How to create this java comparator

Question

I have this class:

public class Test {

  private int    priority;
  private String desciption;
...
}

and I have this arraylist:

Priority: 1, Description: C2
Priority: 2, Description: C1
Priority: 3, Description: C1
Priority: 4, Description: C2

I want this result:

Priority: 1, Description: C2
Priority: 4, Description: C2
Priority: 2, Description: C1
Priority: 3, Description: C1

How I have to do this with comparator? Thank you!

EDIT:

I reply to all of your questions:

@retro: no. description is a string without a specific format. it can be empty, too.

@tobias_k: first "group" by description and than order by priority. priority is always not equals.

@user270349:

this is my try:

int result = 0;
if (o1.getPriority() < o2.getPriority()) result = -1;
else if (o1.getPriority() > o2.getPriority()) result = 1

result *= o1.getDescription().compareTo(o2.getDescription());

return result;

EDIT 2:

another input/output example:

and I have this arraylist:

Priority: 1, Description: C2
Priority: 2, Description: C3
Priority: 3, Description: C1
Priority: 4, Description: C2

I want this result:

Priority: 1, Description: C2
Priority: 4, Description: C2
Priority: 2, Description: C3
Priority: 3, Description: C1

THIS IS MY SOLUTION:

            List<Test> testList = new ArrayList<>();
    testList.add(new Test(4, "C2"));
    testList.add(new Test(2, "C3"));
    testList.add(new Test(3, "C1"));
    testList.add(new Test(1, "C2"));


    Comparator<Test> comparator = new Comparator<Test>() {

        @Override
        public int compare(Test o1, Test o2) {
            int res = o1.getDescription().compareTo(o2.getDescription());
            if (res == 0)
                return o1.getPriority() < o2.getPriority() ? -1 : 1;
            else
                return res;
        }
    };

    Collections.sort(testList, comparator);


    List<String> valoriInseriti = new ArrayList<>();
    List<Test> grouping = new ArrayList<>();
    for (Test t : testList) {
        if (!valoriInseriti.contains(t.getDescription())) {
            valoriInseriti.add(t.getDescription());
            grouping.add(t);
        }
    }


    comparator = new Comparator<Test>() {

        @Override
        public int compare(Test o1, Test o2) {
            return o1.getPriority() < o2.getPriority() ? -1 : 1;
        }
    };

    Collections.sort(grouping, comparator);


    Collections.sort(testList, comparator);

    List<Test> output = new ArrayList<>();
    for (Test t1 : grouping) {
        for (Test t2 : testList) {
            if (t2.getDescription().equals(t1.getDescription())) {
                output.add(t2);
            }
        }
    }

    System.out.println("==============================");
    for (Test t : output)
        System.out.println(t);

Solution 1

Test implements Comparator and override compare() method

  public class Test implements Comparator<Test>{
    private int    priority;
    private String desciption;

    @Override
    public int compare(Test o1, Test o2) {
       // your code here
    }

  }

Solution 2

Comparator<Test> myComparator = new Comparator<Test>() {
    public int compare(Test o1, Test o2) {
        int result = o2.desciption.compareTo(o1.desciption);

        if (result == 0) {
            return o1.priority - o2.priority;
        }

        return result;

    }
};

List<Test> sortedList = Collections.sort(testList, myComparator);

Comments

[tobias_k]

Shouldn't it rather implement Comparable?

[Fabio Mignogna]

this example work with example, but if i have this input Priority: 1, Description: C2 Priority: 2, Description: C3 Priority: 3, Description: C1 Priority: 4, Description: C2 the output should be Priority: 1, Description: C2 Priority: 4, Description: C2 Priority: 2, Description: C3 and your example doesn't work. thank you anyway!

[Fabio Mignogna]

this example work with example, but if i have this input Priority: 1, Description: C2 Priority: 2, Description: C3 Priority: 3, Description: C1 Priority: 4, Description: C2 the output should be Priority: 1, Description: C2 Priority: 4, Description: C2 Priority: 2, Description: C3 and your example doesn't work. thank you anyway!

[Prabhakaran Ramaswamy]

@FabioMignogna Please update your question with your input and expected answer.

[vickirk]

Shouldn't it rather implement Only if this comparison is the logical way to compare then, quite/most often the required order of comparison is required it in a different component and there may well be different sorting requirements.

[tobias_k]

Agreed that Comparable should only be used for the one natural comparison (if any), and any "funny" comparisons should be done with Comparator. But then IMHO the Comparator should be a separate class, and not the class of the object to be compared.

[christopher]

I think, in terms of good design, the Comparator should be a completely different class rather than an anonymous object, but I do think that for the context of the question, this answer would suffice.