Sorting list of objects

Question

I have a list of objects:

List<WorkflowError> workflowErrors = new List<WorkflowError>();

Of which I am wanting to sort alphabetically on the string field errorCode.

I know I have to use

Collections.sort(list,comparator)

and write a custom Comparator:

public class SortByErrorComparator implements Comparator<WorkflowError>
{
    // Comparator logic    
    return 0;
}

I have seen examples of how to do this for a one dimensional list but I can't work out how to begin for a list of objects.

Any help would be appreciated.

Where is the // comparator logic?

adarshr
– adarshr

2012-02-13 12:01:13 +00:00
Commented Feb 13, 2012 at 12:01 — adarshr
– adarshr, Commented Feb 13, 2012 at 12:01

adarshr · Accepted Answer · 2012-02-13 12:09:44Z

9

You need to implement the compare method.

public class SortByErrorComparator implements Comparator<WorkflowError> {
    public int compare(WorkflowError obj1, WorkflowError obj2) {
        return obj1.getErrorCode().compareTo(obj2.getErrorCode());
    }
}

And then, you can simply do:

Collections.sort(list, new SortByErrorComparator()) ;

In the Java world, generally we do it inline using anonymous inner classes as so:

Collections.sort(list, new Comparator<WorkflowError>() {
    public int compare(WorkflowError obj1, WorkflowError obj2) {
        return obj1.getErrorCode().compareTo(obj2.getErrorCode());
    }
});

edited Feb 13, 2012 at 12:09

answered Feb 13, 2012 at 12:03

adarshr

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1 Comment

Fraser Over a year ago

Works perfectly, I didn't know how the compareTo() would behave for strings actually. Thanks a lot.

Sam Felix · Accepted Answer · 2012-02-13 12:00:58Z

0

In your comparator, you have to explain how the specific object should be sorted.

if (obj.att1 < obj2.att2)
   ...

do you get it?

answered Feb 13, 2012 at 12:00

Sam Felix

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Comments

pauljwilliams · Accepted Answer · 2012-02-13 12:02:44Z

0

When sorting the list the sort implementation will use your comparator to compare individual paisr of objects. So what your comparator needs to do is decide, for a given pair of WorkFlowErrors, which comes 'first'.

from what you've said this sounds like just getting the error code from each one and doing a string compairson.

answered Feb 13, 2012 at 12:02

pauljwilliams

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Zack Marrapese · Accepted Answer · 2012-02-13 12:04:02Z

0

If you will always want to compare WorkflowError objects in this manner, the easiest way is to implement Comparable<WorkflowError> on WorkflowError itself.

public int compareTo(WorkflowError that) {
    return this.getErrorCode().compareTo(that.getErrorCode());
}

That's assuming errorCode implements Comparable.

answered Feb 13, 2012 at 12:04

Zack Marrapese

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