Generics/Comparator

Discussion in 'Java' started by Kaiser S., Apr 24, 2007.

  1. Kaiser S.

    Kaiser S. Guest

    Hello,

    here is a short class source with begining of generics. Could you please
    tell me how to use generics with the comparators ?

    import java.util.Comparator;
    import java.util.TreeSet;


    public class TestComparator {

    private static abstract class A {
    }

    private static class B extends A {
    }

    private static class C extends A {
    }

    private static abstract class HandlerOfA {
    public abstract Comparator getComparator();
    }

    private static class HandlerOfB extends HandlerOfA {
    public static class ComparatorOfB implements Comparator<B> {
    public int compare(B b1, B b2) {
    return 0;
    }

    }
    public Comparator getComparator() {
    return new ComparatorOfB();
    }
    }

    private static class HandlerOfC extends HandlerOfA {
    public static class ComparatorOfC implements Comparator<C> {
    public int compare(C c1, C c2) {
    return 0;
    }

    }
    public Comparator getComparator() {
    return new ComparatorOfC();
    }
    }


    public static void main(String[] args) {
    HandlerOfA contA = new HandlerOfB();
    HandlerOfB contB = new HandlerOfB();
    HandlerOfC contC = new HandlerOfC();

    TreeSet<A> treesetA = new TreeSet<A>(contA.getComparator());
    treesetA.add(new B());

    TreeSet<B> treesetB = new TreeSet<B>(contB.getComparator());
    treesetB.add(new B());

    TreeSet<C> treesetC = new TreeSet<C>(contC.getComparator());
    treesetC.add(new C());

    }
    }


    Thanks for any help
     
    Kaiser S., Apr 24, 2007
    #1
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  2. Kaiser S.

    Kaiser S. Guest

    If it's not possible to put generics in comparators, could you tell me
    what is wrong in the design of the classes? What should i change in
    order to have the same fonctionnalities, but with a different design ?
     
    Kaiser S., Apr 25, 2007
    #2
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  3. -----BEGIN PGP SIGNED MESSAGE-----
    Hash: SHA1

    Kaiser S. schreef:
    > If it's not possible to put generics in comparators, could you tell me
    > what is wrong in the design of the classes? What should i change in
    > order to have the same fonctionnalities, but with a different design ?


    Please quote some content if you reply to a message. I now had to look
    for you previous message in order to try to help.

    I pasted it in below.

    There is no problem using generics in comparators, but your design is,
    indeed, flawed. First of all, the semantics is unclear: why are
    HandlerOfB and HandlerOfC extending HandlerOfA?

    That is also the source of the problem: HandlerOfA defines
    getComparator(), which returns a Comparator. Here already you should
    get a compiler warning not to use raw types. But in respect to what
    comes later, it is difficult to generify this. What type of Comparator
    would you return? Comparator<A> won’t work, since then the methods in
    HandlerOfB and HandlerOfC won’t override properly. A possibility is to
    make the method itself generic:

    public abstract <T extends A> Comparator<T> getComparator();

    But that would make it more complicated than it’s worth. From the
    snippet you show in the main method, I see no reason why you would need
    the handlers inherit from each other. So maybe if you explained what
    you really wanted, someone could help you better.

    Read also the last link in my sig, particularly the section ‘describe
    the goal, not the step’.

    > Hello,
    >
    > here is a short class source with begining of generics. Could you please tell me how to use generics with the comparators ?
    >
    > import java.util.Comparator;
    > import java.util.TreeSet;
    >
    >
    > public class TestComparator {
    >
    > private static abstract class A {
    > }
    >
    > private static class B extends A {
    > }
    >
    > private static class C extends A {
    > }
    >
    > private static abstract class HandlerOfA {
    > public abstract Comparator getComparator();
    > }
    >
    > private static class HandlerOfB extends HandlerOfA {
    > public static class ComparatorOfB implements Comparator<B> {
    > public int compare(B b1, B b2) {
    > return 0;
    > }
    >
    > }
    > public Comparator getComparator() {
    > return new ComparatorOfB();
    > }
    > }
    >
    > private static class HandlerOfC extends HandlerOfA {
    > public static class ComparatorOfC implements Comparator<C> {
    > public int compare(C c1, C c2) {
    > return 0;
    > }
    >
    > }
    > public Comparator getComparator() {
    > return new ComparatorOfC();
    > }
    > }
    >
    >
    > public static void main(String[] args) {
    > HandlerOfA contA = new HandlerOfB();
    > HandlerOfB contB = new HandlerOfB();
    > HandlerOfC contC = new HandlerOfC();
    >
    > TreeSet<A> treesetA = new TreeSet<A>(contA.getComparator());
    > treesetA.add(new B());
    >
    > TreeSet<B> treesetB = new TreeSet<B>(contB.getComparator());
    > treesetB.add(new B());
    >
    > TreeSet<C> treesetC = new TreeSet<C>(contC.getComparator());
    > treesetC.add(new C());
    >
    > }
    > }


    H.

    - --
    Hendrik Maryns
    http://tcl.sfs.uni-tuebingen.de/~hendrik/
    ==================
    http://aouw.org
    Ask smart questions, get good answers:
    http://www.catb.org/~esr/faqs/smart-questions.html
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    Hendrik Maryns, Apr 25, 2007
    #3
  4. Kaiser S.

    Kaiser S. Guest

    > There is no problem using generics in comparators, but your design is,
    > indeed, flawed. First of all, the semantics is unclear: why are
    > HandlerOfB and HandlerOfC extending HandlerOfA?


    Well the snippet is a simplified version of the real source code. The
    Handlers have 10/15 methods in common and in a lot of places, i don't
    need to know the actual type of handler.

    > That is also the source of the problem: HandlerOfA defines
    > getComparator(), which returns a Comparator. Here already you should
    > get a compiler warning not to use raw types. But in respect to what
    > comes later, it is difficult to generify this. What type of Comparator
    > would you return? Comparator<A> won’t work, since then the methods in
    > HandlerOfB and HandlerOfC won’t override properly. A possibility is to
    > make the method itself generic:
    >
    > public abstract <T extends A> Comparator<T> getComparator();
    >
    > But that would make it more complicated than it’s worth. From the


    Yes, it's not possible to generify the method.

    > snippet you show in the main method, I see no reason why you would need
    > the handlers inherit from each other. So maybe if you explained what
    > you really wanted, someone could help you better.



    >> import java.util.Comparator;
    >> import java.util.TreeSet;
    >>
    >>
    >> public class TestComparator {
    >>
    >> private static abstract class A {
    >> }
    >>
    >> private static class B extends A {
    >> }
    >>
    >> private static class C extends A {
    >> }
    >>
    >> private static abstract class HandlerOfA {
    >> public abstract Comparator getComparator();
    >> }
    >>
    >> private static class HandlerOfB extends HandlerOfA {
    >> public static class ComparatorOfB implements Comparator<B> {
    >> public int compare(B b1, B b2) {
    >> return 0;
    >> }
    >>
    >> }
    >> public Comparator getComparator() {
    >> return new ComparatorOfB();
    >> }
    >> }
    >>
    >> private static class HandlerOfC extends HandlerOfA {
    >> public static class ComparatorOfC implements Comparator<C> {
    >> public int compare(C c1, C c2) {
    >> return 0;
    >> }
    >>
    >> }
    >> public Comparator getComparator() {
    >> return new ComparatorOfC();
    >> }
    >> }
    >>
    >>
    >> public static void main(String[] args) {
    >> HandlerOfA contA = new HandlerOfB();
    >> HandlerOfB contB = new HandlerOfB();
    >> HandlerOfC contC = new HandlerOfC();
    >>
    >> TreeSet<A> treesetA = new TreeSet<A>(contA.getComparator());
    >> treesetA.add(new B());
    >>
    >> TreeSet<B> treesetB = new TreeSet<B>(contB.getComparator());
    >> treesetB.add(new B());
    >>
    >> TreeSet<C> treesetC = new TreeSet<C>(contC.getComparator());
    >> treesetC.add(new C());
    >>
    >> }
    >> }
     
    Kaiser S., Apr 25, 2007
    #4
  5. Kaiser S.

    Esmond Pitt Guest

    You need a few adjustments:

    private static abstract class HandlerOfA
    {
    public abstract Comparator<? extends A> getComparator();
    }

    private static class HandlerOfB extends HandlerOfA
    {
    public static class ComparatorOfB implements Comparator<B>
    {
    public int compare(B b1, B b2)
    {
    return 0;
    }
    }
    public Comparator<B> getComparator()
    {
    return new ComparatorOfB();
    }
    }

    private static class HandlerOfC extends HandlerOfA
    {
    public static class ComparatorOfC implements Comparator<C>
    {
    public int compare(C c1, C c2)
    {
    return 0;
    }

    }
    public Comparator<C> getComparator()
    {
    return new ComparatorOfC();
    }
    }

    and then in your main:

    HandlerOfA contA = new HandlerOfB();

    TreeSet<A> treesetA = new TreeSet<A>(contA.getComparator());

    This ceases to make sense once the generics are introduced.
     
    Esmond Pitt, Apr 26, 2007
    #5
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