Question

I am trying to write public instance method createParther() that returns an instance of Couple. The method does its job by following these rules: one dancer in the couple must be from aList and the other bList, one dancer in the couple must be male and the other female. Neither dancer in the couple should be partnered already. If it is not possible to create a couple from amongst the unpartnered dancers, then null should be returned. If an instance of Couple is successfully created, both dancers involved in the couple should have their partnered instance variables set to true. I have attempted to list all the aList and bList to together, but then I didn’t know to check the requirements as per above. Can anyone demostrate how this could be acheieved? This is not assignment.

public class FoxDancing
{
 private List<Couple> coupleList;
 private List<Dancer> aList;
 private List<Dancer> bList;

public FoxDancing()
{
   couplesList = new ArrayList<Parthers>();
   aList = new ArrayList<Dancer>();
   bList = new ArrayList<Dancer>();
}

public void fieldLists()
{
   this.addX("Simon","AList",'m');
   this.addX("Jason","AList",'m');
   this.addX("Ian","AList",'m');
   this.addX("Susan","BList",'f');
   this.addX("Helena","BList",'f');
   this.addX("Gina","BList",'f');
}
} 

Answer 1

It looks like you’d want to have a helper method like this:

Dancer findUnpartnered(List<Dancer> list) {
   for (Dancer d : list) {
      if (d.isUnpartnered()) {
          return d;
      }
   }
   return null;
}

This uses a “foreach loop” for a concise, readable iteration over all Dancer in the List<Dancer>.

Then you can write something like this:

Couple createCouple() {
   Dancer a = findUnpartnered(aList);
   Dancer b = findUnpartnered(bList);
   if (a == null || b == null) {
      return null;
   } else {
      a.setPartnered(true);
      b.setPartnered(true);
      return new Couple(a, b);
   }
}

While this should work, note that findUnpartnered is a O(N) linear search. If the list is of any considerable length, consider having alternative data structures, e.g. a Set<Dancer> that partitions the list into unpartnered and partnered subsets.

---

On generics invariance

You’ve written the following:

// snippet from original code
private List<Couple> coupleList;
//...
couplesList = new ArrayList<Parthers>(); // DOES NOT COMPILE!

This will not compile. A List<Partner> (assuming this is what you meant to write) is NOT a List<Couple>. Perhaps you want a new ArrayList<Couple>, or if Partner is a subtype of Couple, then perhaps you want a List<? extends Couple> coupleList.

Related questions

• What is the difference between <E extends Number> and <Number>?

See also

• Java Tutorials/Generics and Subtyping and More Fun With Wildcards
• Angelika Langer’s Java Generics FAQ – What is a bounded wildcard?

---

On enum

You’ve written the following:

// snippet from original code
this.addX("Simon","AList",'m');
this.addX("Susan","BList",'f');
// potentially "bad" use of String and char constants!

I don’t know much about Foxtrot dancing, but if there’s a conceptual A-list and B-list , then you may consider using an enum instead of String markers "AList" and "BList". Similarly, if there can only be male or female dancers, then enum would be much better than 'm' and 'f'.

enum FoxTrotter { A, B; }
enum Sex { MALE, FEMALE; }

See also

• Java Language Guide/Enums

Related questions

• Enumerations: Why? When?