negate the 'isEven' predicate
      LongPredicate isOdd = isEven.negate();
      System.out.println("Is " + number + " odd? " + isOdd.test(number)); // false
   }
}

3. Using with Streams (e.g., `LongStream`):

If we use the LongPredicate with streams, it can be helpful for filtering LongStream.

package org.kodejava.util.function;

import java.util.function.LongPredicate;
import java.util.stream.LongStream;

public class LongPredicateWithStreams {
   public static void main(String[] args) {
      // Generate a stream of numbers from 1 to 10
      LongStream stream = LongStream.rangeClosed(1, 10);

      // Define a LongPredicate to filter even numbers
      LongPredicate isEven = value -> value % 2 == 0;

      // Use 'filter' with LongPredicate
      stream.filter(isEven)
              .forEach(System.out::println); // Prints 2, 4, 6, 8, 10
   }
}

4. Method Reference with `LongPredicate`:

We can also use method references if we have a method that matches the signature of the test method:

package org.kodejava.util.function;

import java.util.function.LongPredicate;

public class LongPredicateMethodReference {
   public static void main(String[] args) {
      LongPredicate isPositive = LongPredicateMethodReference::isPositive;

      long number = 5;
      // true
      System.out.println("Is " + number + " positive? " + isPositive.test(number));
   }

   // Method to check if a number is positive
   public static boolean isPositive(long value) {
      return value > 0;
   }
}

Key Points about `LongPredicate`

The functional method in LongPredicate is boolean test(long value). This method evaluates the predicate against the given long value and returns a boolean result.
It is often used in lambda expressions or method references.
We can combine multiple LongPredicate instances using and, or, and negate methods.

Summary

Use LongPredicate when working with primitive long arguments in functional programming scenarios.
It helps to avoid boxing overhead compared to Predicate<Long>.
We can define simple or complex conditions, combine predicates using and, or, and negate, and use them with streams for concise and readable code.

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Example Usage of LongPredicate

1. Basic Example with Lambda Expression:

2. Combining LongPredicates Using and, or, and negate:

3. Using with Streams (e.g., LongStream):

4. Method Reference with LongPredicate:

Key Points about LongPredicate