How to have one liner code for creating and updating map entries?

Question

In C++, when using maps, one of the ways we can quickly create a new entry and update a map at the same time is by simply doing something like:

dict[key]+=1;

This allows you to bypass checking to see if a particular key already exists, and then creating an if else statement for it.

I was wondering if there was something similar for Java, especially for a put command.

At present I am stuck using two separate put statements. Trying to use a put statement and combining an addition statement with the value parameter hasn't worked.

Answer 1

Java Map has several related methods, depending on what you need. Probably the most generic method is merge. Your example would translate to:

dict.merge(
    key, 
    1, // Set this value if the key didn't exist yet
    (oldValue, value) -> oldValue + value // Add value to old value if the key already existed
);

In one line:

dict.merge(key, 1, (val, increment) -> val + increment);

There are also a few simpler methods for other purposes, e.g. computeIfAbsent.

Answer 2

In c++ when the key is not present in the map, the key, value pair gets added and its value is by default set to 0 [Default value in map], but in Java when the key is not found it returns null.

But it is possible to bypass the checking by setting the default return value to 0,

int key = 2;
HashMap<Integer,Integer> hm = new HashMap<Integer,Integer>();
hm.put(key, hm.getOrDefault(key, 0) + 1);

HashMap_doc

Answer 3

The closest thing to your situation is:

Map<KeyType, AtomicInteger> myMap = new HashMap<>();

newValue = myMap.computeIfAbsent(key, x -> new AtomicInteger()).incrementAndGet();