Stumbling around Google collections

I came across a situation in some code I need to modify where maps were being 'filtered' to create tailored data for different situations. I couldn't come up with the solution I wanted in the moment so I cobbled up an example to see what might work.


As a simple example:

	public class FilterExample {
	private Map<String, String> data = new HashMap<String, String>();
	public FilterExample() {
	data.put("A", "one");
	data.put("B", "two");
	data.put("C", "three");
	data.put("D", "four");
	data.put("E", "five");
	}
	public Map<String, String> getNewMapWithVowelKeys() {
	Map<String, String> result = new HashMap<String, String>();
	for (String keyVal: data.keySet()) {
	if (keyVal.matches("^[aeiouAEIOU].*")) {
	result.put(keyVal, data.get(keyVal));
	}
	}
	return result;
	}
	public Map<String, String> getNewMapWithVowelValues() {
	Map<String, String> result = new HashMap<String, String>();
	for (Map.Entry<String, String> entry: data.entrySet()) {
	if (entry.getValue().matches("^[aeiouAEIOU].*")) {
	result.put(entry.getKey(), entry.getValue());
	}
	}
	return result;
	}
	public Map<String, String> getNewMapWithConsonantKeys() {
	Map<String, String> result = new HashMap<String, String>();
	for (String keyVal: data.keySet()) {
	if (keyVal.matches("^[^aeiouAEIOU].*")) {
	result.put(keyVal, data.get(keyVal));
	}
	}
	return result;
	}
	public Map<String, String> getNewMapWithConsonantValues() {
	Map<String, String> result = new HashMap<String, String>();
	for (Map.Entry<String, String> entry: data.entrySet()) {
	if (entry.getValue().matches("^[^aeiouAEIOU].*")) {
	result.put(entry.getKey(), entry.getValue());
	}
	}
	return result;
	}
	}

view raw OriginalMapAccess.java hosted with ❤ by GitHub

We're looking at two different criteria (string starts with a vowel or a consonant) and two different targets -- the map key or the value.

The looping is duplicated and the determination of whether the values should be in the new map screams for a predicate.

In Google collections this is usually just a filter taking in the collection and the predicate. But 'filter' wasn't available on a Map -- it works on a collection.

Well, first I needed a predicate:

	public class EntryKeyIsVowel implements Predicate<Map.Entry<String,String>> {
	public boolean apply(Map.Entry<String, String> input) {
	return input.getKey().matches("^[aeiouAEIOU].*");
	}
	public boolean equals(Object obj) {
	return this.getClass() == obj.getClass();
	}
	}

view raw EntryKeyIsVowel.java hosted with ❤ by GitHub

That was easy enough.

My first try was to filter the entry set of the map and then add the filtered elements to the result:

	public Map<String, String> getMapFor(Predicate<Map.Entry<String,String>> mapPredicate) {
	Map<String, String> result = newHashMap();
	Collection<Map.Entry<String, String>> newEntries = filter(data.entrySet(), mapPredicate);
	for (Map.Entry<String, String> entry: newEntries) {
	result.put(entry.getKey(), entry.getValue());
	}
	return result;
	}

view raw FilterThenPutResults.java hosted with ❤ by GitHub

Not too happy that I'm looping over something twice in the worst case.
Well, I can just use the predicate without using anything else from Google collections:

	public Map<String, String> anotherMapFor(Predicate<Map.Entry<String,String>> mapPredicate) {
	Map<String, String> result = newHashMap();
	for (Map.Entry<String, String> entry: data.entrySet()) {
	if (mapPredicate.apply(entry)) {
	result.put(entry.getKey(), entry.getValue());
	}
	}
	return result;
	}

view raw LoopAndPutEntry.java hosted with ❤ by GitHub

But, I just felt there should be a better way. Asked a couple of people while I was trying different searches. Finally I found that Maps has a tranform, but it also has different filter methods! Just what I needed.

The one-liner:

	public Map<String, String> transformMapFor(Predicate<Map.Entry<String,String>> mapPredicate) {
	return Maps.filterEntries(data, mapPredicate);
	}

view raw OneLineFilter.java hosted with ❤ by GitHub

That will wrap it up for now.