Why containsKey did not find the key?

Question

I have the following code:

payoffs2exchanges.put(point, exchange); if (!payoffs2exchanges.containsKey(point) ) { game.log.fine("yes"); } else { game.log.fine("no"); }

It outputs "no". In other words, I add the key-value pair to the map, and then, immediately after that I check if the key exist and find out that it does not exist. Why?

I still have the problem with the key. The following code says every time I add a key I add a new key. And I know that it is not the case.

 Integer[] point = new Integer[2]; point[0] = proposerBestScore; point[1] = responderBestScore; game.log.fine("In the getCloudOfPayoffs: found payoffs:" + point[0] + "," + point[1] + ". Exchange: " + exchange[0]+","+exchange[1]+","+exchange[2]+","+exchange[3]+","+exchange[4]); // With the following block we ensure that every options (pair of payoffs) is represented by exchange with minimal number of moves. if (!payoffs2exchanges.containsKey(point)) { payoffs2exchanges.put(point, exchange); game.log.fine("In the getCloudOfPayoffs: this option is new. We add it to the map."); } else { game.log.fine("In the getCloudOfPayoffs: this option is old."); Integer[] exchangeFromMap = payoffs2exchanges.get(point); Integer newSum = 0; Integer oldSum = 0; for (int i=0;i<Design.nColors;i++) { newSum = newSum + Math.abs(exchange[i]); oldSum = oldSum + Math.abs(exchangeFromMap[i]); } if (newSum<oldSum) { game.log.fine("In the getCloudOfPayoffs: the new exchange is better than the old one."); payoffs2exchanges.put(point, exchange); } }

What class is point? Custom class? What class is payoffs2exchanges? — Bert F
– Bert F, Commented Dec 10, 2010 at 15:44
@aioobe, sorry for these manipulations with the question. I put it back. So, you answer is useful. — Roman
– Roman, Commented Dec 10, 2010 at 16:07

aioobe · Accepted Answer · 2010-12-10 16:01:12Z

You're using an Integer[] as key in the map. This is a bad thing, since Java arrays don't implement equals and hashCode as you might expect. See this example:

public class Test { public static void main(String[] args) { Integer[] arr1 = { 1, 2 }; Integer[] arr2 = { 1, 2 }; System.out.println(arr1.equals(arr2)); System.out.println(arr1.hashCode() + " / " + arr2.hashCode()); } }

On my computer it prints:

false 1476323068 / 535746438

My recommendation is to create a custom Point class that properly overrides equals and hashCode (or possibly reuse java.awt.Point if you think that makes sense).

does it mean that containsKey does not work if I use integer array as a key?
Yes, exactly. Maps in general rely heavily on the equals method, and when it comes to for instance HashMap the hashCode is important as well.
but can I still use somHashMap.get(key) if key is an integer array?
No. The get method would call key.hashCode() to figure out in which bucket to look for the value associated with key. If the hash value of key doesn't correspond to the hash value of the key used when inserting the element, the hash map will simply (most likely) look for the value in the wrong bucket.

lijie · Accepted Answer · 2010-12-10 15:44:09Z

It is doing the right thing. containsKey returns true, the ! operator negates it to false, and so it outputs no (the else clause).

pgras · Accepted Answer · 2010-12-10 15:44:26Z

0

Look at your code :) it prints no if the map actually contains the key...

answered Dec 10, 2010 at 15:44

pgras

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Why containsKey did not find the key?

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