CODEWITHSUNDEEP
javalistgenericsjava-8
RobinHood
RobinHood

Reputation: 33

java iterate over map< string, list< string> , getordefault method

I have a map of < string, List< String type>>, What I am trying to do is add a string to map value if list is present, else create a list and add it to the list and insert in map. s1, s2 are strings.

Code:

Map<String, List<String>> map = new HashMap<>();

map.put(s1,(map.getOrDefault(s1, new LinkedList<String>())).add(s2));

Error:

error: incompatible types: boolean cannot be converted to List<String>

What's wrong with this !!!

Upvotes: 3

Views: 7333

Answers (3)

Supp
Supp

Reputation: 1

Map<String, List<String>> map = new HashMap<>();

// gets the value if it is present in the map else initialze the list
List<String> li = map.getOrDefault(s1,new LinkedList<>());

li.add(s2);

map.put(s1,li);

Upvotes: 0

Ravindra Ranwala
Ravindra Ranwala

Reputation: 21124

This call,

(map.getOrDefault(s1, new LinkedList())).add(s2)

Returns a boolean primitive, which can not be casted to a List. That why you are getting this error.

You can solve it like so,

map.compute(s1, (k, v) -> v == null ? new LinkedList<>() : v).add(s2);

The trick here is, map.compute() returns the new value associated with the specified key, and then you can add the s2 String literal into that afterward.

Upvotes: 1

janith1024
janith1024

Reputation: 1042

add method of list 'map.getOrDefault(s1, new LinkedList())).add(s2)' is return boolean so you have to do it in separate line

so try like this

    Map< String, List< String>> map = new HashMap<>();

    List<String> list = map.get(s1);
    if(list == null){
      list = new LinkedList<>();
      map.put(s1,list);
    }
    list.add(s2);

If use java 8 and need to do in single line do like this 

    map.computeIfAbsent(s1, k -> new LinkedList<>()).add(s2);

Upvotes: 10

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