Searching a Hashmap for values of type list, retrieving an element of list and adding it to a new Collection to return

Question

i'm pretty new when it comes to Java but i'll hopefully clear this up.

I currently have one class and within that class i have a TreeMap called "departments" which takes an argument of >:

TreeMap <String, List<String>> department;

Within each department such as HR, Builders etc there are a list of names of people who work there. Such as:

HR: Janet, Jones, Bob

What i'd like to do is search through department to find all departments (keys) that contain someone who's called "bob" for instance and add them to a collection to make a return.

Can anyone help with this, i've been pulling my hair out for a few days! So far i'm this far with the method although clearly nowhere near complete!

public List<String> selectValues( String... aValue)

{

for(String eachDept : department.keySet()){
  Collection<String> peopleInTheDept = department.get(eachDept);
  for(String person : aValue){
      if(peopleInTheDept.contains(person)){
          result.add(person);
      }
  }
}
System.out.println(result);
}

Answer 1

Just like OH GOD SPIDERS predicted, there is a Java 8 stream solution:

TreeMap <String, List<String>> department = new TreeMap<>();
department.put("AB", Arrays.asList("Bob", "Truus", "Miep"));
department.put("CD", Arrays.asList("Jan", "Kees", "Huub"));
department.put("EF", Arrays.asList("Jan", "Piet", "Bert"));
String aValue = "Jan";

Map<String,List<String>> result = department.entrySet().stream()
// filter out those departments that don't contain aValue
  .filter(entry -> entry.getValue().contains(aValue))
// collect the matching departments back into a map 
  .collect(Collectors.toMap(k -> k.getKey(), k -> k.getValue()));

// print the result
result.forEach((k,v)-> System.out.println(k + " " + v.toString()));

Which prints:

EF [Jan, Piet, Bert]
CD [Jan, Kees, Huub]

---

Pre Java 8 solution:

Map<String, List<String>> result2 = new TreeMap<>();

for(String eachDept : department.keySet()){
    List<String> peopleInTheDept = department.get(eachDept);
    if(peopleInTheDept.contains(aValue)){
            result2.put(eachDept, department.get(eachDept));
    }
}
for (String s : result2.keySet()){
    System.out.println(s + " " + result2.get(s));
}

This prints exactly the same as my Java 8 code.