Please help with arrayLists

[StupidStudent]

Hi everyone

I am fairly new to java and ony know the reall basics (if statements, while loops) that kind of stuff however I have started to dabble in the crazy world of lists and collections.... I am using BlueJ (the latest version) and before you say it.......... I know everyone has told me this is not to good, however it is what the uni force use to use...;)
Any way i am writing this code just to add daysOfTheweek to an arraylist and then use another method to print to terminal;

import java.util.*;

public class Collections
{
ArrayList DaysOfTheweek;

public Collections() {
ArrayList DaysOfTheweek = new ArrayList();
}

public void addDays() {

DaysOfTheweek.a dd("Monday");
DaysOfTheweek.a dd("Tuesday");
DaysOfTheweek.a dd("Wednesday") ;
DaysOfTheweek.a dd("Thursday") ;
DaysOfTheweek.a dd("friday");
DaysOfTheweek.a dd("Saturday") ;
DaysOfTheweek.a dd("Sunday");
}

public void printDays() {

int i = 0;
while (i < DaysOfTheweek.s ize()-1) {
System.out.prin tln(""+DaysOfTh eweek.get(i));
i++;
}
}
}

The problem I keep getting is a null pointer execption?? I know this means it is trying to access an empty part of the list. I just cant see where I have gone wrong so PLEASE if anyone can help me out of this it would be much appreciated :)

THANKS !!!!

[prometheuzz]

Couple of tips:

- don't name you class Collections because this is also a class in the java.util package. A better name would be to call it MyCollections or CollectionsDemo;

- use Java's code conventions. The most impotant "rules" are: classes (and interfaces) start with a capital and variables start with a lowercase. So DaysOfTheweek should be better named daysOfTheweek.

Now your problem. Your ArrayList is null because you did not initiate it. You did initiate a local variable called DaysOfTheweek inside your constructor. This variable is only "known" inside that constructor because you added ArrayList in front of it.

Good luck.

[StupidStudent]

Thanks for super quick reply !!

Thanks for rules have changed code accordingly.... .. AWSOME advice works like a dream thanks !!!!!

......just quickly if anyone knows? when i run this code i get a compiler warning stating to recompile -Xlint:unchecked for details.... does anyone know what this means???

Thanks again for all your help

[prometheuzz]

Thanks for super quick reply !!

Thanks for rules have changed code accordingly.... .. AWSOME advice works like a dream thanks !!!!!

You're welcome.

......just quickly if anyone knows? when i run this code i get a compiler warning stating to recompile -Xlint:unchecked for details.... does anyone know what this means???

Thanks again for all your help

That's because you're using Java version 1.5 (or higher) and are not using generics.
Pre-Java 1.5 when writing:
[CODE=java]ArrayList list = new ArrayList();[/CODE]
you could add any Object (Integer's, String's, ...) in that list. And when accessing an element, you had to cast it back like this:
[CODE=java]String anElement = (String)list.ge t(4);[/CODE]

But now, since Java 1.5, you can write it like this:
[CODE=java]ArrayList<Strin g> list = new ArrayList<Strin g>(); // we can only store Strings in it
// ...
String anElement = list.get(4); // no need to cast since the compiler "knows" it is a String[/CODE]

To get back to your compiler warning: if you use Java 1.5 (or higher) and NOT use generics (the <...> brackets), the compiler gives you a warning about it.

Good luck.

[StupidStudent]

Thanks again got an exam tomorow so this has been awsome info life saver!!!

[r035198x]

Thanks again got an exam tomorow so this has been awsome info life saver!!!

Well, good luck with your exam StupidStudent.