Question

I have a String Vector that contains data like this :

5:34, 5:38, 17:21, 22:11, …

If i try to merge this using Collections.sort( … ); it will appear like this :

17:21, 22:11, 5:34, 5:38

Actually i want it to appear like this :

5:34, 5:38, 17:21, 22:11

So i want to sort the elements according to the number before the colon “:” then if some elements have the same number before “:” then sort them according to the number after the “:”.

What is the simplest way to do this ?

Answer 1

You could either create a custom Comparator to split the String and parse it into two ints, or create a bespoke class to represent each String and store that in the Collection instead. I favour the latter approach as you only incur the overhead of splitting / parsing the String once; e.g.

public class Data implements Comparable<Data> {
  private final int prefix;
  private final int suffix;

  public Data(String str) {
    String[] arr = str.split(":");

    if (arr.length != 2) {
      throw new IllegalArgumentException();
    }

    this.prefix = Integer.parseInt(arr[0]);
    this.suffix = Integer.parseInt(arr[1]);
  }

  public int compareTo(Data data) {
    // Should really avoid subtraction in case of overflow but done to keep code brief.
    int ret = this.prefix - data.prefix;

    if (ret == 0) {
      ret = this.suffix - data.suffix;
    }

    return ret;
  }

  // TODO: Implement equals and hashCode (equals to be consistent with compareTo).

  public String toString() { return String.format("%d:%d", prefix, suffix); }
}

Then it’s simply a case of storing some Data objects in your Collection; e.g.

List<Data> l = new ArrayList<Data>();
l.add(new Data("13:56"));
l.add(new Data("100:16"));
l.add(new Data("9:1"));
Collections.sort(l);

One more thing – You mention you’re using a Vector. You should try to avoid using Vector / Hashtable as these have been superseded by List / Map, which were introduced as part of the Collections Framework in JDK 1.2.