Java8 Create Map grouping by key contained in values - collections

Question

I have the following two lists of String:
{APPLE, ORANGE, BANANA} //call it keyList
{APPLE123, ORANGEXXX, 1APPLE, APPLEEEE} //call it valueList
Desired output is an HashMap<String, List<String>> like this:
<APPLE, {APPLE123, 1APPLE, APPLEEEE}>
<ORANGE, {ORANGEXXX}>
<BANANA, {}> //also <key, null> is accepted
I have implemented this solution(it works)
HashMap<String, List<String>> myMap = new HashMap<>();
keyList.forEach(key -> {
List<String> values = valueList.stream()
.filter(value -> value.contains(key))
.collect(Collectors.toList());
myMap.put(key, values);
});
Given the assumption that a value is related to only one key (it's a constraint of my domain), is this the best solution in java8 , in terms of performance and/or code cleaning ?
Can it be tuned in some way?

Answer 1

If you can safely assume that each value is associated with a key, and only one key, you can go into the following direction:
Pattern p = Pattern.compile(String.join("|", keyList));
Map<String, List<String>> map = valueList.stream()
.collect(Collectors.groupingBy(s -> {
Matcher m = p.matcher(s);
if(!m.find()) throw new AssertionError();
return m.group();
}));
map.forEach((k,v) -> System.out.println(k+": "+v));
If the keys may contain special characters which could get misinterpreted as regex constructs, you can change the preparation code to
Pattern p = Pattern.compile(
keyList.stream().map(Pattern::quote).collect(Collectors.joining("|")));
The collect operation does only create the groups for existing values. If you really need all keys to be present, you can use
Map<String, List<String>> map = valueList.stream()
.collect(Collectors.groupingBy(s -> {
Matcher m = p.matcher(s);
if(!m.find()) throw new AssertionError();
return m.group();
},
HashMap::new, // ensure mutable map
Collectors.toList()
));
keyList.forEach(key -> map.putIfAbsent(key, Collections.emptyList()));
or
Pattern p = Pattern.compile(
keyList.stream().map(Pattern::quote)
.collect(Collectors.joining("|", ".*(", ").*")));
Map<String, List<String>> map = valueList.stream()
.map(p::matcher)
.filter(Matcher::matches)
.collect(Collectors.groupingBy(m -> m.group(1),
HashMap::new, // ensure mutable map
Collectors.mapping(Matcher::group, Collectors.toList())
));
keyList.forEach(key -> map.putIfAbsent(key, Collections.emptyList()));