CODEWITHSUNDEEP
javaarrayssorting
biggdman
biggdman

Reputation: 2106

Sort array in java, by class field

Could someone please help me out with the following problem in java. I have a simple class defined as below:

public class Expr {
  public long total_apparitions;
  public String expression=new String();

  public Expr(long total,String expr){
    this.total_apparitions=total;
    this.expression=expr;
  }

  public void increment(long aparitions){
    total_apparitions+=aparitions;
  }
}

I want to sort an array of Expr objects by the total_apparitions field, using the Arrays.sort built-in function. How do I specify to the Arrays.sort function the comparison factor? Thanks a lot.

Upvotes: 0

Views: 176

Answers (3)

Jason Braucht
Jason Braucht

Reputation: 2368

Make Expr implement java.lang.Comparable

Edit - Should have provided an example (others already did). Here's a full sample using generics.

public class Expr implements Comparable<Expr>
{

    public long total_apparitions;
    public String expression = new String();

    public Expr(long total, String expr)
    {
        this.total_apparitions = total;
        this.expression = expr;

    }

    public void increment(long aparitions)
    {
        total_apparitions += aparitions;
    }

    public int compareTo(Expr o)
    {
        if (total_apparitions > o.total_apparitions)
        {
            return 1;
        }
        else if (total_apparitions < o.total_apparitions)
        {
            return -1;
        }
        else
        {
            return 0;
        }
    }
}

Upvotes: 4

Simon Nickerson
Simon Nickerson

Reputation: 43169

As an alternative to implementing Comparable, you can pass a Comparator instance to the Arrays.sort() method. The advantage of doing it this way is that it allows you to have different concepts of sorting an array of objects of this type (say you might want to sort by the name later, in which case you just need a different implementation of the comparator).

For example:

public class ByApparationsComparator implements Comparator<Expr> {
  public int compare(Expr first, Expr second) {
    if (first.total_apparitions > second.total_apparitions) {
      return 1;
    } else if (first.total_apparitions < second.total_apparitions) {
      return -1;
    } else {
      return 0;
    }
  }
}

Then you can say:

Arrays.sort(exprArray, new ByApparationsComparator());

Upvotes: 3

Jon Egeland
Jon Egeland

Reputation: 12613

As @Jason Braucht said, implement Comparable like this:

public class Expr implements Comparable {
  ...
  public int compareTo(Object o) {
    if(this.total_apparitions > ((Expr) o).total_apparitions)
      return 1;
    else if(this.total_apparitions < ((Expr) o).total_apparitions)
      return -1;
    return 0;
  }
}

Upvotes: 5

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