List casting interface is failing in Java

Question

I have the following interfaces:

public interface CustomWebElement extends WebElement
. . . methods

In the following places, when I try to cast WebElement to CustomWebElement things are fine:

CustomWebElement a = (CustomWebElement) element.findElement(by); //findElement return WebElement

but the calls to findElements method which returns List<WebElement> casting is failing:

List<CustomWebElement> a = (List<CustomWebElement>) element.findElements(by);

giving me exception:

Inconvertable types; Cannot cast List<WebElement> to List<CustomWebElement>

why List cast is failing in this case?

score 3 · Answer 1 · answered Oct 06 '14 at 06:32

3

Please take a look at http://docs.oracle.com/javase/tutorial/java/generics/

CustomWebElement extends WebElement does NOT mean that List<CustomWebElement> extends List<WebElement>

answered Oct 06 '14 at 06:32

Atul

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score 1 · Accepted Answer · answered Oct 06 '14 at 06:34

1

A List<WebElement> is an object type in its own right as is List<CustomWebElement>, and List<CustomWebElement> does not extend List<WebElement>

answered Oct 06 '14 at 06:34

Scary Wombat

44,617
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score 0 · Answer 3 · answered Oct 06 '14 at 06:40

It is because List<WebElement> is neither a subtype nor supertype of List<CustomWebElement>. For more information, you can read Item25 from Effective Java of Joshua Bloch. I am copying and pasting from his book

Arrays differ from generic types in two important ways. First, arrays are covariant. This scary-sounding word means simply that if Sub is a subtype of Super, then the array type Sub[] is a subtype of Super[]. Generics, by contrast, are invariant: for any two distinct types Type1 and Type2, List is neither a subtype nor a supertype of List

score 0 · Answer 4 · answered Oct 06 '14 at 06:43

0

Use of wildcards while using generics may help Like

List<? extends WebElement>

answered Oct 06 '14 at 06:43

Dhan

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List casting interface is failing in Java

4 Answers4