从地图交换密钥 <Key, List<Values> &gt;

Question

I'm searching a solution for this problem(it is for an exam):

I have a Map < String, SortedSet < String > > operators populated by a function

public void addOperator(String operatorName, String... destinationNames) throws ProposalException {
    if(operators.containsKey((operatorName))){
        throw new ProposalException("Operator " + operatorName + "already into system!");
    }
    else{
        SortedSet<String> destinationstemp=new TreeSet<>();
        for(String s: destinationNames){
            if(s!=null){
                destinationstemp.add(s);
            }
        }
        operators.put(operatorName, destinationstemp);


    }

Now, i want to create a new Map < String, SortedSet < String > > destinations that has as key the destinationName and as values the operatorNames related.

How can i make this out?

PS: this one up there is the usage of the methods and the not-in-code part is the output wanted. Sorry for the bad formattation of the code. ph is the instance of the façade pattern class

    public SortedSet<String> getDestOperators(String destinationName) {...}//method that returns the **destinations** values related to destinationName}
ph.addOperator("op3","london","rome");
 ph.addOperator("op2","london","berlin");
 ph.addOperator("op5","berlin","rome","madrid");
ph.addOperator("op1","london","madrid","berlin");
 ph.addOperator("op10","rome");
 ph.addOperator("op4","madrid","berlin"); 
 System.out.println(ph.getDestOperators("madrid"));

Output: [op1, op4, op5]

Answer 1

you need to go through each entry in your map and check if inner set contains the value you are checking against,

public SortedSet<String> getDestOperators(String destinationName) {
   Set<String> result = new HashSet<String>();
   for(Map.Entry<String,Set<String>> entry : operators.getValues()){

      if(entry.getValue().contains(destinationName)){
          results.add(entry.getKey());
      }
   }

  return result;
}

Answer 2

To get your example output a simple one-liner with streams:

List<String> result = operators.entrySet().stream().filter(entry -> entry.getValue().contains(destinationName)).map(Entry::getKey).sorted().collect(Collectors.toList());

or here for better readability spread over multiple lines:

List<String> result = operators
      .entrySet()
      .stream()
      .filter(entry -> entry.getValue().contains(destinationName))
      .map(Entry::getKey)
      .sorted()
      .collect(Collectors.toList());

A more complex one-liner if you want to "reverse" the mapping as described in your text:

Map<String, List<String>> result = operators.entrySet().stream().flatMap(entry -> entry.getValue().stream().collect(Collectors.toMap(Function.identity(), o -> Arrays.asList(entry.getKey()))).entrySet().stream()).collect(Collectors.toMap(Entry::getKey, Entry::getValue, (a, b) -> Stream.of(a, b).flatMap(List::stream).sorted().collect(Collectors.toList())));

or here for better readability spread over multiple lines:

Map<String, List<String>> result2 = operators
      .entrySet()
      .stream()
      .flatMap(entry -> entry
            .getValue()
            .stream()
            .collect(Collectors.toMap(Function.identity(),
                                      o -> Arrays.asList(entry.getKey())))
            .entrySet()
            .stream())
      .collect(Collectors.toMap(Entry::getKey,
                                Entry::getValue,
                                (a, b) -> Stream.of(a, b)
                                      .flatMap(List::stream)
                                      .sorted()
                                      .collect(Collectors.toList())));

Answer 3

What you need to do, is loop over each operator, and then loop over all entries in the list, if value from the list is not yet present in your output map, you add it, else you modify its colection of operators. Here is some code for you:

origin.forEach((key, list) -> {list.forEach(city -> {
            if(result.containsKey(city))
                result.get(city).add(key);
            else{
                SortedSet<String> set = new TreeSet<>();
                set.add(key);
                result.put(city, set);
                });
        });