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• Is `List<Dog>` a subclass of `List<Animal>`? Why aren't Java's generics implicitly polymorphic?

  - by froadie

  I'm a bit confused about how Java generics handle inheritance / polymorphism. Assume the following hierarchy - Animal (Parent) Dog - Cat (Children) So suppose I have a method doSomething(List<Animal> animals). By all the rules of inheritance and polymorphism, I would assume that a List<Dog> is a List<Animal> and a List<Cat> is a List<Animal> - and so either one could be passed to this method. Not so. If I want to achieve this behavior, I have to explicitly tell the method to accept a list of any subset of Animal by saying doSomething(List<? extends Animal> animals). I understand that this is Java's behavior. My question is why? Why is polymorphism generally implicit, but when it comes to generics it must be specified?

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• How to get the first non-null value in Java?

  - by froadie

  Is there a Java equivalent of SQL's COALESCE function? That is, is there any way to return the first non-null value of several variables? e.g. Double a = null; Double b = 4.4; Double c = null; I want to somehow have a statement that will return the first non-null value of a, b, and c - in this case, it would return b, or 4.4. (Something like the sql method - return COALESCE(a,b,c)). I know that I can do it explicitly with something like: return a != null ? a : (b != null ? b : c) But I wondered if there was any built-in, accepted function to accomplish this.

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• How do I resolve this scope issue in VB .NET?

  - by froadie

  I have a code structure something like this: For row = 1 To numRows Dim arrayIndex As Integer = 0 For column As Integer = bucketStartColumn To bucketEndColumn ' whatever code arrayIndex = arrayIndex + 1 Next Next Dim arrayIndex As Integer = 0 For column As Integer = bucketStartColumn To bucketEndColumn ' whatever code arrayIndex = arrayIndex + 1 Next Not exactly the code, so I don't really need suggestions about refactoring, but my problem is this - with this code I get a compiler error for the first Dim arrayIndex As Integer = 0 - "Variable 'arrayIndex' hides a variable in an enclosing block." As far as I can tell, arrayIndex is local to the first for loop and shouldn't exist by the time we reach the second loop. If I try to change the second declaration of arrayIndex to arrayIndex = 0, I get the error "Name 'arrayIndex' is not declared", as I expected. So is it visible, or not? Does this have something to do with the Dim keyword? Any suggestions of how to get around this, other than naming the second index variable something else?

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• What's the scope of a Python variable declared in an if statement?

  - by froadie

  I'm new to Python, so this is probably a simple scoping question. The following code in a Python file (module) is confusing me slightly: if __name__ == '__main__': x = 1 print x In other languages I've worked in, this code would throw an exception, as the x variable is local to the if statement and should not exist outside of it. But this code executes, and prints 1. Can anyone explain this behavior? Are all variables declared in a module global/available to the entire module?

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• Why can't I access the instance.__class__ attribute in Python?

  - by froadie

  I'm new to Python, and I know I must be missing something pretty simple, but why doesn't this very, very simple code work? class myClass: pass testObject = myClass print testObject.__class__ I get the following error: AttributeError: class myClass has no attribute '__class__' Doesn't every object in Python have a __class__ attribute?

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• Interview Question: What are the characteristics of a good programmer?

  - by froadie

  I was asked this question on an interview a few months ago - "What would you say are the characteristics of a good programmer?" What would you answer to this? What, as a hiring manager, would you be looking for in an answer? I did get the job and am currently working with this company, although I don't know if that means I answered what they wanted to hear, but here were a couple of the points I mentioned: Passion PATIENCE Logic Teamwork etc. I'm especially curious to hear from the hiring side of things what would impress as an answer...

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• What's the best way to replace the first letter of a string in Java?

  - by froadie

  I'm trying to convert the first letter of a string to lowercase. I know there's a capitalize method, but I want to accomplish the opposite. This is the code I used: value.substring(0,1).toLowerCase() + value.substring(1) Effective, but feels a bit manual. Are there any other ways to do it? Any better ways? Any Java string functions that do it for you? I was thinking of using something like a replace function, but Java's replace doesn't accept an index as a parameter. You have to pass the actual character/substring. Another way I can think of doing it is something like: value.replaceFirst(value.charAt(0), value.charAt(0).toLowerCase()) Except that replaceFirst expects 2 strings, so the value.charAt(0)s would probably need to be replaced with value.substring(0,1)s. Is this any better? Does it matter? Is there any standard way to do this?

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• Why isn't this infinite recursion? How does default variable initialization work in VB.NET?

  - by froadie

  I just made an interesting mistake in my code: Dim endColumn As Integer = startColumn + endColumn - 1 The code was actually supposed to be: Dim endColumn As Integer = startColumn + numColumns - 1 The interesting thing is, I would think that this code should be recursive and loop indefinitely, as the initialization of endColumn sort of calls itself. However, it seems that the code just treats the uninitialized variable as a 0 and so I get startColumn + 0 - 1. What is happening here behind the scenes? When does a variable get assigned a default value?

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• Which is the better way to simulate optional parameters in Java?

  - by froadie

  I have a Java method that takes 3 parameters, and I'd like it to also have a 4th "optional" parameter. I know that Java doesn't support optional parameters directly, so I coded in a 4th parameter and when I don't want to pass it I pass null. (And then the method checks for null before using it.) I know this is kind of clunky... but the other way is to overload the method which will result in quite a bit of duplication. Which is the better way to implement optional method parameters in Java: using a nullable parameter, or overloading? And why?

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• In Python, is it better to use list comprehensions or for-each loops?

  - by froadie

  Which of the following is better to use and why? Method 1: for k, v in os.environ.items() print "%s=%s" % (k, v) Method 2: print "\n".join(["%s=%s" % (k, v) for k,v in os.environ.items()]) I tend to lead towards the first as more understandable, but that might just be because I'm new to Python and list comprehensions are still somewhat foreign to me. Is the second way considered more Pythonic? I'm assuming there's no performance difference, but I may be wrong. What would be the advantages and disadvantages of these 2 techniques? (Code taken from Dive into Python)

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• I created a Python egg!!! Now... what do I do with it?

  - by froadie

  I've finally figured out how to create a Python egg and gotten it to work. Now... what do I do with it? How do I use it? How do I ensure that everything was correctly included? (Simple steps please... not just redirection to another site. I've googled, but it's confusing me, and I was hoping someone could explain it in a couple of simple bullet points or sentences.)

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• Is it bad practice to initialize a variable to a dummy value?

  - by froadie

  This question is a result of the answers to this question that I just asked. It was claimed that this code is "ugly" because it initializes a variable to a value that will never be read: String tempName = null; try{ tempName = buildFileName(); } catch(Exception e){ ... System.exit(1); } FILE_NAME = tempName; Is this indeed bad practice? Should one avoid initializing variables to dummy values that will never actually be used? (EDIT - And what about initializing a String variable to "" before a loop that will concatenate values to the String...? Or is this in a separate category? e.g. String whatever = ""; for(String str : someCollection){ whatever += str; } )

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• Should programmers use boolean variables to "document" their code?

  - by froadie

  I'm reading McConell's Code Complete, and he discusses using boolean variables to document your code. For example, instead of: if((elementIndex < 0) || (MAX_ELEMENTS < elementIndex) || (elementIndex == lastElementIndex)){ ... } He suggests: finished = ((elementIndex < 0) || (MAX_ELEMENTS < elementIndex)); repeatedEntry = (elementIndex == lastElementIndex); if(finished || repeatedEntry){ ... } This strikes me as logical, good practice, and very self-documenting. However, I'm hesitant to start using this technique regularly as I've almost never come across it; and perhaps it would be confusing just by virtue of being rare. However, my experience is not very vast yet, so I'm interested in hearing programmers' opinion of this technique, and I'd be curious to know if anyone uses this technique regularly or has seen it often when reading code. Is this a worthwhile convention/style/technique to adopt? Will other programmers understand and appreciate it, or consider it strange?

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• Why aren't Java's generics implicitly polymorphic?

  - by froadie

  I'm a bit confused about how Java generics handle inheritance / polymorphism. Assume the following hierarchy - Animal (Parent) Dog - Cat (Children) So suppose I have a method doSomething(List<Animal> animals). By all the rules of inheritance and polymorphism, I would assume that a List<Dog> is a List<Animal> and a List<Cat> is a List<Animal> - and so either one could be passed to this method. Not so. If I want to achieve this behavior, I have to explicitly tell the method to accept a list of any subset of Animal by saying doSomething(List<? extends Animal> animals). I understand that this is Java's behavior. My question is why? Why is polymorphism generally implicit, but when it comes to generics it must be specified?

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• Can you have 2 completely independent instances of Eclipse running at the same time?

  - by froadie

  I'm sure there isn't really a way to do this, but figured it doesn't hurt to ask... I use Eclipse a lot. I'm currently using it for both Java and Python (with PyDev). I often find that I have one project open, with lots of files, say in Java... And then for some reason I have to switch to a Python project for a bit. I want to leave my Java project the way it is, and I don't just want to open tons of Python files in the same place because then I have too much open at once and get a headache and confused. Is there any way I can just leave the Java project exactly the way it is, and sort of open a completely new session of Eclipse? (sort of the way you can do with a browser) Or is this just wishful thinking?

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