Vectors and Collections and Hashtables - which to choose

[Rachel Swailes]

I have this class

public class Contacts{
String name, email;
int referenceID

etc...
}

These contacts get stored to a file and then read back into a Vector when I want to use them as I want to display them in a JList.

Because I want the JList to display the Contacts alphabetically by name, I would like the Vector to sort alphabetically by name field. Is this possible without writing a full sort method myself? Or am I going to have to use another type of Collection.

Any thoughts or suggestions would be great!
Thanks,
Rachel

[Ernest Friedman-Hill]

Vector implements the java.util.List interface, and there are a couple of static sort() methods in java.util.Collections which will sort a List -- so you can just use those.

[Rachel Swailes]

Thanks.

I've tried sorting a Vector when it just has the Contact's name in it.
example
myVec.elementAt(0) would be Rachel
Sorting that kind of Vector is fine.

When I try to sort a vector with the actual Contact in it
example
myVec.elementAt(0) would the the Contact Object with the name Rachel
Applying Collections.sort(myVec) gives me ...


Exception in thread "main" java.lang.ClassCastException: satstar.data.AddressCon
tactPerson
at java.util.Arrays.mergeSort(Arrays.java:1124)
at java.util.Arrays.sort(Arrays.java:1074)
at java.util.Collections.sort(Collections.java:110)
at satstar.data.AddressContactHandler.getAllContactsAddressContactHandl
er.java:109)
at satstar.data.AddressContactHandler.main(AddressContactHandler.java:15
7)


Why does this happen?

[Ernest Friedman-Hill]

If you look at the Javadoc for the one-argument sort method, you see

All elements in the list must implement the Comparable interface.

This interface has a method compareTo() in it. String implements this interface already, but your Contact class doesn't. All you need to do is to declare that the class implements this interface, and then forward the implementation to the name member variable (given that you want to sort on names

public class Contacts implements Comparable {
  ...
  public int compareTo(Object o) {    
    return name.compareTo(((Contact) o).name);
  }
}

This method will throw ClassCastException if your Vector contains anything other than Contacts objects, and if name is ever null, it can throw NullPointerException, so you may want to improve it a little, but this gives you the idea.

[Rachel Swailes]

Thanks! I'll give that a try. I've never used the Comparable interface - something new is always fun!

Thanks again.
Rachel

[Chengwei Lee]

You could also use the comparator, its quite easy too.