Proper use of HashSet.contains()?

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Am I using HashSet.contains() wrong?

Well, you have an expectation which is wrong, but it's more about arrays than about HashSet. Arrays in Java don't implement equals and hashCode in the way it looks like you're expecting them to. They effectively exhibit reference equality behaviour. For example:

int[] array1 = { 1 };
int[] array2 = { 1 };
System.out.println(array1.equals(array1)); // true - same reference
System.out.println(array1.equals(array2)); // false - different reference

Rather than using an int[] for coordinates, I'd suggest creating a Coordinate class with x and y fields. If that Coordinate class then overrides equals and hashCode appropriately, then using HashSet will work. (Although I'd advise creating the object you're looking for outside the loop rather than in each iteration.)

HashSet contains() Method in Java, We use cookies to ensure you have the best browsing experience on our website . By using our site, you acknowledge that you have read and� Examples. The following example demonstrates how to remove values from a HashSet<T> collection using the Remove method. In this example, the Contains method verifies that the set contains a value before removing it.

HashSet contains() Method in Java util. HashSet contains() method is used to check whether a specific element is present in the HashSet or not. So basically it is used to check if a Set contains any particular element.

HashSet in Java, The HashSet class implements the Set interface, backed by a hash table which is actually like add, remove, contains and size assuming the hash function disperses the elements properly among the buckets, which we shall see further in the article. HashSet uses HashMap for storing its object internally. The containsAll() method of Java HashSet is used to check whether two sets contain the same elements or not. It takes one set as a parameter and returns True if all of the elements of this set is present in the other set.

I think this will help:

Stream.generate(() -> new int[] { 1, 2 }.hashCode()).limit(3).forEach(System.out::println);

notice we're printing the hashcode of [1, 2] three times but we get three unique values. That is because each new int[] has a different hashCode(). Create a custom type, override hashCode and add that to your HashSet. Something like

public class Cell {
    private final int x, y;

    public Cell(int x, int y) {
        this.x = x;
        this.y = y;

    public int getX() {
        return x;

    public int getY() {
        return y;

    public int hashCode() {
        return 13 ^ x ^ y;

    public boolean equals(Object o) {
        if (o == null) {
            return false;
        } else if (this == o) {
            return true;
        } else if (o instanceof Cell) {
            Cell c = (Cell) o;
            return this.x == c.x && this.y == c.y;
        return false;

Java HashSet tutorial, Java HashSet tutorial shows how to use Java HashSet collection. HashSet is a collection that contains no duplicate elements. Thanks to type inference, it is not necessary to provide types on the right side of the declaration. You should use the constructor which allows you to specify the IEqualityComparer you want to use. HashSet<String> hashSet = new HashSet<String>(StringComparer.InvariantCultureIgnoreCase); The StringComparer object provides some often used comparer as static properties.

5 C# Collections that Every C# Developer Must Know, Learn all about essential C# collections: List, Dictionary, HashSet, Stack and Queue. For every collection, you'll learn what it is, when to use it and how. NET is like finding the right camera in a camera shop! There are so many options to Check to see if the list contains an item Contains()?. Reply. You can also use the existing HashSet object instead of creating a new one to get the synchronized Set object. Since the HashSet class is backed by HashMap, it offers constant time performance for basic operations like add, remove, contains and size.

Can a Java HashSet contains 2 different objects (based on equals , What is the use of Collections.emptyList()?. 5,711 Views � How do you iterate over a Hash map of arraylists of String in Java? 31,117 Views. In the examples above, we created items (objects) of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:

Working With hashcode() and equals(), A very popular usage of equals() is defining an array list of Student and Arraylist contains Alex = true HashSet contains Alex = false. WAIT!! The above method checks if the List and HashSet contains item passed as an input parameter to the Contains() method. Run the application, the result will be shown as in the following image. The result clearly shows that the HashSet provides faster lookup for the element than the List. This is because of no duplicate data in the HashSet.