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vj# : Some understanding questions...
Hi!
I am still a beginner with Java/VJ#.NET/ASP.NET, and now I have some understanding questions: 1. private, public and protected: 1.1. If I want to use my variables only inside of my class-file they must be private? 1.2. Is this correct: If I want to use variables or methods from outside the class-file thy must be public? 1.3. What means protected? 2. Sometimes I heard "If the page request is not a postback". What does this mean? 3. What means static? 3.1. I know that I could use staic variables only from staitc mehtods. But for what is "static"? 4. Exception handling with try/catch: 4.1. If I want to catch all Exceptions, I do this with "catch(Exception ex) {...}"? 4.2. Else I could specify the exact Exception "catch(OleDbException) {...}"? Thanks for any help!!! Greetings, christian.
Hi!
I forgot: 5. "this.": 5.1. Why should I use "this.". In my Java-Learning-Book, and in manny samples "this." is used, but I tried my code with and without it, and I got every thime the same result -> NO ERROR... Kind Greetings, Christian.
Dear Christian-Josef Schrattenthaler,
[quoted text, click to view] >1.1. If I want to use my variables only inside of my class-file they must be private?They should be. That is not compulsory, but it makes sense to declare them private. [quoted text, click to view] >1.2. Is this correct: If I want to use variables or methods from outside the class-file thy must be public?If the other class file is in the same namespace, then you can access only "public" or "protected" methods and variables. Also, you can declare a variable private and write public accessor methods to it, like private int myPrivateNumber; public int get_myPrivateNumber(){ return myPrivateNumber; } public void set_PrivateNumber(int value){ myPrivateNumber = value; } Using this method, you can verify whether the value assigned from outside is valid or not before you actually assign it to a variable. [quoted text, click to view] >1.3. What means protected? protected variables or methods can be accessed only from the same class or the same namespace (package in Java). [quoted text, click to view] >3. What means static? This is large topic, but briefly, static means that the particular value does not depend on what particular instance of the class you are investigating. If you have one class, MyClass and 3 objects of type MyClass named objOne, objTwo and objThree, then a static variable myStatVar will have the same value for all of objOne, objTwo and objThree, and can be accessed by calling MyClass.myStatVar. [quoted text, click to view] >3.1. I know that I could use staic variables only from staitc mehtods. No! It's the other way round! Static methods can only access static variables, but static variables can be accessed from non-static contexts as well. [quoted text, click to view] >4.1. If I want to catch all Exceptions, I do this with "catch(Exception ex) {...}"?4.2. Else I could specify the exact Exception "catch(OleDbException) {...}" Yes, absolutely true. Also, you can get the error message by calling ex.getMessage(); [quoted text, click to view] > 5. "this.": > 5.1. Why should I use "this.". In my Java-Learning-Book, and in manny > samples "this." is used, but I tried my code with and without it, and I > got every thime the same result -> NO ERROR... "this" is especially useful when you are dealing with more than one classes. Suppose you have two classes MyTopClass and MyBtmClass. MyTopClass creates an object of type MyBtmClass and calls a method of that class. Suppose that method needs to call a method of MyTopClass. This can be achieved by passing the function the object "this", and then that function can use that object reference to call functions in MyTopClass This has been a long post... Hope it helps -- Sushovan, a crazy junior [quoted text, click to view] "Christian-Josef Schrattenthaler" wrote:
> Hi! > > I forgot: > > 5. "this.": > 5.1. Why should I use "this.". In my Java-Learning-Book, and in manny > samples "this." is used, but I tried my code with and without it, and I > got every thime the same result -> NO ERROR... > > Kind Greetings, > Christian. >
[quoted text, click to view]
> 5. "this.": > 5.1. Why should I use "this.". In my Java-Learning-Book, and in manny > samples "this." is used, but I tried my code with and without it, and I > got every thime the same result -> NO ERROR... suppose you have the following code: public class Test { private int person; public void tst(int person){ this.person=person; } } you need "this" only in such scenarios, to differentiate class fields from local variables (or function parameters) that have the same name (the local scope has higher priority in name resolution, so a plain "person" would mean the function parameter "person", not the class field "person" of course, it's much better to use this pattern: public class Test { private int person; public void tst(int thePerson){ person=thePerson; } } in Borland-style you'd see "aPerson", but I prefer to write "thePerson" cheers, George ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ George Birbilis <birbilis@kagi.com> Microsoft Most Valuable Professional MVP J# for 2004, 2005 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + QuickTime (Delphi & ActiveX: VB, PowerPoint, .NET) + Plugs (InterProcess/Internet communication) http://www.kagi.com/birbilis + Robotics http://www.mech.upatras.gr/~robgroup .........................................................................
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[quoted text, click to view] > >1.1. If I want to use my variables only inside of my class-file they > must be private? > > They should be. That is not compulsory, but it makes sense to declare them > private. if you don't add an access specifier they're package-private, that is any class that has the same package name can access them (e.g. the gr.cti.eslate.X class would be able to access such fields of gr.cti.eslate.Y class) [quoted text, click to view] > >1.2. Is this correct: If I want to use variables or methods from > outside the class-file thy must be public? > > If the other class file is in the same namespace, then you can access only > "public" or "protected" methods and variables. not sure if in J# you can access "protected" fields of a class from another class that has same namespace or not. In Java you should be able to do so only from class descendents, so this is problematic security-wise if this is true (esp. when porting existing Java code to J#). Unless you can somehow seal a namespace (as you can do with packages in Java), maybe by using a signed assembly so that no other third-party code uses it apart from the code you want. Since there's the notion of "package private" too as I describe above, not sure why there should be separate namespace behaviour. To be sure try it and see what it does [quoted text, click to view] > Also, you can declare a variable private and write public accessor methods > to it, like > > private int myPrivateNumber; > public int get_myPrivateNumber(){ > return myPrivateNumber; > } > public void set_PrivateNumber(int value){ > myPrivateNumber = value; > } > > Using this method, you can verify whether the value assigned from outside is > valid or not before you actually assign it to a variable. also see the J# documentation on "properties". There are two get/set patterns in J#, the JavaBeans-style (getXX/setXX) and the .NET style (get_XX/set_XX), which differ a bit (the 2nd one I think is understood by J# as accessors for .NET class properties and it exposes such automatically [e.g. you could write myObject.xx=5 in C# or VB.net then instead of writing myObject.setXX(5) if you wanted]) [quoted text, click to view] > >1.3. What means protected? > > protected variables or methods can be accessed only from the same class or > the same namespace (package in Java). see above my comment on "package private" and "protected". They should be different things in languages like Object Pascal, indeed all protected class members are accessible by other classes inside the same unit, but that shouldn't be true in Java [quoted text, click to view] > >3. What means static? > > This is large topic, but briefly, static means that the particular value > does not depend on what particular instance of the class you are > investigating. If you have one class, MyClass and 3 objects of type MyClass > named objOne, objTwo and objThree, then a static variable myStatVar will have > the same value for all of objOne, objTwo and objThree, and can be accessed by > calling MyClass.myStatVar. there's also the notion of "static" function parameters, but they're rarely used. See "Thinking in Java" free book by Bruce Eckel (www.eckelobjects.com) [quoted text, click to view] > >4.1. If I want to catch all Exceptions, I do this with "catch(Exception > ex) {...}"? > 4.2. Else I could specify the exact Exception "catch(OleDbException) > {...}" > > Yes, absolutely true. Also, you can get the error message by calling > ex.getMessage(); Unfortunately you can't catch .NET exceptions with "catch(Exception ex)" I think. This is bad since that pattern is used a lot by Java programmers. Try "catch(Throwable t)" instead, but that way you'll catch "Error" descendents too (Exception and Error are descendents of Throwable class, and J# compiler is understand "catch(Throwable t)" as catch-all [.NET exceptions included] if I remember well, but not "catch(Exception e)" too) -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ George Birbilis <birbilis@kagi.com> Microsoft Most Valuable Professional MVP J# for 2004, 2005 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + QuickTime (Delphi & ActiveX: VB, PowerPoint, .NET) + Plugs (InterProcess/Internet communication) http://www.kagi.com/birbilis + Robotics http://www.mech.upatras.gr/~robgroup .........................................................................
I've asked the J# team on this and will let you know about it
if anyone has time to checkout "Thinking in Java" book from www.eckelobjects.com or the Java specification from java.sun.com on this, please post your findings as a reply [quoted text, click to view] > > If the other class file is in the same namespace, then you can access only > > "public" or "protected" methods and variables. > > not sure if in J# you can access "protected" fields of a class from another > class that has same namespace or not. In Java you should be able to do so > only from class descendents, so this is problematic security-wise if this is > true (esp. when porting existing Java code to J#). Unless you can somehow > seal a namespace (as you can do with packages in Java), maybe by using a > signed assembly so that no other third-party code uses it apart from the > code you want. Since there's the notion of "package private" too as I > describe above, not sure why there should be separate namespace behaviour. > To be sure try it and see what it does
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