Question

1. How can I improve performance of
   this chunk of code ?
2. What would be unit test case for the given problem statement ?

Code:

    public class SlowDictionary {
        private final Map<String,String> dict = new HashMap<String,String>();
        public synchronized String translate (String word)
        throws IllegalArgumentException {
            if (!dict.containsKey(word)) {
                throw new IllegalArgumentException(word + " not found.");
            }
            return dict.get(word);
        }

        public synchronized void addToDictionary (String word, String translation) 
            throws IllegalArgumentException {
            if (dict.containsKey(word)) {
                throw new IllegalArgumentException(word + " already exists.");
            }
            dict.put(word,translation);
        }

        public synchronized Set<String> getAllWords () {    
            return dict.keySet();
        }
    }

Answer 1

First thing you want to do is get rid of all synchronized key words.

The easiest way of doing that is be declaring dict to be a ConcurrentHashMap:

private final ConcurrentMap<String,String> dict = new ConcurrentHashMap<String,String>();

Doing this you can immidiatly remove the synchronized part of translate so it would look like:

 public String translate (String word) throws IllegalArgumentException { ..

The reason for this is the contract in which the CCHM holds regarding up to date reads.

Finally, add to dictionary can look like:

 public void addToDictionary (String word, String translation) throws IllegalArgumentException {
            if (dict.putIfAbsent(word,translation)!=null) {
                throw new IllegalArgumentException(word + " already exists.");
            }
        }

Also remove synchronized from getAllWords as well.

Edit: After thinking about tom’s comment. Having a double look up in this ‘exception case’ probably isnt worth it. If the case didnt throw an exception then it would be appropriate.