Programming Forums > Application Development > Java

JDStud6 · Dec 1st, 2005, 9:20 AM

for(int i=0;i<points.size();i++) 
{
	g.fillOval(
				((MyPoint)points.get(i)).x,
				((MyPoint)points.get(i)).y,
				dotSize, dotSize);
}

try that

JD

groovicus · Dec 1st, 2005, 9:21 AM

Why would he want to do that?

JDStud6 · Dec 1st, 2005, 9:26 AM

are you asking because you don't know, or because you think he should know?

haha

JD

groovicus · Dec 1st, 2005, 9:51 AM

I'm asking because he doesn't know, which is why I suggested that he read the API instead of just giving him code.. then he may have been able to figure out on his own how to create the constructor, which libraries were necessary, and why one needs to do what you told him to do. And if not, he would have had questions about reading APIs, which would have allowed him to learn more on his own.

So to answer your question, I asked because I think he should know why you are recommending that he do what you said to do. Otherwise the next time that he goes to .get something, he will be stuck again, having learned nothing now.

If it were me, I would have explained that an arraylist is really just a list of pointers that point to objects in memory, and it has no idea what kind of objects that they are. In order to use those objects, one must cast the object (which is just a way of telling the arraylist what it is) back into what they originally were so that the arraylist knows what to do with them. This is a crude (and not completely correct) explanation, but usually enough to make people understand.

I would also explain that depending on the version of java one is using, and that since it is bad practice to store more than one datatype in a single collection, one can save themselves the trouble of having to tell the arraylist what it is removing each time by telling it ahead of time what sort of objects it will be holding, and one can do that (for example), in the following manner:

(Toggle Plain Text)

ArrayList<myObject> a = new ArrayList();

Now my arraylist knows it is only going to hold myObjects, and won't let you put anything else in there but myObjects. It will always know what to do with them.

*****

But that is just me...

JDStud6 · Dec 1st, 2005, 9:57 AM

uhh....yea, what he said

JD

(ps - if you don't understand, research 'casting' and look at the API for arraylist)

Writlaus · Dec 1st, 2005, 1:18 PM

Ah thank you all, it finally works. Seeing those dots appear is a beautiful sight...

I knew it had to do with the fact that I didn't specify what type of object was going to be in the ArrayList, but I didn't know how to specify that. But now I do, so it's all good =)

titaniumdecoy · Dec 1st, 2005, 2:38 PM

Quote:

Originally Posted by andro

Make sure to import java.util.ArrayList.

(Toggle Plain Text)

ArrayList<Integer> myList = new ArrayList<Integer>();

and the ArrayList api...
http://java.sun.com/j2se/1.5.0/docs/...ArrayList.html

Thanks for your help. Is it possible to do the equivalent of this but store primitive int data types, for example ArrayList<int>? I looked up "autoboxing" but I'm not sure that's what I want. I need something that will run very fast, which is why I don't want to have to keep converting from Integer to int and so forth.

andro · Dec 1st, 2005, 7:31 PM

Integer is int.

titaniumdecoy · Dec 1st, 2005, 7:37 PM

No, one is a basic data type and the other is an object.

groovicus · Dec 1st, 2005, 8:17 PM

There are a couple of approaches that I can think of, but all of them require some sort of conversion, whether it is handled by the programmer, or by the JVM, the conversions are still taking place. And it could very well be that I don't have a clue as to what I am talking about.. in which case this is an opportunity for me to learn also.

You can use autoboxing, which only means that the conversion is done for you. It is still happening, and it may or may not be very efficient.

You can create your own add/get methods that do the conversions for you.

How big of a list of are you wanting to handle? Over a million? 3 million? A billion? Maybe there is an alternate means of doing what you want to do. All in all, the conversions don't take hardly any time, and I can't think of any way right off that one could seriously optimize the process.

Dec 1st, 2005, 9:20 AM	#11
JDStud6 Programmer Join Date: Jan 2005 Location: Charleston, SC www.wareonearth.com Posts: 57 Rep Power: 4	(Toggle Plain Text) for(int i=0;i<points.size();i++) { g.fillOval( ((MyPoint)points.get(i)).x, ((MyPoint)points.get(i)).y, dotSize, dotSize); } for(int i=0;i<points.size();i++) { g.fillOval( ((MyPoint)points.get(i)).x, ((MyPoint)points.get(i)).y, dotSize, dotSize); } try that JD

Dec 1st, 2005, 9:21 AM	#12
groovicus Programmer Join Date: Nov 2004 Posts: 84 Rep Power: 4	Why would he want to do that? __________________ HijackThis Team-SFDC

Dec 1st, 2005, 9:51 AM	#14
groovicus Programmer Join Date: Nov 2004 Posts: 84 Rep Power: 4	I'm asking because he doesn't know, which is why I suggested that he read the API instead of just giving him code.. then he may have been able to figure out on his own how to create the constructor, which libraries were necessary, and why one needs to do what you told him to do. And if not, he would have had questions about reading APIs, which would have allowed him to learn more on his own. So to answer your question, I asked because I think he should know why you are recommending that he do what you said to do. Otherwise the next time that he goes to .get something, he will be stuck again, having learned nothing now. If it were me, I would have explained that an arraylist is really just a list of pointers that point to objects in memory, and it has no idea what kind of objects that they are. In order to use those objects, one must cast the object (which is just a way of telling the arraylist what it is) back into what they originally were so that the arraylist knows what to do with them. This is a crude (and not completely correct) explanation, but usually enough to make people understand. I would also explain that depending on the version of java one is using, and that since it is bad practice to store more than one datatype in a single collection, one can save themselves the trouble of having to tell the arraylist what it is removing each time by telling it ahead of time what sort of objects it will be holding, and one can do that (for example), in the following manner: (Toggle Plain Text) ArrayList<myObject> a = new ArrayList(); ArrayList<myObject> a = new ArrayList(); Now my arraylist knows it is only going to hold myObjects, and won't let you put anything else in there but myObjects. It will always know what to do with them. ***** But that is just me... __________________ HijackThis Team-SFDC

Dec 1st, 2005, 8:17 PM	#20
groovicus Programmer Join Date: Nov 2004 Posts: 84 Rep Power: 4	There are a couple of approaches that I can think of, but all of them require some sort of conversion, whether it is handled by the programmer, or by the JVM, the conversions are still taking place. And it could very well be that I don't have a clue as to what I am talking about.. in which case this is an opportunity for me to learn also. You can use autoboxing, which only means that the conversion is done for you. It is still happening, and it may or may not be very efficient. You can create your own add/get methods that do the conversions for you. How big of a list of are you wanting to handle? Over a million? 3 million? A billion? Maybe there is an alternate means of doing what you want to do. All in all, the conversions don't take hardly any time, and I can't think of any way right off that one could seriously optimize the process. __________________ HijackThis Team-SFDC

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Dec 1st, 2005, 9:26 AM	#13
JDStud6 Programmer Join Date: Jan 2005 Location: Charleston, SC www.wareonearth.com Posts: 57 Rep Power: 4	are you asking because you don't know, or because you think he should know? haha JD

Dec 1st, 2005, 9:57 AM	#15
JDStud6 Programmer Join Date: Jan 2005 Location: Charleston, SC www.wareonearth.com Posts: 57 Rep Power: 4	uhh....yea, what he said JD (ps - if you don't understand, research 'casting' and look at the API for arraylist)

Dec 1st, 2005, 1:18 PM	#16
Writlaus Hobbyist Programmer Join Date: Nov 2005 Posts: 149 Rep Power: 3	Ah thank you all, it finally works. Seeing those dots appear is a beautiful sight... I knew it had to do with the fact that I didn't specify what type of object was going to be in the ArrayList, but I didn't know how to specify that. But now I do, so it's all good =)

Dec 1st, 2005, 7:31 PM	#18
andro Professional Programmer Join Date: Oct 2005 Location: California Posts: 309 Rep Power: 3	Integer is int.

Dec 1st, 2005, 7:37 PM	#19
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 855 Rep Power: 3	No, one is a basic data type and the other is an object.

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Programming Forums > Application Development > Java

Java array functions?