Optional filter() method in Java with examples

The filter() method of java.util.Optional class in Java is used to filter the value of this Optional instance by matching it with the given Predicate, and then return the filtered Optional instance. If there is no value present in this Optional instance, then this method returns an empty Optional instance.

Syntax:

public Optional<T> 
  filter(Predicale<T> predicate)

Parameters: This method accepts predicate as parameter of type Predicate to filter an Optional instance with this.

Return value: This method returns the filtered Optional instance. If there is no value present in this Optional instance, then this method returns an empty Optional instance.

Exception: This method throws NullPointerException if the specified predicate is null.

Below programs illustrate filter() method:
Program 1:

// Java program to demonstrate
// Optional.filter() method

import java.util.*;

public class GFG {

    public static void main(String[] args)
    {

        // create a Optional
        Optional<Integer> op
            = Optional.of(9456);

        // print value
        System.out.println("Optional: "
                           + op);

        // filter the value
        System.out.println("Filtered value "
                           + "for odd or even: "
                           + op
                                 .filter(num
                                         -> num % 2 == 0));
    }
}

Output:

Optional: Optional[9456]
Filtered value for odd or even: Optional[9456]

Program 2:

// Java program to demonstrate
// Optional.filter() method

import java.util.*;

public class GFG {

    public static void main(String[] args)
    {

        // create a Optional
        Optional<Integer> op
            = Optional.empty();

        // print value
        System.out.println("Optional: "
                           + op);

        try {

            // filter the value
            System.out.println("Filtered value "
                               + "for odd or even: "
                               + op
                                     .filter(num
                                             -> num % 2 == 0));
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

Output:

Optional: Optional.empty
Filtered value for odd or even: Optional.empty

Reference: https://docs.oracle.com/javase/9/docs/api/java/util/Optional.html#filter-java.util.function.Predicate-