Java 8 Collectors Class with Examples

Java 8 Tutorial

Introduction

The Collectors class in Java 8 is part of the java.util.stream package and provides various utility methods to collect the results of stream operations. It's primarily used with the Stream.collect() method to convert a stream into a different form, such as a List, Set, Map, or even a concatenated String. Collectors simplify tasks like grouping, partitioning, and reducing data from streams.

In this tutorial, we will explore several commonly used Collectors methods with examples.

Example 1: Collect to a List
Example 2: Collect to a Set
Example 3: Collect to a Map
Example 4: Joining Elements into a String
Example 5: Summing Elements
Example 6: Averaging Values
Example 7: Grouping Elements by a Key
Example 8: Partitioning Elements by a Predicate
Example 9: Counting Elements
Example 10: Collecting and Reducing Elements

Example 1: Collect to a List

The most common use of the Collectors class is to collect elements from a stream into a List.

Code Example

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

public class CollectToListExample {
    public static void main(String[] args) {
        List<String> cities = Arrays.asList("Mumbai", "Delhi", "Bangalore", "Chennai");

        // Collecting the stream into a List
        List<String> cityList = cities.stream().collect(Collectors.toList());

        System.out.println(cityList);
    }
}

Output

[Mumbai, Delhi, Bangalore, Chennai]

Explanation

We use Collectors.toList() to collect the stream elements into a List.

Example 2: Collect to a Set

You can use Collectors.toSet() to collect elements from a stream into a Set, which removes duplicates.

Code Example

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

public class CollectToSetExample {
    public static void main(String[] args) {
        List<String> cities = Arrays.asList("Mumbai", "Delhi", "Mumbai", "Bangalore");

        // Collecting the stream into a Set
        Set<String> citySet = cities.stream().collect(Collectors.toSet());

        System.out.println(citySet);
    }
}

Output

[Mumbai, Delhi, Bangalore]

Explanation

The Collectors.toSet() method collects the stream elements into a Set, removing duplicate entries.

Example 3: Collect to a Map

You can collect elements from a stream into a Map using Collectors.toMap(). In this example, we will map city names to their lengths.

Code Example

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

public class CollectToMapExample {
    public static void main(String[] args) {
        List<String> cities = Arrays.asList("Mumbai", "Delhi", "Bangalore");

        // Collecting the stream into a Map (city name -> city name length)
        Map<String, Integer> cityMap = cities.stream()
                                             .collect(Collectors.toMap(city -> city, city -> city.length()));

        System.out.println(cityMap);
    }
}

Output

{Mumbai=6, Delhi=5, Bangalore=9}

Explanation

The Collectors.toMap() method creates a Map where the city names are the keys and their lengths are the values.

Example 4: Joining Elements into a String

You can use Collectors.joining() to concatenate the elements of a stream into a single String.

Code Example

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

public class CollectJoiningExample {
    public static void main(String[] args) {
        List<String> cities = Arrays.asList("Mumbai", "Delhi", "Bangalore", "Chennai");

        // Joining elements into a single String
        String joinedCities = cities.stream().collect(Collectors.joining(", "));

        System.out.println(joinedCities);
    }
}

Output

Mumbai, Delhi, Bangalore, Chennai

Explanation

The Collectors.joining() method concatenates the city names, separated by a comma and a space.

Example 5: Summing Elements

You can sum the numeric elements of a stream using Collectors.summingInt(), summingLong(), or summingDouble().

Code Example

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

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

        // Summing all integers in the list
        int sum = numbers.stream().collect(Collectors.summingInt(Integer::intValue));

        System.out.println("Sum: " + sum);
    }
}

Output

Sum: 15

Explanation

The Collectors.summingInt() method is used to sum the integer values in the stream.

Example 6: Averaging Values

To calculate the average of elements, you can use Collectors.averagingInt(), averagingLong(), or averagingDouble().

Code Example

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

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

        // Calculating the average of all integers in the list
        double average = numbers.stream().collect(Collectors.averagingInt(Integer::intValue));

        System.out.println("Average: " + average);
    }
}

Output

Average: 3.0

Explanation

The Collectors.averagingInt() method calculates the average of the elements in the stream.

Example 7: Grouping Elements by a Key

You can use Collectors.groupingBy() to group elements of a stream by a specific key.

Code Example

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

public class CollectGroupingByExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Amit", "Deepa", "Rahul", "Akhil");

        // Grouping names by the first letter
        Map<Character, List<String>> groupedByLetter = names.stream()
                                                            .collect(Collectors.groupingBy(name -> name.charAt(0)));

        System.out.println(groupedByLetter);
    }
}

Output

{A=[Amit, Akhil], D=[Deepa], R=[Rahul]}

Explanation

The Collectors.groupingBy() method groups the names by the first letter of each name.

Example 8: Partitioning Elements by a Predicate

You can partition elements into two groups based on a predicate using Collectors.partitioningBy().

Code Example

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

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

        // Partitioning numbers into even and odd
        Map<Boolean, List<Integer>> partitioned = numbers.stream()
                                                         .collect(Collectors.partitioningBy(num -> num % 2 == 0));

        System.out.println(partitioned);
    }
}

Output

{false=[1, 3, 5], true=[2, 4, 6]}

Explanation

The Collectors.partitioningBy() method splits the numbers into two groups: even and odd.

Example 9: Counting Elements

To count the number of elements in a stream, use Collectors.counting().

Code Example

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

public class CollectCountingExample {
    public static void main(String[] args) {
        List<String> cities = Arrays.asList("Mumbai", "Delhi", "Bangalore", "Chennai");

        // Counting the number of cities
        long count = cities.stream().collect(Collectors.counting());

        System.out.println("Number of cities: " + count);
    }
}

Output

Number of cities: 4

Explanation

The Collectors.counting() method returns the number of elements in the stream.

Example 10: Collecting and Reducing Elements

You can use Collectors.reducing() to reduce elements in a stream using a custom binary operator.

Code Example

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

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

        // Reducing the numbers to their sum
        int sum = numbers.stream().collect(Collectors.reducing(0, (a, b) -> a + b));

        System

.out.println("Sum using reducing: " + sum);
    }
}

Output

Sum using reducing: 15

Explanation

The Collectors.reducing() method reduces the stream's elements using a custom binary operator (sum in this case).

Conclusion

The Collectors class in Java 8 provides powerful methods for working with streams. With Collectors, you can transform, group, partition, and perform various operations on stream elements, making it easier to process data in a functional and concise way. These examples cover a range of common use cases, such as collecting to lists, maps, grouping, and more.

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