Java ArrayList Sort on the basis of dynamic objects fields?

Question

I am getting a issue i have below code

public void columnsList(List<TableRecord> records){

    for(TableRecord record : records){
        Table table = record.getTable();
        //Do sort here on stampDate
        Field[] fields = table.fields();
        for(Field field : fields){
            field.getName();
            record.getValue(field);

        }
    }

}

and records object contain Object of different class type

List<TableRecord> records = new List<TableRecord>();
records.add(new AddressRecord());
records.add(new CityRecord());
records.add(new UserRecord());

Now how i need to sort them by stampDate variable which is in each class how can we do it when we have different classes in list

aUserHimself · Accepted Answer · 2013-09-24 07:12:25Z

2

If your code above is correct, that means AddressRecord, CityRecord and UserRecord all extend TableRecord:

class AddressRecord extends TableRecord {
    // other fields and methods here
}
class CityRecord extends TableRecord {
    // other fields and methods here
}
class UserRecord extends TableRecord {
    // other fields and methods here
}

You only have to write a Comparator for this class. It should look something like this:

class TableRecord {
    private Date timeStamp;

    public Date getTimeStamp() {
        return timeStamp;
    }
// other fields and methods here
}

class RecordStampDateComparator implements Comparator<TableRecord>{

    public int compare(TableRecord tr1, TableRecord tr2) {
        Date tr1Date = tr1.getTimeStamp();
        Date tr2Date = tr2.getTimeStamp();   
        return tr1Date.compareTo(tr2Date);
    }
}

edited Sep 24, 2013 at 7:12

answered Sep 24, 2013 at 6:54

aUserHimself

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Comments

agad · Accepted Answer · 2013-09-24 09:00:04Z

1

If you can't change the classes write your comparator( Comparator<Object>), that will try to find the field stampDate and compare them. Than use it for sorting the list. Comparator implementation:

import java.util.Comparator;
import java.util.Date;


public class StampDateComparator implements Comparator<Object> {

@Override
public int compare(Object o1, Object o2) {

    try {
        Date d1 = (Date) o1.getClass().getDeclaredField("stampDate").get(o1);
        Date d2 = (Date) o2.getClass().getDeclaredField("stampDate").get(o2);
        return compare(d1, d2);
    } catch (SecurityException e) {
        throw new RuntimeException(e);
    } catch (NoSuchFieldException e) {
        throw new RuntimeException("Missing variable stampDate");
    }catch (ClassCastException e) {
        throw new RuntimeException("stampDate is not a Date");
    } catch (IllegalArgumentException e) {
        //shoud not happen
        throw new RuntimeException(e);
    } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
    }
}

}

edited Sep 24, 2013 at 9:00

answered Sep 24, 2013 at 6:54

agad

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5 Comments

agad Over a year ago

@Hariom But you can mark it as correct answer ;-). Thanks anyway :-)

Hariom Over a year ago

Yes i will one more question I have to sort on the basis of date will this code do that

agad Over a year ago

Collections.sort(records, new StampDateComparator); does it. Of course assuming the stampDate is java.util.Date.

Hariom Over a year ago

What is records here its a List?

agad Over a year ago

Yes, please look at java.util.Collections

Viktor Ozerov · Accepted Answer · 2013-09-24 06:56:40Z

1

Just write abstract class Record with protected field stampDate, that implements Comparable and override compareTo method.

public abstract class Record implements Comparable<Record> {
    protected Date stampDate;

    @Override
    public int compareTo(Record anotherRecord){
        return this.stampDate.compareTo(anotherRecord.stampDate);
    }
}

Then extend this class with your record classes:

public class AddressRecord extends Record{
...
}

public class CityRecord extends Record{
...
}

public class UserRecord extends Record{
...
}

edited Sep 24, 2013 at 6:56

answered Sep 24, 2013 at 6:50

Viktor Ozerov

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4 Comments

aUserHimself Over a year ago

Not quite right.Comparator is more suitable in this case, since Comparable imposes a natural ordering.

Viktor Ozerov Over a year ago

@aUserHimself, maybe that's not quite right, but this example is still usable. Let the TS choose =)

aUserHimself Over a year ago

It doesn't really matter that TableRecord cannot be modified, you can use Comparator. The main problem is whether you can extend it to define AddressRecord, CityRecord and UserRecord or not.

Viktor Ozerov Over a year ago

@Hariom, so? You can import this class. You also can do that: public class UserRecord extends com.example.Record

Masudul · Accepted Answer · 2013-09-24 06:55:40Z

0

Use following Comparator class on List.

class TableRecordCompare implements Comparator<TableRecord>{
     if(TableRecord instanceof AddressRecord){
       // return compareTo for sample data of address.
     }
     else if(TableRecord instanceof CityRecord){
     // return compareTo for sample data of CityRecord.
      }
     else{
     // return compareTo for sample data of UserRecord.
    }

}

answered Sep 24, 2013 at 6:55

Masudul

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1 Comment

Hariom Over a year ago

'else if(TableRecord inteseof CityRecord)' is not possible here can be 100 of class i have to compare

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