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# arraylist group by id.

liulca ruggiero
Greenhorn
Posts: 15
how to group the data in numeric format in an array I thank you in advance

Set<coordinates> group = new LinkedHashSet<coordinates>();

//group id = 1 . id x y
group.add(new coordinates(1, 50, 20));
group.add(new coordinates(1, 70, 30));
group.add(new coordinates(1, 50, 40));

//group id = 2
group.add(new coordinates(2, 100, 20));
group.add(new coordinates(2, 130, 30));
group.add(new coordinates(2, 140, 40));
group.add(new coordinates(2, 140, 40));

//the result should be the following

group id = 1 x={50,70,50}; y={20,30.40};

group id = 2 x={100,130,140,140}; y={20,30,40,40};

Michael Dunn
Ranch Hand
Posts: 4632
trhis is no different to your other recent thread

http://www.coderanch.com/t/598886/GUI/java/drawing-polygons

and you were told there how to do it.

are you just waiting for someone to post the code for you?

liulca ruggiero
Greenhorn
Posts: 15
I apologize but this problem I just can not solve, you can help I thank you in advance

Michael Dunn
Ranch Hand
Posts: 4632
I've already given you the steps, and a rough pseudo code for the separation.

I can only help further if you post your attempt at following those steps

liulca ruggiero
Greenhorn
Posts: 15
I am doing as you tell me error thank you for your patience.

ArrayList[] j = new ArrayList[2];

for (coordinates n : group) {
j[n.getID()-1].add(n);
System.out.println(j);
}

error
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

Michael Dunn
Ranch Hand
Posts: 4632
new ArrayList[2];

OK, that's the declaration, where's the class instance?
class instance = new .....();//notice the ()

liulca ruggiero
Greenhorn
Posts: 15
I'm sorry but I do not understand what you mean by class instance = new ..... () ;/ / notice the ()
you can give a few examples I thank you in advance asking for your help

Michael Dunn
Ranch Hand
Posts: 4632
> you can give a few examples I thank you in advance asking for your help

not nasty here, but have you ever written a java program?

if so, what do you think these are

= new JButton[];
= new JButton();

Campbell Ritchie
Sheriff
Posts: 50241
79
Michael Dunn wrote:trhis is no different to your other recent thread

http://www.coderanch.com/t/598886/GUI/java/drawing-polygons

and you were told there how to do it. . . .
Please read this. I am closing this discussion to avoid duplication of effort.