how to convert Map<Integer, List<String>> to Map<Integer, Set<String>>

Question

Map<Integer, List<String>> mapList = new TreeMap<>();
Map<Integer, Set<String>> mapSet = new TreeMap<>();
        Set<String> set = new TreeSet<>();
        for (Map.Entry<Integer, List<String>> entry : entriesSortedByValues(mapList)) {
            set.addAll(entry.getValue());
            mapSet.put(entry.getKey(), set);
        }

It doesnt work. I want to copy mapList to mapSet.

You have to create a new set for each iteration, not one set for all iterations. If you would have debugged your code, you would have seen that you keep constantly writing to the same set. — Smutje
– Smutje, Commented May 26, 2014 at 9:32

Alexis C. · Accepted Answer · 2014-05-26 09:49:08Z

4

You keep adding and modifying the same object for all the keys.

What you currently do can be viewed as this:

enter image description here

So to fix that, create a new Set at each iteration of the loop.

Map<Integer, List<String>> mapList = new TreeMap<>();
Map<Integer, Set<String>> mapSet = new TreeMap<>();
for (Map.Entry<Integer, List<String>> entry : entriesSortedByValues(mapList)) {
    Set<String> set = new TreeSet<>();
    set.addAll(entry.getValue());
    mapSet.put(entry.getKey(), set);
}

Also you could use the constructor that takes a collection as parameter.

Map<Integer, List<String>> mapList = new TreeMap<>();
Map<Integer, Set<String>> mapSet = new TreeMap<>();
for (Map.Entry<Integer, List<String>> entry : entriesSortedByValues(mapList)) {
    mapSet.put(entry.getKey(), new TreeSet<>(entry.getValue()));
}

edited May 26, 2014 at 9:49

answered May 26, 2014 at 9:32

Alexis C.

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Comments

Abimaran Kugathasan · Accepted Answer · 2014-05-26 10:07:46Z

3

You can convert a List to Set using HashSet's constructor like, new HashSet<String>(myList);

Map<Integer, List<String>> mapList = new TreeMap<>();
Map<Integer, Set<String>> mapSet = new TreeMap<>();

   for (Map.Entry<Integer, List<String>> entry : entriesSortedByValues(mapList)) {
        Set<String> set = new HashSet<String>(entry.getValue());
        mapSet.put(entry.getKey(), set);
   }

edited May 26, 2014 at 10:07

answered May 26, 2014 at 9:33

Abimaran Kugathasan

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Comments

Aniket Thakur · Accepted Answer · 2014-05-26 09:32:48Z

0

Add Set in your for loop

    for (Map.Entry<Integer, List<String>> entry : entriesSortedByValues(mapList)) {
        Set<String> set = new TreeSet<>();
        set.addAll(entry.getValue());
        mapSet.put(entry.getKey(), set);
    }

answered May 26, 2014 at 9:32

Aniket Thakur

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