JUnit test case casting wrong elements in generic methods

Question

I have 2 methods.

 public static <T extends Comparable<T>> T[] sort(T[] array) {}

And

 public static <T extends Comparable<T>> T[] sort(T[] array,
            Comparator<? super T> c) {}

Implemantation w/o any erros, but after JUnit test case i got an error on T[] s =...

JUnit says:

java.lang.object cannot be cast to java.lang.Comparable;

In the test case JUnit gives me also this line next to the error:

assertArrayEquals(a1, ComparisonCountingSort.sort(ar1));

My question here is to know what i can fix here to have an succesful JUnit test case.

Take a look at this post: http://stackoverflow.com/questions/1817524/generic-arrays-in-java. Now, what are you trying to do with the Array s? Is that code in your productive code or in the Unit test? — Martin, Nov 07 '13 at 10:29
No,its the productive code. I post the Unit test case in start post.Thanks — James Rich, Nov 07 '13 at 10:36
@JamesRich you should accept one of the answers as correct if they worked for you. — Ruslans Uralovs, Nov 12 '13 at 11:40

Alan Spencer · Answer 1 · 2013-11-07T13:02:05.813

3

If you want an array of nulls of the correct type and length

T[] s = Arrays.copyOf(Arrays.copyOf(array, 0), array.length);

If you are ok with an exact copy, you can use

T[] s = Arrays.copyOf(array,  array.length);

edited Nov 07 '13 at 13:02

answered Nov 07 '13 at 11:02

Alan Spencer

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I tested the second one. It works fine. Can you explain why it works in the methods? – James Rich Nov 07 '13 at 11:39
Under the hood, this method also uses 'System.arraycopy'. The trick is how it creates an array with a generic type... This is done by finding the _Component Type_ of the incoming array 'array.getClass().getComponentType()' and dynamically creating a new array of the same type 'Array.newInstance(componentType)' - I find it quite useful to dive into the JDK source once in a while to understand what is really going on, and maybe using a JUnit test and/or debugger to see the details... – Alan Spencer Nov 07 '13 at 12:36
To clarify... This approach actually creates an array of the correct type, and in the calling code an attempt to cast the array to the non-generic type will work. If you 'new Comparable(...' This is actually a _Comparable_ array - you can add _any_ Comparable to it, so, you can't cast it to a specific type as it may contain something that is not that type... – Alan Spencer Nov 07 '13 at 13:00

score 1 · Answer 2 · answered Nov 07 '13 at 10:39

1

Since T is a type which extends Comparable it should be

@SuppressWarnings("unchecked")
T[] s = (T[]) new Comparable[array.length];

Reason being an Object type cannot be cast to Comparable since Object IS NOT A Comparable

answered Nov 07 '13 at 10:39

sanbhat

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I changed it. Passed the test succesfully. 0,13 s I ask only if i have to change it on both methods. – James Rich Nov 07 '13 at 10:43

score 0 · Answer 3 · answered Nov 07 '13 at 10:18

0

Collections of object cannot be casted to collection of T. What is more you cannot do this: T x = new T(). You can provide dependency to class that is able to create object of type T. To copy your array you can use System.arraycopy

answered Nov 07 '13 at 10:18

Areo

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Thanks for the answers. I dont get the last part of your post. Maybe i post the whole implementation,maybe it is easier for me to understand. – James Rich Nov 07 '13 at 10:31

JUnit test case casting wrong elements in generic methods

3 Answers3