I've started java recently and this is what is bothering me.  consider this code.  class Superclass{ void print(){ System.out.print("hi"); } }  class Subclass extends Superclass{ void Print(){ System.out.print("HI"); } }  class RandomTestDrive{ public static void main(String[] args){ Superclass a = new Subclass(); a.print(); } }   so my question here is, why would anyone declare their reference variable as superclass and object as subclass.  since you cannot use the methods of subclass from a reference variable variable that is of type superclass, why do it in the first place?  why not just make the reference variable of the same type as the object?
685
I have seen this mentioned a few times and I am not clear on what it means. When and why would you do this?   I know what interfaces do, but the fact I am not clear on this makes me think I am missing out on using them correctly.   Is it just so if you were to do:  IInterface classRef = new ObjectWhatever()   You could use any class that implements IInterface? When would you need to do that? The only thing I can think of is if you have a method and you are unsure of what object will be passed except for it implementing IInterface. I cannot think how often you would need to do that.   Also, how could you write a method that takes in an object that implements an interface? Is that possible?
330
The primitive types (number, string, etc.) are passed by value, but objects are unknown, because they can be both passed-by-value (in case we consider that a variable holding an object is in fact a reference to the object) and passed-by-reference (when we consider that the variable to the object holds the object itself).  Although it doesn't really matter at the end, I want to know what is the correct way to present the arguments passing conventions. Is there an excerpt from JavaScript specification, which defines what should be the semantics regarding this?
107554
Is it possible in C# to have a Struct with a member variable which is a Class type?  If so, where does the information get stored, on the Stack, the Heap, or both?
34848
I used to use the implicit call of toString when wanting some debug info about an object, because in case of the object is null it does not throw an Exception.  For instance:    System.out.println("obj: "+obj);   instead of:  System.out.println("obj: "+obj.toString());   Is there any difference apart from the null case? Can the latter case work, when the former does not?  Edit: What exactly is done, in case of the implicit call?
89062
What is the meaning of "virtual" inheritance?  I saw the following code, and didn't understand the meaning of the keyword virtual in the following context:   class A {}; class B : public virtual A;
23909
Looking for reviewers to suggest optimization and improvements, and to evaluate coding practices.  final class Variables {     private final int x;     private final int y;      Variables(int x2, int y2) {         this.x = x2;         this.y = y2;     }      public int getX() {         return x;     }      public int getY() {         return y;     }      @Override public String toString()  {          return "x = " + x + " y = " + y;     } }  public final class FindMissing {      private FindMissing() {};      /**  * Takes an input a sorted array with two variables that repeat in the  * range. Remaining variable are consecutive. and returns two variables that repeats  *   * eg: for a range of 1,2,3,4, a valid input is [1, 4], [1, 3] etc.  *   * @param a : the sorted array.  */ /*  * Why is start needed ?  * If start is not given then if a1 is empty how do we know if missing numbers were  * [1, 2] or [4, 5] ??.   */ public static Variables sortedConsecutiveTwoMissing (int[] a, int startOfRange) {     if (a == null) throw new NullPointerException("a1 cannot be null. ");      int low = startOfRange;     // calculating the highest value of the range. it needs to be 2 more than length since all elememts are consecutive.     int high = startOfRange + (a.length - 1) + 2;      int i = 0;     boolean foundX = false;     int x = 0;       while (i &lt; a.length) {         if (a[i] != low) {             if (foundX) {                 return new Variables(x, low);             } else {                 x = low;                 foundX = true;             }         } else {             i++;         }         low++;     }      return new Variables(x, high);  }  public static void main(String[] args) {         // unsortedConsecutiveTwoMissing         int[] a6 = {1, 4};         System.out.println("Expected 2 and 3, Actual " + sortedConsecutiveTwoMissing(a6, 1));         // unsortedConsecutiveTwoMissing         int[] a7 = {-4, -1};         System.out.println("Expected -3 and -2, Actual " + sortedConsecutiveTwoMissing(a7, -4));         int[] a8 = {-4, -3, -2, -1};         System.out.println("Expected 0 and 1, Actual " + sortedConsecutiveTwoMissing(a8, -4));         int[] a9 = {5};         System.out.println("Expected 4 and 6, Actual " + sortedConsecutiveTwoMissing(a9, 4));         }      }
7441
Was there any reason why the designers of Java felt that local variables should not be given a default value? Seriously, if instance variables can be given a default value, then why can't we do the same for local variables?  And it also leads to problems as explained in :     Well this rule is most frustrating when trying to close a resource in a finally block. If I instantiate the resource inside a try, but try to close it within the finally, I get this error. If I move the instantiation outside the try, I get another error stating that a it must be within a try.      Very frustrating.
90464
We noticed that lots of bugs in our software developed in C# (or Java) cause a NullReferenceException.  Is there a reason why "null" has even been included in the language?  After all, if there were no "null", I would have no bug, right?  In other words, what feature in the language couldn't work without null?
43586
What is the difference between the following class methods?  Is it that one is static and the other is not?  class Test(object):   def method_one(self):     print "Called method_one"    def method_two():     print "Called method_two"  a_test = Test() a_test.method_one() a_test.method_two()
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How do you decide between passing arguments to a method versus simply declaring them as object instance variables that are visible to all of the object's methods? I prefer keeping instance variables in a list at the end of the Class, but this list gets longer as my program grows. I figure if a variable is passed often enough it should just be visible to all methods that need it, but then I wonder, &quot;if everything is public there will be no need for passing anything at all!&quot;
123439
Ok, bear with me guys and girls as I'm learning.  Here's my question.  I can't figure out why I can't override a method from a parent class.  Here's the code from the base class (yes, I pilfered the java code from an OOP book and am trying to rewrite it in C#).  using System;  public class MoodyObject {     protected String getMood()     {         return "moody";     }      public void queryMood()     {         Console.WriteLine("I feel " + getMood() + " today!");     } }   and here are my other 2 objects that inherit the base class (MoodyObject):  using System; using System.Collections.Generic; using System.Linq; using System.Text;  namespace ConsoleApplication1 {     public class SadObject: MoodyObject     {         protected String getMood()         {             return "sad";         }          //specialization         public void cry()         {             Console.WriteLine("wah...boohoo");         }     } }   And:  using System; using System.Collections.Generic; using System.Linq; using System.Text;  namespace ConsoleApplication1 {     public class HappyObject: MoodyObject     {         protected String getMood()         {             return "happy";         }          public void laugh()         {             Console.WriteLine("hehehehehehe.");         }     } }   and here is my main:  using System; using System.Collections.Generic; using System.Linq; using System.Text;  namespace ConsoleApplication1 {     class Program     {         static void Main(string[] args)         {             MoodyObject moodyObject = new MoodyObject();             SadObject sadObject = new SadObject();             HappyObject happyObject = new HappyObject();              Console.WriteLine("How does the moody object feel today?");             moodyObject.queryMood();             Console.WriteLine("");             Console.WriteLine("How does the sad object feel today?");             sadObject.queryMood();             sadObject.cry();             Console.WriteLine("");             Console.WriteLine("How does the happy object feel today?");             happyObject.queryMood();             happyObject.laugh();         }     } }   As you can see, pretty basic stuff, but here's the output:     How does the moody object feel today?   I feel moody today!      How does the sad object feel today? I   feel moody today! wah...boohoo      How does the happy object feel today?   I feel moody today! hehehehehehe.   Press any key to continue . . .   Not as I expected.  I've tried to make the base method virtual and calling override when trying to override it and that just gets me this error "cannot override inherited member 'MoodyObject.getMood()' because it is not marked virtual, abstract, or override".  I also tried it without the virtual and override and it thinks I'm trying to hide the base method.  Again, I'm new to OOP and would appreciate any guidance.  EDITED TO ADD:  I found it!  The MoodyObject.cs was only a "solution item" in the solution explorer as opposed to a "ConsoleApplication1" item.  I dragged it down to where it belonged in the solution explorer and voila!  It works now.  I marked Luc's answer below as the answer because he offered the help I needed to get to where I have it resolved... I'm learning so much here.  It's amazing and you guys and girls are crazy smart!
78860
The question was about plain  functions, not  static methods, as clarified in comments.  I understand what a static variable is, but what is a static function?  And why is it that if I declare a function, let's say void print_matrix, in let's say a.c (WITHOUT a.h) and include "a.c" -  I get "print_matrix@@....) already defined in a.obj", BUT if I declare it as static void print_matrix then it compiles?  UPDATE Just to clear things up - I know that including .c is bad, as many of you pointed out. I just do it to temporarily clear space in main.c until I have a better idea of how to group all those functions into proper .h and .c files. Just a temporary, quick solution.
114445
So I understand what a static method or field is, I am just wondering when to use them. That is, when writing code what design lends itself to using static methods and fields.   One common pattern is to use static methods as a static factory, but this could just as easily be done by overloading a constructor. Correct? For example:  var bmp = System.Drawing.Bitmap.LoadFromFile("Image01.jpg");   As for static fields, is creating singelton-objects their best use?
81621
If I write the following class:  public class Example {        int j;       int k;        public Example(int j, int k) {            j = j;            k = k;       }        public static void main(String[] args) {            Example exm = new Example(1,2);            System.out.println(exm.j);            System.out.println(exm.k);       }  }   The program compiles, but when I run the program, the main method will print out two 0s. I know that in order to say that I want to initialize the instance variables in the constructor I have to write:  this.j = j; this.k = k;   But if I don't write it, then which variable is evaluated (or considered) in the constructor (on the left and on the write hand side of the expressions)? Is is the argument or the instance variable? Does it make a difference?  Are there other cases where the use of this is obligatory?
30943
I think there must be something subtle going on here that I don't know about. Consider the following:  public class Foo&lt;T&gt; {   private T[] a = (T[]) new Object[5];    public Foo() {     // Add some elements to a   }    public T[] getA() {     return a;   } }   Suppose that your main method contains the following:  Foo&lt;Double&gt; f = new Foo&lt;Double&gt;(); Double[] d = f.getA();   You will get a CastClassException with the message java.lang.Object cannot be cast to java.lang.Double.  Can anyone tell me why? My understanding of ClassCastException is that it is thrown when you try to cast an object to a type that cannot be casted. That is, to a subclass of which it is not an instance (to quote the documentation). e.g.:  Object o = new Double(3.); Double d = (Double) o; // Working cast String s = (String) o; // ClassCastException   And it seems I can do this. If a was just a T instead of an array T[], we can get a and cast it without a problem. Why do arrays break this?  Thanks.
54427
I have two classes: Action and MyAction. The latter is declared as:  class MyAction extends Action {/* some methods here */}   All I need is method in the Action class (only in it, because there will be a lot of inherited classes, and I don’t want to implement this method in all of them), which will return classname from a static call. Here is what I’m talking about:  Class Action {  function n(){/* something */} }   And when I call it:  MyAction::n(); // it should return "MyAction"   But each declaration in the parent class has access only to the parent class __CLASS__ variable, which has the value “Action”.  Is there any possible way to do this?
6413
When programming interfaces, I've found I'm doing a lot of casting or object type conversion.  Is there a difference between these two methods of conversion?  If so, is there a cost difference or how does this affect my program?  public interface IMyInterface {     void AMethod(); }  public class MyClass : IMyInterface {     public void AMethod()     {        //Do work     }      // Other helper methods.... }  public class Implementation {     IMyInterface _MyObj;     MyClass _myCls1;     MyClass _myCls2;      public Implementation()     {         _MyObj = new MyClass();          // What is the difference here:         _myCls1 = (MyClass)_MyObj;         _myCls2 = (_MyObj as MyClass);     } }   Also, what is "in general" the preferred method?