Java HashMap

A HashMap is a class in the Java Collections Framework that implements the Map interface and stores key-value pairs. It uses a hash table to store the elements, which allows for fast lookups, insertions, and deletions based on the key.

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

HashMap<String, Integer> map = new HashMap<>();

In this example, the map variable is a HashMap object that maps strings to integers. The HashMap class has two generic type parameters: the type of the keys (String in this case) and the type of the values (Integer in this case).

You can add elements to the HashMap using the put method.

For example:

map.put("apple", 1);
map.put("banana", 2);
map.put("orange", 3);

To get the value of an element in the HashMap, you can use the get method and specify the key of the element.

For example:

int value = map.get("apple"); // returns 1

You can use the size method of the HashMap class to get the number of elements in the map.

For example:

int size = map.size(); // returns 3

You can use the isEmpty method of the HashMap class to check if the map is empty.

For example:

if (map.isEmpty()) {
    // the map is empty
}

You can use the clear method of the HashMap class to remove all the elements from the map.

For example:

map.clear(); // removes all the elements from the map

You can use the keySet method of the HashMap class to get a Set of the keys in the map.

For example:

Set<String> keys = map.keySet();

You can use the values method of the HashMap class to get a collection of the values in the map.