Question

I wondered how I could change ordering-direction only on some contitional. In my case Strings starting with ‘BB’ should be ordered in the other direction, everything else should be ordered as usual.

My Test-Class:

public class StringTest {


    public static void main(String[] args) {
        SomeClass someClass1= new SomeClass("AA");
        SomeClass someClass2= new SomeClass("AB");
        SomeClass someClass3= new SomeClass("CB4");
        SomeClass someClass4= new SomeClass("BB7");
        SomeClass someClass5= new SomeClass("BB9");
        SomeClass someClass6= new SomeClass("BB3");
        SomeClass someClass7= new SomeClass("CB3");

        List<SomeClass> list = new ArrayList<SomeClass>();
        list.add(someClass1);
        list.add(someClass2);
        list.add(someClass3);
        list.add(someClass4);
        list.add(someClass5);
        list.add(someClass6);
        list.add(someClass7);

        Collections.sort(list);

        for (SomeClass someClass : list) {
            System.out.println(someClass.getSomeField());
        }
    }

}

My Comparator:

public class SomeClass implements Comparable<SomeClass>
{
  private String someField;

  public int compareTo(final SomeClass o)
  {
    int res = 0;

    if (someField.startsWith("BB"))
    {
      res = o.someField.compareTo(someField);
    }
    else
    {
      res = someField.compareTo(o.someField);
    }
    return res;
  }

}

My desired output:
AA
AB
BB9
BB7
BB3
CB3
CB4

The actual result so far:
AA
AB
CB3
BB9
BB7
BB3
CB4

Jonny

Answer 1

You need to make sure your Comparator applies the different sorting only when both elements start with “BB”. Right now your Comparator applies the different sorting even if you compare “BB9” with “CB3” and therefore the latter is being sorted in front of BB9.

public class SomeClass implements Comparable<SomeClass>
{
   private String someField;

   public int compareTo(final SomeClass o)
   {
     int res = 0;

     if (someField.startsWith("BB") && o.someField.startsWith("BB"))
     {
       res = o.someField.compareTo(someField);
     }
     else
     {
       res = someField.compareTo(o.someField);
     }
     return res;
   }
}