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Thread: covariant return type

  1. #1
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    Default covariant return type

    public class MyMainClass {
    public static void main (String[] args) {
    MySuperClass msup=new MySuperClass();
    MySubClass msub=new MySubClass();
    //msup.a=10;
    msub.a=5;
    //System.out.println(msup.a +" "+ msub.a);
    WithMethod wm=new WithMethod();
    MySuperClass msc=new MySuperClass();
    msc=wm.theMethod(msub);
    System.out.println("Returned value : " + msc.a);
    }
    }

    class MySuperClass {
    int a;
    }

    class MySubClass {
    int a;
    }

    class WithMethod {
    public MySuperClass theMethod(MySubClass mscl) {
    mscl.a+=9;
    return mscl;
    }
    }


    Fails with the error :

    C:\Documents and Settings\Administrator\Desktop\Java Programs>javac MyMainClass.
    java
    MyMainClass.java:26: incompatible types
    found : MySubClass
    required: MySuperClass
    return mscl;
    ^
    1 error

    why is covariant return not working ???


  2. #2
    Crazy Cat Lady KevinWorkman's Avatar
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    Default Re: covariant return type

    I've moved this thread to a more appropriate forum. For more information, read this: The Member Introductions Forum is for saying hi, NOT for technical questions!

    Also, when posting code, please remember to use the highlight tags.
    Useful links: How to Ask Questions the Smart Way | Use Code Tags | Java Tutorials
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  3. #3
    Super Moderator Norm's Avatar
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    Default Re: covariant return type

    incompatible types
    found : MySubClass
    required: MySuperClass
    The compiler's error message explains the problem.
    Do you understand about data types? The compiler will NOT allow you to use a variable of the wrong type.
    What type does the method's definition say that the method returns?
    What type is the mscl variable?
    Are they the same or compatible? The compiler does not think they are.

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