Java > Streams API

Streams API in Java Collections

The Streams API in Java provides a functional approach to processing collections of data. It allows for efficient operations like filtering, mapping, sorting, and aggregation in a declarative manner.

Why Use Streams?

• Improves code readability
• Reduces boilerplate code
• Supports parallel processing for better performance
• Works seamlessly with collections

Creating a Stream

Streams can be created from various sources, such as Lists, Sets, Maps, and Arrays.

Example: Creating a Stream from a List

import java.util.*;
import java.util.stream.*;

public class StreamExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Ajay", "Balu", "Chaitanya", "Dinesh");
        
        Stream<String> nameStream = names.stream();
        nameStream.forEach(System.out::println);
    }
}

Common Stream Operations

1. Filtering

The filter() method is used to select elements that match a given condition.

List<String> names = Arrays.asList("Ajay", "Balu", "Chaitanya", "Dinesh");
names.stream()
     .filter(name -> name.startsWith("A"))
     .forEach(System.out::println);

2. Mapping

The map() function transforms each element in a stream.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
numbers.stream()
       .map(n -> n * n)
       .forEach(System.out::println);

3. Sorting

Streams allow sorting using sorted().

List<String> names = Arrays.asList("Chaitanya", "Ajay", "Dinesh", "Balu");
names.stream()
     .sorted()
     .forEach(System.out::println);

4. Collecting Results

To convert a stream back into a list, use collect(Collectors.toList()).

List<Integer> numbers = Arrays.asList(5, 3, 8, 1, 2);
List<Integer> sortedNumbers = numbers.stream()
                                     .sorted()
                                     .collect(Collectors.toList());
System.out.println(sortedNumbers);

5. Reducing

reduce() is used to accumulate stream elements into a single result.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int sum = numbers.stream()
                 .reduce(0, Integer::sum);
System.out.println("Sum: " + sum);

6. Grouping Elements

The Collectors.groupingBy() method is used to group elements based on a property.

import java.util.*;
import java.util.stream.*;

public class GroupingExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Ajay", "Balu", "Chaitanya", "Dinesh", "Arjun", "Bhaskar");
        
        Map<Character, List<String>> groupedNames = names.stream()
            .collect(Collectors.groupingBy(name -> name.charAt(0)));
        
        System.out.println(groupedNames);
    }
}

Parallel Streams

For large datasets, parallel streams can improve performance by utilizing multiple CPU cores.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
numbers.parallelStream()
       .map(n -> n * n)
       .forEach(System.out::println);

Conclusion

The Streams API provides a powerful, functional approach to working with collections in Java. By leveraging streams, developers can write cleaner, more efficient, and parallelizable code.

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🔗 Related Topics:

• Collections
• ArrayList

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