HashMap in Java

In Java, a HashMap is a collection that implements the Map interface and uses a hash table to store key-value pairs. This means that you can store and retrieve elements based on their keys in constant time, which provides a very efficient way to manage data.

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The HashMap uses an array of linked lists to store the key-value pairs. When you add an element to the map, the hash code of the key is used to compute an index into the array. The key-value pair is then added to the linked list at that index.

When you retrieve an element from the map, the hash code of the key is used to compute the index into the array, and then the linked list at that index is searched for the key. If the key is found, the corresponding value is returned.

Here are some key features of a HashMap in Java:

1. Unique keys: Each key in a HashMap must be unique. If you try to add a key that already exists in the HashMap, the value associated with the key will be updated to the new value.
2. Null keys and values: A HashMap implementation allows null keys and values. This means that you can add key-value pairs where the key or value is null.
3. Iteration: You can iterate through the key-value pairs in a HashMap using a for-each loop or an iterator.

Here’s an example of how to create and use a HashMap in Java:

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import java.util.HashMap;
import java.util.Map;

public class HashMapExample {
    public static void main(String[] args) {
        // create a new HashMap
        Map<String, Integer> scores = new HashMap<>();

        // add some key-value pairs to the map
        scores.put("Alice", 100);
        scores.put("Bob", 90);
        scores.put("Charlie", 80);

        // get the value associated with a key
        int aliceScore = scores.get("Alice");
        System.out.println("Alice's score is " + aliceScore);

        // iterate through the key-value pairs in the map
        for (Map.Entry<String, Integer> entry : scores.entrySet()) {
            String name = entry.getKey();
            int score = entry.getValue();
            System.out.println(name + " scored " + score + " points.");
        }
    }
}

In this example, we create a new HashMap and add some key-value pairs to it. We then retrieve the value associated with the “Alice” key using the get() method, and iterate through all the key-value pairs in the map using a for-each loop over the entrySet().