How to convert a HashSet into an array in Java

In this article we will discuss how to convert a HashSet in to an array.

Each collection that provides a member function toArray() i.e.

 public <T> T[] toArray(T[] a)

It iterates through all the elements of Collection (HashSet in our case) and set the each elements in the passed argument array []a. But if array [] a ‘s length is less than the Collection’s size then it will create a new array of required size and initialize elements in it.

Checkout the following example to convert HashSet into an array.

package com.thispointer.java.collections.hashsets;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Iterator;

public class Example5 {

	public static void main(String[] args) {

		// Create a new Set of String objects
		HashSet<String> setOfStrs = new HashSet<>();

		// Add elements to a HashSet
		setOfStrs.add("hello");
		setOfStrs.add("abc");
		setOfStrs.add("time");
		setOfStrs.add("Hi");

		// Create an array of size equivalent to Set size
		// String [] arrOfStrs = new String[setOfStrs.size()];
		String[] arrOfStrs = new String[5];

		// Initialize the array.
		String[] recvArr = setOfStrs.toArray(arrOfStrs);

		// Print the Set
		for (String data : recvArr)
			System.out.println(data);

	}

}

Output:

Hi
abc
hello
time
null

As you can see the last element is null. It is because we created the array of size 5 but in HashSet there were only 4 elements. Remaining elements in array will be set to null.

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Remember if passed array’s length is less than the Collection’s size then it will internally create a new array of required size, then set elements in it and then returns it. Therefore, always use the array returned by toArray() not the array passed.