Java competition, simple example, multi-threaded hash map not secure

Question

I’m using a hash map to assign the voto at the id of a voter. But the class has several threads operating on top of the method addVote(...) and getVoteCount(...) , and the result returned is always unconscious, although he gave used the version synchronized hash map, Can anyone explain why not this thread safe yet? Any suggestions on how I can fix

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class SimpleVoteCounter {
    private final Map<Integer, Integer> electionsResults = 
            Collections.synchronizedMap(new HashMap<>());

    protected void addVote(Integer id){
        Integer old = electionsResults.get(id);
        int value = (old == null) ? 1 : old + 1;
        electionsResults.put(id, value);
    }

    protected Integer getVoteCount(Integer id){
        return electionsResults.get(id);
    }
}

Answer 1

You don’t need to sync the HashMap nor adopt ConcurrentHashMap. Just synchronize the methods that access it:

public class SimpleVoteCounter {
    private final Map<Integer, Integer> electionsResults = new HashMap<>();
    private final Object lock = new Object();

    protected void addVote(Integer id){
        synchronized(lock) {
            Integer old = electionsResults.get(id);
            int value = (old == null) ? 1 : old + 1;
            electionsResults.put(id, value);
        }
    }

    protected Integer getVoteCount(Integer id){
        synchronized(lock) {
            return electionsResults.get(id);
        }
    }
}

I adopted an object lock apart as a matter of preference, you can synchronize the method itself (protected synchronized void addVote()) or even synchronize on itself HashMap (synchronized(electionsResults) { ... }) which equals.