In Java, you can use the Optional API to filter values based on a condition using the filter method. The filter method takes a predicate as an argument and applies it to the value contained in the Optional. If the predicate evaluates to true, the Optional is returned unchanged. If the predicate evaluates to false, an empty Optional is returned.

Here’s an example:

Example:

package org.kodejava.util;

import java.util.Optional;

public class OptionalFilterExample {
   public static void main(String[] args) {
      // Create an Optional with a value
      Optional<String> optionalValue = Optional.of("hello");

      // Filter the Optional value based on a condition
      Optional<String> filteredValue = optionalValue.filter(value -> value.length() > 3);

      // If the value passes the filter, print it
      filteredValue.ifPresent(System.out::println); // Output: hello

      // Example where the filter does not match
      Optional<String> emptyValue = optionalValue.filter(value -> value.length() > 10);
      System.out.println(emptyValue.isPresent()); // Output: false
   }
}

Explanation:

1. Initial Value: The Optional is created with the value "hello".
2. Filtering: The filter method takes a predicate (value -> value.length() > 3) and applies it to the contained value.
   • If the predicate is true (length is greater than 3), the Optional retains the value.
   • If the predicate is false (e.g., length is less than 10), the result is an empty Optional.
3. Accessing Results: The ifPresent method is used to print the value if it is still present, or use isPresent to evaluate if the result is empty.

Summary:

• Use Optional.filter(Predicate<T>) to conditionally retain the value in an Optional.
• If the predicate fails, the Optional becomes empty.
• Combine Optional with ifPresent, isPresent, or orElse to handle the filtered result.