Find size of a Java Collection (Set, List, Map, etc)

This post will discuss how to find the size of a Java Collection (Set, List, Map, etc.).

1. Using `size()` method

Since all Java Collections implement the Collection interface, they all have the size() method that returns the number of elements in the collection.

import java.util.Collection;

import java.util.Set;

public class Main

{

public static void main(String[] args)

{

Collection<Integer> collection = Set.of(4, 2, 3, 1, 5);

System.out.println("The collection contains " + collection.size() + " elements");

}

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Output:

The collection contains 5 elements

Note that an exception will be thrown for a null input.

import java.util.Collection;

public class Main

{

public static void main(String[] args)

{

Collection<Integer> collection = null;

System.out.println("The collection contains " + collection.size() + " elements");

}

Output:

Exception in thread “main” java.lang.NullPointerException
at Main.main(Main.java:8)

2. Using Apache Commons Collections

To get a null-safe version, you may use the CollectionUtils.size() method provided by Apache Commons Collections.

import org.apache.commons.collections4.CollectionUtils;

import java.util.Collection;

import java.util.Set;

public class Main

{

public static void main(String[] args)

{

Collection<Integer> collection = Set.of(4, 2, 3, 1, 5);

int size = CollectionUtils.size(collection);

System.out.println("The collection contains " + size + " elements");

}

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Output:

The collection contains 5 elements

The following code demonstrates how this method tries to handle null input gracefully.

import org.apache.commons.collections4.CollectionUtils;

import java.util.Collection;

public class Main

{

public static void main(String[] args)

{

Collection<Integer> collection = null;

int n = CollectionUtils.size(collection);

System.out.println("The collection contains " + n + " elements");

}

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Output:

The collection contains 0 elements

3. Using `Stream.count()` method

Another possibility is using Java 8 Streams. The idea is to get a stream over the elements in the collection and the count of elements in the stream using the count() method.

import java.util.Collection;

import java.util.List;

public class Main

{

public static void main(String[] args)

{

Collection<Integer> collection = List.of(4, 2, 3, 1, 5);

long n = collection.stream().count();

System.out.println("The collection contains " + n + " elements");

}

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Output:

The collection contains 5 elements

That’s all about finding the size of a Java Collection.

Find size of a Java Collection (Set, List, Map, etc)

1. Using size() method

2. Using Apache Commons Collections

3. Using Stream.count() method

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1. Using `size()` method

3. Using `Stream.count()` method