Using Java 8 lambda to convert Map of Lists to different Map

Question

I have data that is like this:

List<Map<String, String>> = // psuedocode
[
 {
  "ID": "a",
  "Value": "val1"
 },
 {
  "ID": "b",
  "Value": "val2"
 },
]

I want to turn this to

Map<String, String> = // more pseudocode
{
 "a": "val1",
 "b": "val2"
}

I have something like this so far:

    Map<String, String> output = myList.stream()
            .flatMap(m -> m.entrySet().stream())
            .collect(Collectors.toMap())

Can someone please guide me? The alternative is to use regular for loops, but I am trying to use lambdas.

Rather than pseudo-code, it would be helpful if you could come up with a minimal reproducible example. That would stop everyone else having to do the duplicate work of creating a small test. The fact that your code so far doesn't refer to "ID" or "Value" is somewhat surprising... it sounds like you should be mapping each Map<String, String> into a single key-value pair, basically... — Jon Skeet
– Jon Skeet, Commented Jul 15, 2016 at 2:20

erickson · Accepted Answer · 2016-07-15 02:39:53Z

5

You need to map each Map instance in the list to a key and a value, then collect into a single Map instance.

Map<String, String> output = myList.stream()
  .collect(Collectors.toMap(m -> m.get("ID"), m -> m.get("Value")));

This will throw an exception if an "ID" is duplicated. There are overloaded methods that give you more control over this behavior.

answered Jul 15, 2016 at 2:39

erickson

271k59 gold badges406 silver badges502 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Jude Niroshan · Accepted Answer · 2016-07-16 06:38:35Z

0

while above solution is great, I got the result as follows:

//declared a result map
Map<String, String> result = new ConcurrentHashMap<String, String>();

list.parallelStream().forEach( m -> result.put(m.get("id"), m.get("value")));

You could be safe in concurrent processing situations.

answered Jul 16, 2016 at 6:38

Jude Niroshan

4,4809 gold badges43 silver badges68 bronze badges

Collectives™ on Stack Overflow

Using Java 8 lambda to convert Map of Lists to different Map

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related