Partial search in HashMap

Question

I need to create phone book kind of thing. It contains name & number. Now when I type letters matching list should be returned. For the example given below, when I type

Answer 1

This calls for a Trie data structure. See this question for java implementations. I used this one.

Answer 2

Yeah, a HashMap is not the right data structure for this. As Bozho said, a Trie would be the right one.

With Java's on-board tools, a TreeMap (or any SortedMap, actually) could be used:

public <V> SortedMap<String, V> filterPrefix(SortedMap<String,V> baseMap, String prefix) {
    if(prefix.length() > 0) {
        char nextLetter = prefix.charAt(prefix.length() -1) + 1;
        String end = prefix.substring(0, prefix.length()-1) + nextLetter;
        return baseMap.subMap(prefix, end);
    }
    return baseMap;
}

The output would even be sorted by key.

Here an usage example:

SortedMap<String, String> nameNum = new TreeMap<String, String>();
// put your phone numbers

String prefix = ...;
for(Map.Entry<String,String> entry : filterPrefix(nameNum, prefix).entrySet()) {
    System.out.println(entry);
}

If you want your prefix filter to not be depending on case differences, use a suitable Comparator for your map (like a Collator with a suitable strength setting, or String.CASE_INSENSITIVE_ORDER).

Answer 3

Put it all in a MultiMap (or just store a List as the value in your HashMap). For "Brown", store:

"B"->["Brown"]
"BR"->["Brown"]
"BRO"->["Brown"]

If you later add "Bradley":

"B"->["Brown", "Bradley"]
"BR"->["Brown", "Bradley"]
"BRO"->["Brown"]
"BRA"->["Bradley"]

etc...

then have another map to map "Brown" or "Bradley" to the phone number.

Answer 4

Remove all values which doesn't contain key part:

yourMap.keySet().removeIf(key -> !key.contains(keyPart));

Or regex:

yourMap.keySet().removeIf(key -> !key.matches(".*keyPart.*"));

Or filter stream and collect to a new map:

yourMap.entrySet().stream().filter(e -> e.getKey().contains(keyPart)).collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()));

Answer 5

Use guava Multimap will ease your solution.

The key is first letter of name, the value is a Collection containing all name-phone pair which name is started with the key(first letter).

Example:

    public void test(){
      //firstLetter -> list of name-phone pair
      Multimap<String, Pair> mMap =  ArrayListMultimap.create();

      put(mMap, "Brown",  "+1236389023");
      put(mMap, "Bob",    "+1236389023");
      put(mMap, "Harmer", "+1236389023");
      put(mMap, "Harris", "+1236389023");
      put(mMap, "Hawken", "+1236389023");
      put(mMap, "Hosler", "+1236389023");

      //Test
      System.out.println(mMap.get("H"));
   }

   void put(Multimap<String, Pair> mMap, String name, String phone){
      mMap.put(name.substring(0,1), new Pair(name, phone));
   }

   public static class Pair{
      String name;
      String phone;

      public Pair(String name, String phone) {
         this.name = name;
         this.phone = phone;
      }

      @Override
      public String toString() {
         return "Pair [name="+name+", phone="+phone+"]";
      }

}