How to update a value, given a key in a hashmap?

map.put(key, map.get(key) + 1);

should be fine. It will update the value for the existing mapping. Note that this uses auto-boxing. With the help of map.get(key) we get the value of corresponding key, then you can update with your requirement. Here I am updating to increment value by 1.

The simplified Java 8 way:

map.put(key, map.getOrDefault(key, 0) + 1);

This uses the method of HashMap that retrieves the value for a key, but if the key can't be retrieved it returns the specified default value (in this case a '0').

This is supported within core Java: HashMap<K,V> getOrDefault(Object key, V defaultValue)

Java 8 way:

You can use computeIfPresent method and supply it a mapping function, which will be called to compute a new value based on existing one.

For example,

Map<String, Integer> words = new HashMap<>();
words.put("hello", 3);
words.put("world", 4);
words.computeIfPresent("hello", (k, v) -> v + 1);
System.out.println(words.get("hello"));

Alternatevely, you could use merge method, where 1 is the default value and function increments existing value by 1:

words.merge("hello", 1, Integer::sum);

In addition, there is a bunch of other useful methods, such as putIfAbsent, getOrDefault, forEach, etc.