Java, sorting ArrayList using comparator

Question

I want to sort my list of Task object by two criterion. First it is sorted by leftEntryTime and then by timeNeededToBeProcessed.

Can someone help me?

So far I did sorting by leftEntryTime but how I can sort it by timeNeededToBeProcessed in the same Comparator class?

 Task(int procesID, int neededTime, int waitingTime, int leftEntryTime) {
    this.procesID = procesID;
    this.timeNeededToBeProcessed = neededTime;
    this.waitingTime = waitingTime;
    this.leftEntryTime = leftEntryTime;
 }

This is my Comparator:

import java.util.Comparator;

public class TimeEntryComparator implements Comparator<Task> {

    @Override
    public int compare(Task o1, Task o2) {
        int entryTime;
        int taskTime = o1.getLeftEntryTime()- o2.getLeftEntryTime();

        if (taskTime > 0){
            return 1;
        }

        if (taskTime < 0) {
            return -1;
        }
        return 0;
    }
}

then I sort it by:

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

add one more if? what is the issue? are you getting unexpected results? please share them — Kalpesh Soni
– Kalpesh Soni, Commented Oct 23, 2014 at 14:57
let me shorten your code: public int compare(Task o1, Task o2) { return (o1.getLeftEntryTime() - o2.getLeftEntryTime()); } — msrd0
– msrd0, Commented Oct 23, 2014 at 14:58
What is the unit of time stored in leftEntryTime and timeNeededToBeProcessed ? — ortis
– ortis, Commented Oct 23, 2014 at 15:00

Giovanni · Accepted Answer · 2014-10-23 14:58:37Z

2

something like:

public class TimeEntryComparator implements Comparator<Task> {
    @Override
    public int compare(Task o1, Task o2) {
        if (o1.getLeftEntryTime() == o2.getLeftEntryTime())
            return o1.getTimeNeededToBeProcessed() - o2.getTimeNeededToBeProcessed();
        return o1.getLeftEntryTime() - o2.getLeftEntryTime();
    }
}

answered Oct 23, 2014 at 14:58

Giovanni

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Comments

siki · Accepted Answer · 2014-10-23 15:11:30Z

2

Using some new Java 8 functionality, it's actually pretty easy and elegant:

Comparator<Task> byLeftEntryTime = (t1, t2) -> Integer.compare(t1.getLeftEntryTime(), t2.getLeftEntryTime());
Comparator<Task> byTimeNeededToBeProcessed = (t1, t2) -> Integer.compare(t1.getTimeNeededToBeProcessed(), t2.getTimeNeededToBeProcessed());

list.sort(byLeftEntryTime.thenComparing(byTimeNeededToBeProcessed));

answered Oct 23, 2014 at 15:11

siki

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Konstantin Yovkov · Accepted Answer · 2014-10-23 14:58:08Z

1

I believe you have to do:

@Override
public int compare(Task o1, Task o2) {
    if (o1.getLeftEntryTime() == o2.getLeftEntryTime()) {
        return o1.getTimeNeededToBeProcessed - o2.getTimeNeededToBeProcessed();
    } else {
        return o1.getLeftEntryTime() - o2.getLeftEntryTime();
    }
}

This will compare the Task objects by LeftEntryTime and if the leftEntryTime properties are equal, it will compare their timeNeededToBeProcessed properties.

answered Oct 23, 2014 at 14:58

Konstantin Yovkov

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Comments

BarrySW19 · Accepted Answer · 2014-10-23 15:00:21Z

0

When the comparison of the leftEntryTime values gives 0 (i.e. they are equal), then continue to compare by timeNeededToBeProcessed, e.g.:

@Override
public int compare(Task o1, Task o2) {
    int compare = o1.getLeftEntryTime()- o2.getLeftEntryTime();
    if(compare != 0) {
        return compare;
    }

    return o1.getTimeNeededToBeProcessed() - o2.getTimeNeededToBeProcessed();
}

answered Oct 23, 2014 at 15:00

BarrySW19

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John Bollinger · Accepted Answer · 2014-10-23 15:00:33Z

I think you're saying that you want to sort your Task objects by a compound key consisting of their leftEntryTime and timeNeededToBeProcessed fields. A Comparator that represents such an ordering would look like this:

import java.util.Comparator;

public class TimeEntryComparator implements Comparator<Task> {

    @Override
    public int compare(Task o1, Task o2) {
        int entryTime;
        int taskTime = o1.getLeftEntryTime()- o2.getLeftEntryTime();

        if (taskTime > 0){
            return 1;
        } else if (taskTime < 0) {
            return -1;
        } else {
            int neededTime = o1.getTimeNeededToBeProcessed()
                    - o2.getTimeNeededToBeProcessed();

            if (neededTime > 0) {
                return 1;
            } else if (neededTime < 0) {
                return -1;
            } else {
                return 0;
            }
        }
    }
}

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