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emmoore · Guest

Hello everybody. I am rather new to Java, and I'm struggling understanding a
program I have to write that needs to use a Comparator. I really don't
understand how to use one or where to put it.

My task is to write a method sort(d) that will sort an ArrayList of points by
x value if d=0 and by y value if d=1. I think I've managed to create the
ArrayList of points needed, but I feel as if I've hit a complete impasse when
it comes to figuring out how to sort in the necessary way.

I've tried doing this a few different ways, all of which are wrong, but maybe
you might help steer me in the correct direction.

One try:

public static ArrayList<Point> sort(final int d)
{
Comparator MyLittlePointComparator = new Comparator()
{
public int compare(Object o1, Object o2)
{
return compare( (Point)o1, (Point)o2);
}
public int compare(Point p, Point q)
{
int diff;

if (d==0)
diff = p.x - q.x;
else if (d==1)
diff = p.y - q.y;

return diff;
}
}
return Collections.sort(this_use_array, MyLittlePointComparator());

Another try:
public static ArrayList<Point> sort(final int d)
{
public class MyLittlePointComparator extends PS7File.EqComparator
implements Comparator<Point>
{
public int compare(Point p, Point q)
{
if (d == 0)
{
double pee = p.getX();
double kyew = q.getX();
}
else if (d == 1)
{
double pee = p.getY();
double kyew = q.getY();

}
if (pee < kyew) return -1;
if (pee == kyew) return 0;
return 1;
}
}
return Collections.sort(this_use_array, new MyLittlePointComparator());
}
}

I just have no idea where to even begin fixing this. The task seems so simple
(just sorting by x and y value), but I feel so lost. Thanks so much in
advance for your time and help.

Roedy Green · Guest

On Fri, 17 Mar 2006 01:40:45 GMT, "emmoore" <u19771@uwe> wrote, quoted
or indirectly quoted someone who said :

Knute Johnson · Guest

emmoore wrote:

Roedy Green · Guest

On Fri, 17 Mar 2006 01:40:45 GMT, "emmoore" <u19771@uwe> wrote, quoted
or indirectly quoted someone who said :

Patricia Shanahan · Guest

emmoore wrote:

emmoore via JavaKB.com · Guest

Thank you everybody for your suggestions (I am VERY appreciative). I have
been reading through both the links, and I think they are helping. I also
took the suggestion of making two different comparators (here I attempted
just the one for X values). Perhaps I am still going about it the wrong way,
but I managed to get a piece of code that finally compiled.

import java.io.*;
import java.util.*;
import java.awt.*;

public class PointCollection2
{
static ArrayList<Point> this_use_array;

/*Using the method provided in the PS7File class to turn a file into an
ArrayList of points*/
public static ArrayList<Point> PointCollection(File desired_file)
{
PS7File for_this_use = new PS7File(); //creates an object of type PS7File
so that the methods can be called in this class

ArrayList<Point> this_use_array = for_this_use.points_from_file
(desired_file);

return this_use_array; //the ArrayList of points needed for sorting is
created
}

public static ArrayList<Point> sort(int d)
{
Double pee, kyew;

if (d == 0)
{
class MyLittlePointComparator extends PS7File.EqComparator implements
Comparator<Point>
{
public int compare(Point p, Point q)
{
Double pee = p.getX();
Double kyew = q.getX();
return (pee).compareTo(kyew);
}
}
Collections.sort(this_use_array, new MyLittlePointComparator());
}
return this_use_array;
}
}

I write a tester program that also compiles.
import java.io.*;
import java.util.*;
import java.awt.*;

public class PointCollectionTest
{
public static void main(String[] arg)
{
PointCollection2 tester = new PointCollection2();

File tester_file = new File("C:\\Documents and Settings\\Elizabeth\\My
Documents\\Programming\\Programs\\randompoints.lst");

ArrayList<Point> tester_array = tester.PointCollection(tester_file);

ArrayList<Point> sorted_array = tester.sort(0);

System.out.print(sorted_array);
}
}

I'm not having much luck when I actually try to implement the program after
compiling. I get a very nasty red error message in Dr. Java that reads:
NullPointerException:
at java.util.Collections.sort(Unknown Source)
at PointCollection2.sort(PointCollection2.java:34)
at PointCollectionTest.main(PointCollectionTest.java:15)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)

I might have an idea what is wrong. I want to sort the arrayList (here,
this_use_array) made from the file of points with the method PointCollection.
I thought I fixed the problem by declaring this_use_array at the top before
writing any methods, but I guess that doesn't work?

I am not sure how to fix this.

--
Message posted via http://www.javakb.com

Oliver Wong · Guest

"emmoore via JavaKB.com" <u19771@uwe> wrote in message
news:5d73ad3410c15@uwe...