How to Convert a Map<String, List<String>> to Map<String, String> in java 8

Question

I have a map like

key= ["a1", "a2", "a3"] value = [["a1.value1", "a1.value2"],["a2.value1", "a2.value2"]]

the resulting Map should be like

key = ["a1", "a2", "a3"] value = ["a1.value1, a1.value2", "a2.value1, a2.value2"]

How can we use Collectors.joining as an intermediate step ?

Welcome to Stack Overflow! Please take the tour and visit our help center to learn what kinds of questions are appropriate for this site. If you can edit your question to fit the requirements of this site, please do so. — Joe C
– Joe C, Commented Nov 17, 2018 at 22:24
Not sure I understand your question... are you trying to map multiple values to the same key in a Map, i.e. have multiple Map entries per a specific key? In a Java Map, the keys are a set, meaning no single key value can repeat. — John Camerin
– John Camerin, Commented Nov 17, 2018 at 22:27

Ousmane D. · Accepted Answer · 2018-11-17 22:33:37Z

How can we use Collectors.joining as an intermediate step ?

You mean, in the collecting phase...

Yes, you can:

Map<String, String> result = source.entrySet() .stream() .collect(toMap(Map.Entry::getKey, e -> e.getValue().stream().collect(joining(", "))));

but, better to use String.join:

Map<String, String> result = source.entrySet() .stream() .collect(toMap(Map.Entry::getKey, e -> String.join(", ", e.getValue())));

or none stream variant:

Map<String, String> resultSet = new HashMap<>(); source.forEach((k, v) -> resultSet.put(k, String.join(",", v)));

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