Java Collections HashMap

In Java, a `HashMap` is a collection that stores key-value pairs. It is part of the Java Collections Framework and provides a fast and efficient way of storing and retrieving data using keys. Here are some basics of the `HashMap` in Java:

1. Creating a `HashMap`: To create a `HashMap`, you can use the `HashMap` class and specify the types of objects you want to use for the keys and values. For example:

HashMap scores = new HashMap();

Here, a `HashMap` is created with keys of type `String` and values of type `Integer`.

2. Adding elements: You can add key-value pairs to a `HashMap` using the `put` method. For example:

scores.put("Alice", 95);
scores.put("Bob", 80);
scores.put("Charlie", 75);

Here, three key-value pairs are added to the `HashMap`.

3. Accessing elements: You can access values in a `HashMap` using the `get` method and specifying the key of the value you want to access. For example:

int aliceScore = scores.get("Alice");

Here, the value associated with the key “Alice” is accessed and stored in the variable `aliceScore`.

4. Removing elements: You can remove key-value pairs from a `HashMap` using the `remove` method and specifying the key of the pair you want to remove. For example:

scores.remove("Bob");

Here, the key-value pair associated with the key “Bob” is removed from the `HashMap`.

5. Checking for key existence: You can check if a key exists in a `HashMap` using the `containsKey` method. For example:

boolean containsAlice = scores.containsKey("Alice");

Here, the variable `containsAlice` is set to `true` if the key “Alice” exists in the `HashMap`.

6. Iterating over elements: You can iterate over the key-value pairs in a `HashMap` using a for-each loop and the `entrySet` method. For example:

for (Map.Entry entry : scores.entrySet()) {
    String name = entry.getKey();
    int score = entry.getValue();
    System.out.println(name + ": " + score);
}

Here, each key-value pair in the `HashMap` is printed to the console.

The `HashMap` in Java is a powerful data structure that allows you to store and manipulate key-value pairs in a fast and efficient way. By understanding the basics of the `HashMap`, you can write more efficient and effective code that can handle complex data structures.