How to directly initialize a HashMap in Java?

To directly initialize a HashMap in Java, you can use the put() method to add elements to the map. Here's an example:

Map<String, Integer> map = new HashMap<>();
map.put("key1", 1);
map.put("key2", 2);
map.put("key3", 3);

This creates a HashMap with three key-value pairs: {"key1", 1}, {"key2", 2}, and {"key3", 3}.

Alternatively, you can use the putAll() method to add all of the elements from another map to the HashMap:

Map<String, Integer> map1 = new HashMap<>();
map1.put("key1", 1);
map1.put("key2", 2);

Map<String, Integer> map2 = new HashMap<>();
map2.put("key3", 3);
map2.put("key4", 4);

map1.putAll(map2);

This creates a map1 with four key-value pairs: {"key1", 1}, {"key2", 2}, {"key3", 3}, and {"key4", 4}.

You can also use the HashMap constructor that takes a Map as an argument to create a new HashMap and initialize it with the elements from another map:

Map<String, Integer> map1 = new HashMap<>();
map1.put("key1", 1);
map1.put("key2", 2);

Map<String, Integer> map2 = new HashMap<>(map1);

This creates a new map2 with the same key-value pairs as map1.

Finally, you can use the Map.of() method to create an immutable map with a fixed set of elements:

Map<String, Integer> map = Map.of("key1", 1, "key2", 2, "key3", 3);

This creates an immutable map with three key-value pairs: {"key1", 1}, {"key2", 2}, and {"key3", 3}.