Map and Filter functions in Java

The map and filter methods in Java are part of the Streams API, introduced in Java 8. These methods are used for processing collections in a functional and declarative manner.

Map

• The map method is used to transform each element in a stream by applying a function to it.
• It produces a new stream with the transformed elements.

Example 1: Convert a list of strings to their lengths

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class MapExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
        
        // Transform names to their lengths
        List<Integer> lengths = names.stream()
                                     .map(String::length) // Get the length of each string
                                     .collect(Collectors.toList());
        
        System.out.println(lengths); // Output: [5, 3, 7]
    }
}

Example 2: Multiply a list of numbers by 2

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class MapExample2 {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

        // Double each number
        List<Integer> doubledNumbers = numbers.stream()
                                              .map(n -> n * 2) // Multiply each number by 2
                                              .collect(Collectors.toList());

        System.out.println(doubledNumbers); // Output: [2, 4, 6, 8, 10]
    }
}

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Filter

• The filter method is used to select elements from a stream that match a given condition (predicate).
• It produces a new stream with only the elements that pass the test.

Example 1: Filter even numbers from a list

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

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

        // Filter even numbers
        List<Integer> evenNumbers = numbers.stream()
                                           .filter(n -> n % 2 == 0) // Keep numbers divisible by 2
                                           .collect(Collectors.toList());

        System.out.println(evenNumbers); // Output: [2, 4, 6]
    }
}

Example 2: Filter names starting with ‘A’

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class FilterExample2 {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Alice", "Bob", "Andrew", "Charlie");

        // Filter names starting with 'A'
        List<String> filteredNames = names.stream()
                                          .filter(name -> name.startsWith("A"))
                                          .collect(Collectors.toList());

        System.out.println(filteredNames); // Output: [Alice, Andrew]
    }
}

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Combining map and filter

You can chain map and filter together to perform complex operations in one pipeline.

Example: Find the square of even numbers

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

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

        // Filter even numbers and square them
        List<Integer> squaresOfEvenNumbers = numbers.stream()
                                                    .filter(n -> n % 2 == 0) // Keep even numbers
                                                    .map(n -> n * n) // Square the even numbers
                                                    .collect(Collectors.toList());

        System.out.println(squaresOfEvenNumbers); // Output: [4, 16, 36]
    }
}