Java Filters

In Java, a filter is a function that takes an input and returns a boolean value indicating whether the input meets certain criteria. Filters are commonly used to select a subset of data based on certain criteria.

There are several ways to implement filters in Java. Here are some common approaches:

• Using a Predicate: A Predicate is a functional interface that represents a function that takes an input and returns a boolean value. You can use a Predicate to define a filter by implementing the test() method. Here's an example:

import java.util.function.Predicate;

public class Main {
  public static void main(String[] args) {
    // Define a filter that checks if a number is even
    Predicate<Integer> isEven = n -> n % 2 == 0;

    // Test the filter
    System.out.println(isEven.test(4));  // prints "true"
    System.out.println(isEven.test(5));  // prints "false"
  }
}

• Using a lambda expression: You can use a lambda expression to define a filter inline. Here's an example:

import java.util.Arrays;
import java.util.List;

public class Main {
  public static void main(String[] args) {
    List<String> list = Arrays.asList("a", "b", "c", "d", "e");

    // Filter the list to get only strings that start with "b"
    list.stream()
      .filter(s -> s.startsWith("b"))
      .forEach(System.out::println);  // prints "b"
  }
}

• Using a method reference: You can use a method reference to define a filter that uses an existing method. Here's an example:

import java.util.Arrays;
import java.util.List;

public class Main {
  public static boolean isEven(int n) {
    return n % 2 == 0;
  }

  public static void main(String[] args) {
    List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6);

    // Filter the list to get only even numbers
    list.stream()
      .filter(Main::isEven)
      .forEach(System.out::println);  // prints "2 4 6"
  }
}

These are just a few examples of how you can use filters in Java. You can find more information about filters and other functional programming concepts in the Java documentation.