Convert Set to array in Java
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));
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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));
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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));
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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));
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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));
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⮚ 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));
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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));
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⮚ 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));
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That’s all about converting Set to array in Java.
