Sorting two dimensional array

[Vic Newman]

Is it possible to sort a 2 dimensional array with Arrays.sort()? It works great with a one dimensional array. I have a simple 2 dimensional array of strings [5] [10]. I want to sort on the first element.

[Neil Laurance]

I don't think you can directly use sort, but someting like this should be a good starting point:

import java.util.*;
public class Sorting2D {
  public static void main(String[] args) {
    String[][] strings = {
      { "c third line","2","3" },
      { "a first line", "6","3" },
      { "b second line","4","1" },
      { "d fourth line","1","2" }
    };
    ArrayList handles = new ArrayList();
    for(int i = 0; i < strings.length ; i++)
      handles.add(strings[i]);
    Collections.sort(handles, new Comparator() {
      public int compare(Object o1, Object o2) {
        String[] a = (String[])o1;
        String[] b = (String[])o2;
        if(a.length == 0 && b.length == 0)
          return 0;
        if(a.length == 0 && b.length != 0)
          return 1;
        if(a.length != 0 && b.length == 0)
          return -1;
        return a[0].compareTo(b[0]);
      }
      public boolean equals(Object o) {
        return this == o;
      }
    });
    for(Iterator it = handles.iterator(); it.hasNext(); ) {
      String[] a = (String[])it.next();
      for(int i = 0; i < a.length ; i++)
        System.out.print(a[i] + ", ");
      System.out.println();
    }
  }
}

According to my Java in a Nutshell, in most cases the Comparator.compare method should be implemented to return 0 only when o1.equals(o2) returns true. My example doesn't abide by that rule, so there may be a better way to sort this...
Cheers, Neil

[Tom Hughes]

a simple way to implement the equals method would be

  
public boolean equals(Object o) {
  return compare(this, o) == 0;
}

I often do this with Comparators, it is not the most efficient but it is good enough 99% of cases.

[Ilja Preuss]

Originally posted by Neil Laurance:
I don't think you can directly use sort[...]

You can - there is also an Array.sort method that gets a Comparator.

[Andrew Lit]

hi,
i would be appreciated if someone explained me what this line means and what it does?
also how it is concerned with the array, which you want to sort?

    Collections.sort(...);

thanx

[Neil Laurance]

Originally posted by Ilja Preuss:
You can - there is also an Array.sort method that gets a Comparator.

In that case, you can use:

import java.util.*;
public class Sorting2D {
  public static void main(String[] args) {
    String[][] strings = {
      { "c third line","2","3" },
      { "a first line", "6","3" },
      { "b second line","4","1" },
      { "d fourth line","1","2" }
    };
    Arrays.sort(strings, new Comparator() {
      public int compare(Object o1, Object o2) {
        String[] a = (String[])o1;
        String[] b = (String[])o2;
        if(a.length == 0 && b.length == 0)
          return 0;
        if(a.length == 0 && b.length != 0)
          return 1;
        if(a.length != 0 && b.length == 0)
          return -1;
        return a[0].compareTo(b[0]);
      }
      public boolean equals(Object o) {
        return this == o;
      }
    });
    for(int i = 0; i < strings.length ; i++) {
      for(int j = 0; j < strings[i].length; j++)
        System.out.print(strings[i][j] + ", " );
      System.out.println();
    }
  }
}

Cheers, Neil