How do I create a generic Set?

In another post of Java Generics you have seen how to create a generic collection using List in this example you will learn how to make a generic Set. Making a Set generic means that we want it to only hold objects of the defined type. We can declare and instantiate a generic Set like the following code.

Set<String> colors = new HashSet<>();

We use the angle brackets to define the type. We need also to define the type in the instantiation part, but using the diamond operator remove the duplicate and Java will infer the type it self. This declaration and instantiation will give you a Set object that holds a reference to String objects. If you tried to ask it to hold other type of object such as Date or Integer you will get a compile time error.

Set<String> colors = new HashSet<>();

//colors.add(new Date()); // Compile time error!

The code for adding items to a set is look the same with the old way we code. But again, one thing you will get here for free is that the compiler will check to see if you add the correct type to the Set. If we remove the remark on the last line from the snippet above we will get a compile time error. Because we are trying to store a Date into a Set of type String.

Now, let see how we can iterate the contents of the Set. First we’ll do it using the Iterator. And here is the code snippet.

Iterator iterator = colors.iterator();
while (iterator.hasNext()) {

When using a generic Set it will know that the will return a type of String so that you don’t have use the cast operator. Which of course will make you code looks cleaner and more readable. We can also using the for-each loop when iterating the Set as can be seen in the following example.

for (String color : colors) {

How do I convert Set into Array?

package org.kodejava.example.util;

import java.util.*;

public class SetToArray {
    public static void main(String[] args) {
        // Create a java.util.Set object and add some integers into the Set.
        Set<Integer> numberSet = new HashSet<>();

        // Converting a java.util.Set into an array can be done by creating a
        // java.util.List object from the Set and then convert it into an array
        // by calling the toArray() method on the list object.
        List<Integer> numberList = new ArrayList<>(numberSet);
        Integer[] numbers = numberList.toArray(new Integer[numberList.size()]);

        // Display the content of numbers array.
        for (int i = 0; i < numbers.length; i++) {
            Integer number = numbers[i];
            System.out.print(number + (i < numbers.length - 1 ? ", " : "\n"));

        // Display the content of numbers array using the for-each loop.
        for (Integer number : numbers) {
            System.out.print(number + ", ");