Convert Set to array in Java

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Convert Set to array in Java

This post will discuss how to convert a set to an array in plain Java, Java 8, and the Guava library.

1. Naive solution

A naive solution is to iterate over the given set and copy each encountered element in the Integer array one by one.

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Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5)); Integer[] array = new Integer[set.size()]; int k = 0; for (Integer i: set) { array[k++] = i; } System.out.println(Arrays.toString(array));

2. Using `Set.toArray()` method

Set interface provides the toArray() method that returns an Object array containing the elements of the set.

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Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5)); Object[] array = set.toArray(); System.out.println(Arrays.toString(array));

The JVM doesn’t know the desired object type, so the toArray() method returns an Object[]. We can pass a typed array to the overloaded toArray(T[] a) method to let JVM know your desired object type.

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Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5)); Integer[] array = set.toArray(new Integer[set.size()]); System.out.println(Arrays.toString(array));

We can also pass an empty array of the specified type, and JVM will allocate the necessary memory:

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Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5)); Integer[] array = set.toArray(new Integer[0]); System.out.println(Arrays.toString(array));

3. Using Java 8

In Java 8, we can use the Stream to convert a set to an array. The idea is to convert a given set to stream using Set.stream() method and use Stream.toArray() method to return an array containing the stream elements. There are two ways to do so:

⮚ Using Streams with method reference

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Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5)); Integer[] array = set.stream().toArray(Integer[]::new); System.out.println(Arrays.toString(array));

⮚ Using Streams with lambda expression

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Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5)); Integer[] array = set.stream().toArray(n -> new Integer[n]); System.out.println(Arrays.toString(array));

4. Using Guava Library

⮚ Using FluentIterable class:

The FluentIterable is an expanded Iterable API that provides similar functionality as Java 8’s Stream. We can get a fluent iterable that wraps an iterable set and returns an array containing all the elements from the fluent iterable.

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Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5)); Integer[] array = FluentIterable.from(set).toArray(Integer.class); System.out.println(Arrays.toString(array));

⮚ Using Iterables class:

Guava’s Iterables class also provides the toArray() method that copies an iterable’s elements into an array.

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Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5)); Integer[] array = Iterables.toArray(set, Integer.class); System.out.println(Arrays.toString(array));

That’s all about converting Set to array in Java.

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