HashMap in Java

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 HashMap is a collection class in Java that implements the Map interface, which allows you to store key-value pairs in a hash table. It uses a hash function to generate a hash code for each key, which is used to index the value in the underlying array.

Here's an example of how to create a HashMap in Java:

typescript
import java.util.HashMap;

public class Example {
    public static void main(String[] args) {
        // Create a new HashMap
        HashMap<String, Integer> myHashMap = new HashMap<>();

        // Add some key-value pairs
        myHashMap.put("John", 25);
        myHashMap.put("Jane", 30);
        myHashMap.put("Bob", 20);

        // Retrieve a value based on a key
        System.out.println("John's age is " + myHashMap.get("John"));

        // Iterate over the keys and values in the map
        for (String key : myHashMap.keySet()) {
            System.out.println(key + " is " + myHashMap.get(key) + " years old.");
        }
    }
}

In this example, we create a HashMap with String keys and Integer values. We then add three key-value pairs to the map, retrieve the value associated with the key "John", and iterate over all the key-value pairs in the map.

HashMap provides constant-time performance for basic operations like get() and put(), but the performance can degrade when the hash table needs to be resized. It is also not thread-safe, so if you need to use it in a multi-threaded environment, you should use a ConcurrentHashMap instead.

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