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Thread: Problem with inheritence? arguments not applicable

  1. #1
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    Default Problem with inheritence? arguments not applicable

    Hey there!

    I'm working on a project where I have the following classes and interfaces:

    public interface I_SolutionCarrier
    public class Agent
    public class Actor extends Agent implements I_SolutionCarrier
    public class ProblemRastrigin implements I_Problem

    ProblemRastrigin contains a method that uses a Solution to calculate a value.
    It can get this solution by calling the getSolution() method which is defined in I_SolutionCarrier and implemented by Actor.


    Now I'd like to have a sort function in the class ProblemRastrigin that uses the getSolution() method to sort a LinkedList of actors. In the future however, different types of agents will carry a solution and thus the getSolution() method will be implemented in the Agent class.

    Therefore I'd like the sort method in problemRastrigin to be Sort(LinkedList<I_SolutionCarrier>) instead of sort(LinkedList<Actor>). In principle this works fine, but for this to work I have to cast all elements of a LinkedList<Actor> to get a LinkedList<I_SolutionCarrier> before I can sort.

    Someone told me it was possible to write a method sort(LinkedList<? extends I_SolutionCarrier> list)

    But when I do this, I get errors in the swap method of the sort algorithm (quicksort)
    private  void swap(LinkedList<? extends I_SolutionCarrier> list, int i, int j) {
    		   I_SolutionCarrier tmp = list.get(i);
    		   list.set(i, list.get(j));
    		   list.set(j, tmp); 
    	   }

    Both calls to list.set give the following erros:
    The method set(int, capture#17-of ? extends I_SolutionCarrier) in the type LinkedList<capture#17-of ? extends I_SolutionCarrier> is not applicable for the arguments (int, capture#18-of ? extends I_SolutionCarrier)
    Any ideas on how to get this working?

    Thanks in advance!
    Last edited by rbk; March 29th, 2011 at 07:11 AM.


  2. #2
    Administrator copeg's Avatar
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    Default Re: Problem with inheritence? arguments not applicable

    See Wildcards (The Java™ Tutorials > Learning the Java Language > Generics)
    In the case you posted since you do not need access to the objects themselves (just the list), don't parametize the list
            @SuppressWarnings("unchecked")
    	private void swap(List list, int a , int b ){
    		Object s = list.get(a);
    		list.set(a, list.get(b));
    		list.set(b, s);
    	}

  3. The Following User Says Thank You to copeg For This Useful Post:

    rbk (March 29th, 2011)

  4. #3
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    Thumbs up Re: Problem with inheritence? arguments not applicable

    Quote Originally Posted by copeg View Post
    See Wildcards (The Java™ Tutorials > Learning the Java Language > Generics)
    In the case you posted since you do not need access to the objects themselves (just the list), don't parametize the list
            @SuppressWarnings("unchecked")
    	private void swap(List list, int a , int b ){
    		Object s = list.get(a);
    		list.set(a, list.get(b));
    		list.set(b, s);
    	}
    I was new to the wildcard thing. Using the wildcard at all the sort methods except the swap method (where I use the generic type) did the trick.

    Thanks a lot!

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