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 H, a list containing Harmer, Harris, Hawken, Hosler should be returned. When type Ha then list containing only Harmer, Harris, Hawken should be returned.

  Map<String, String> nameNum = new HashMap<String, String>();

  nameNum.put("Brown", "[phone]");
  nameNum.put("Bob", "[phone]");
  nameNum.put("Harmer", "[phone]");
  nameNum.put("Harris", "[phone]");
  nameNum.put("Hawken", "[phone]");
  nameNum.put("Hosler", "[phone]");

Any idea how achieve it? Thanks in advance.

Answer 1

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).