Java - initialize HashSet at construction

In this article, we would like to show you how to initialize HashSet at the construction in Java.

Quick solution:

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Set<String> letters = new HashSet<>(Arrays.asList("A", "B", "C"));

In this example, to initialize letters HashSet at the construction time, we use Arrays.asList() method in the following way:

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import java.util.*;

​

public class Example {

​

    public static void main(String[] args) {

        Set<String> letters = new HashSet<>(Arrays.asList("A", "B", "C"));

​

        System.out.println(letters);

    }

}

Output:

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[A, B, C]

Note:

This approache is inefficient since it needs to create an array, convert it to a list, and use it to create a HashSet.

In this example, we use double curly braces to initialize letters HashSet on its creation.

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import java.util.*;

​

public class Example {

​

    public static void main(String[] args) {

        Set<String> letters = new HashSet<>() {{

            add("A");

            add("B");

            add("C");

        }};

​

        System.out.println(letters);

    }

}

Output:

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[A, B, C]

Note:

This approache is inefficient since it needs to create anonymous class each time it's called