如何在java中为Map中的特定键增加值？

Question

In C++ if I declare a map like std::map m

then I can increment the value for a specific key in the map in this way

m[key]++

In Java I declare a map

Map<Integer, Integer> m = new HashMap<>();

and I increment the value for a specific key in this way:

m.put(key, m.get(key).intValue() + 1)

My question: Is there any shortcut or better way to do this?

Answer 1

您可以使用计算（Java 8+）：

m.compute(key, (k, v) -> v + 1);

Answer 2

I've always preferred to use a mutable int for these problems. So the code ends up looking like...

m.get(key).increment()

This avoids the unnecessary put overhead (which is small).

Answer 3

You don't need the .intValue() because of autoboxing, but apart from that, there is no better way to do this.

m.put(key, m.get(key) + 1)

The reason (or the problem) is that Java decided not to let classes implement their own operators (like it is possible in C++).

Answer 4

There is a problem with the accepted answer that if there is no value for a requested key, we'll get an exception. Here are a few ways to resolve the issue using Java 8 features:

1. Using Map.merge()

m.merge(key, 1, Integer::sum)

If the key doesn't exist, put 1 as value, otherwise sum 1 to the value given for the key.

2. Using Map.putIfAbsent()

m.putIfAbsent(key, 1);
m.put(key, m.get(key) + 1);

Similar to other answers, but clearly says you need to init the value first.

3. Using Map.compute()

m.compute(key, (k, v) -> v == null ? 1 : v + 1)

Always computing the value, providing 1 if there is nothing there yet.

4. Using Map.getOrDefault()

m.put(key, m.getOrDefault(key, 0) + 1)

Gets the value if it exists or a default value otherwise, and returns that value plus one.

Answer 5

您可以删除intValue调用并依靠自动拆箱。