Map and Filter
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.
- The
mapmethod 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]
}
}
- The
filtermethod 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]
}
}
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]
}
}