Thread: Hello world

Of course you can't return a private object. That's the whole poing of OO! You can't just expose your private parts to the world! It's illegal!

Seriously, that breaks encapsulation. There is no reason to return private members/expose them to the rest of the program. This defeats the entire purpose of OO design.

Quzah.

Code:

class Date 
{ 
	// insert usual datestuff here 
public: 
	const char* AsText() const; 
	void Change( int, int, int ); 
}; 

class Person 
{ 
private: 
	Date Birthdate; 
public: 
	const Date& GetBirthdate() const 
		{ return Birthdate; } 
	void SetBirthdate( const Date& d ) 
		{ Birthdate = d; } 
};

There is a slight problem with your code. Since this is a birthday, you don't want it changing. Face it, people's birthday's never change. As such:

Code:

class Date
{   private int d,m,yr;
    public void Date( int day, int month, int year )
    { d = day; m = month; y = year; }
}

class BirthDate extends Date
{   public Date GetBirthDate( )
     {   return new Date( this.d, this.m, this.y );
    }
    public void ShowBirthDate( )
    {   System.out.println( "" + this.d + "/" + this.m + "/" + this.y );
    }
}

class Person
{   private BirthDate b;
    //more personal information

    public Person( int day, int month, int year )
    {   b = new BirthDate( day, month, year );
    }

    public ShowBirthday( )
    { b.ShowBirthDate( ); }
}

class MyProggie
{   public static void main( String [] args )
    {   Person p = new Person( 4, 11, 2001 );

        p.ShowBirthDay( );
    }
}

That is how the class should be designed. More like that anyway. Yours was poor class design, in that, you are extracting information from the class in a way you shouldn't. A class should be self contained. Anything that you need from the class, it should be able to provide you with access functions.

Yes, it is more work, more lines of code, and takes longer to write;
but, if you want your classes working the way they should, then that (above) is how you'd go about it.

Granted, I just threw that together, so there are probably ways it could be done better. (For example, I should have made the functions return a string, rather than have called println inside it. I was just hacking this together real quick, thus the result.)

Additionally, my Date class could have been done better. For example, I could have used a 'final' setDate class in Birthday, to prevent the date from being changed, something along that line.

Quzah.

>> iB.getA( ).setX( 333 );
>
>I admitedly don't have a java compiler handy at home. A well
> implemented compiler should either scream and shout and
> wave every flag available, or silently step aside and commit
> sepuku when compiling this line. If it produces executable code
> from this line, it's crap.

Actually, I'm using the Sun Java compiler (1.3 i believe) and it
compiles without error or warning, and executes correctly. The
reason it works is that 'setX' is a public method.

I could have also just done:

iB.iA.setX( 333 );

In either case, the end result is the same. 'iA' calls 'setX'.

In your example using 'final', this means that instance of the class is final. How that exactly works, I am not entirely sure. Perhaps you cannot do:

final c myC = new C( ); //give C an instance
myC = new C( ); //give it a new one

I believe that's what you're looking at there.

If you want an unchangable class, you declare the class as final:

final class C {
}

This means that you can't exctend that class. Thus:

class D extends C { //this is invalid
}

Anyway, going from the book I have, "Core Java 2", which is used for certification, they state that 'final' is basicly the same as 'const'. I'm at work now, and the book is at home so without going to http://java.sun.com and hunting around for it, that's the best I can do for now.

The best analogy I can guess at here (I don't know enough Java to say with100% certainty.) is that basicly, everything in Java is a pointer. This is why you have to use: 'x = new X( );' all the time.

If you try and give C a new value, once it's already been given one:

c C = new c( );
C = new c( ); //second value

It will fail. I tried:

Code:

C:\WINDOWS\Desktop>javac MyApp2.java
MyApp2.java:18: Can't assign a value to a final variable: C
          C = new c( );
          ^
1 error

So basicly, it's like making the pointer constant, in that, you can give it a value once, and from then on, it cannot be changed.

And I believe that's how "Core Jave:Volume 1" describes it. "final" means (for instances, I suspect) that you can give it a value once, and from then on, you cannot change it. Thus, if you want your methods to be "constant", then you have to declare the method as final. If you want a data member constant, you have to declare it final.

A final method cannot be overridden either, at least I believe so.
I wish we had a Java board...

Quzah.

> If your iB.getA( ) had returned a constant A object then you
> wouldn't have been able to call setX( 333 ) on it, so iB.getA
> ( ).setX( 333 ); should be illegal.

Hm... Yup. It would seem so. However, it compiles and runs. I
really don't have the answer to why. Let's try...

Code:

class A { //class A
	private int x; //some data here
	public int getX( ) { return x; }
	public void setX( int y ) { x = y; }
}

class B {
	private A iA; //private instance clA
	public B( ) { iA = new A( ); }
	public final A getA( ) { return iA; }
}

class MyApp {
	public static void main( String[] args ) {
		B iB = new B( );
		iB.getA( ).setX( 333 );
		System.out.println( "A.x is " + iB.getA( ).getX( ) );
		iB.getA( ).setX( 444 );
		System.out.println( "A.x is " + iB.getA( ).getX( ) );
		iB.iA.setX( 555 );
		System.out.println( "A.x is " + iB.getA( ).getX( ) );
	}
}

MyApp.java:22: Variable iA in class B not accessible from class MyApp.
                iB.iA.setX( 555 );
                  ^

Ok, so what exactly is this saying? We cannot access 'iA' because we're in another class while doing it? Got me. I thought this'd work. Hell, it should work, IMO. Examine:

iB = public
iA = private member of iB

Since it's private, 'iB' should be able to access it, but other classes cannot. But, when we do:

iB.getA( ).setX( ... );

This works because we're returning an iA object, and having it call it's own functions. Eeh.. I need a Java board.

> You are modifying an object that has been returned from a Get()
> method and this method doesn't protect it. If you wanted
> exclusive control of an 'A' object from within' B', and wanted to
> provide external read-only access to it, how would 'B' know
> when this 'A' is being modified?

I'm not sure. My lunch is over, so I'll mull this around abit, and see what I can come up with...

Quzah.

http://java.sun.com/docs/books/tutor...sscontrol.html

Well I've been tinkering with this, and I'm not sure I'm liking what I find. (What I find, is that it isn't working the way I thought it should be working.)

Code:

class A {
    private int x; //should be only available to this class's members
    public int getX( ) { return x; } //should be available to everyone
    protected void setX( int y ) { x = y; } //only this and subclasses
}
class B {
    public A pubA = new A( ); //everyone can play with this
    private A priA = new A( ); //only B's members?
    protected A proA = new A( ); //only this and B's subclasses

    public A getpubA( ) { return pubA; }
    private A getpriA( ) { return priA; }
    protected A getproA( ) { return proA; }
}
class MiApp {
	public static void main( String [] args ) {
		B iB = new B( );

		// All is legal. We're strictly public here. Free for all.
		iB.pubA.setX( 1 );
		System.out.println( "iB.pubA.getX( ) == " + iB.pubA.getX( ) );

		//error, iB does not have access to 'priA' in class 'MiApp'.
		//error for these next two lines.
		iB.priA.setX( 2 );
		System.out.println( "iB.priA.getX( ) == " + iB.priA.getX( ) );

		//This leads me to the following conclusion:
		// 'MiApp' is the one that the compiler believes is accessing
		// the inner workings of 'iB'. In other words, just because
		// 'MiApp' __HAS A__ 'iB', does not mean that it can access
		// the contents of 'iB'.

		//This is also legal, because with 'protected', we extend access
		//to those classes that 'have a' instance of said class. Actually,
		//I believe it works because we're in the same 'package', (ie: in
		//this case, the same file. An external file that had a 'B'
		//instance would fail here? Haven't tried it.
		iB.proA.setX( 3 );
		System.out.println( "iB.proA.getX( ) == " + iB.proA.getX( ) );
	}
}

How does that relate with us returning data and the 'final' keyword? I'll get to work on that now that I __think__ I understand how the access works.

Quzah.

> public void(?) setA (A obj) { iA = obj; } // assuming...

Yep, this is correct. Or would be rather. This, in Java, is just like this in C++:

Code:

class A {
public:
    A( ) { obj = new Object( ); } //can't remember if you use () here

    void setObj( Obj o ) { if( obj ) delete( obj ); obj = o; }
private:
    Object *ob;
};

As far as passing by reference, pretty much. However, I'd be really really surprised if there wasn't a way to prevent changes when you didn't want them. (ie: const/final) I just am not sure how yet :P

Quzah.