Sort array in java, by class field

Question

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.

Jon Egeland · Accepted Answer · 2011-12-09 22:43:01Z

5

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;
  }
}

answered Dec 9, 2011 at 22:43

Jon Egeland

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Comments

Jason Braucht · Accepted Answer · 2011-12-09 22:51:46Z

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;
        }
    }
}

Simon Nickerson · Accepted Answer · 2011-12-09 22:44:12Z

3

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());

answered Dec 9, 2011 at 22:44

Simon Nickerson

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2 Comments

Jon Egeland Over a year ago

but this makes his calling code look crowded. Implementing Comparable makes everything easier to read, and allows him to easily call the sort whenever he needs to.

Simon Nickerson Over a year ago

It really depends if you might want to sort by something else at some later stage. If there's only one natural sort order, then implementing Comparable is the way to go.

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