Playing with Arrays

[Stefan Jaeschke]

Hi experts,
i am playing a little bit around with arrays an I am stuck on this (my intention was to create some code as a framework for sorting experiments):

class MyNumbers {
 double a[] = new double[100];
 void fillArray () {
 for (int i = 0; i < 100; i++)
 a[i] = Math.random();
 }
 void showMe() {
 System.out.println(a[1]);
 }
 }
 
 public class MyArray {
 public static void main (String [] args) {
 MyNumbers z = new MyNumbers();
 z.fillArray();
 z.showMe();
 }
}

The Code works fine but I wanted to put some flexibility into it:

class MyNumbers {
double a[];
void fillArray (int howMany) {
double a[] = new double[howMany];
for (int i = 0; i < howMany; i++)
a[i] = Math.random();
}
void showMe() {
System.out.println(a[1]);
}
}
 
public class MyArray {
public static void main (String [] args) {
int sizeArray = 1000;
MyNumbers z = new MyNumbers();
z.fillArray(sizeArray);
z.showMe(); //bad idea!!!
}
}

Filling the Array still works fine but unfortunatly the showMe-Method raises an Exception: java.lang.NullPointerException (the array is empty - isn't it?).

So how can someone help me? I am looking for a way to fill and use the array without declaring the array-length inside the code. Any ideas (Thanks in advance)?


Bye Stefan

[Keith Lynn]

The problem is that the declaration of a inside the method fillArray shadows the instance variable a.

[sven studde]

You declared the member variable 'a' here:

class MyNumbers {
double a[];

but you never assigned anything to the member variable. This code:

void fillArray (int howMany) {
double a[] = new double[howMany];
for (int i = 0; i < howMany; i++)
a[i] = Math.random();
}

creates another variable called 'a' that is local to the fillArray() function. When the fillArray() function terminates, that local variable is destroyed. You used different code in your first example. Compare your two examples to see the difference.

[Cameron Wallace McKenzie]

I once asked my boss about arrays.

She told me to use vectors instead.

Heh..I kill me.

(Arrays == a raise)

Sorry. Wrong forum for my humour.

-Cameron McKenzie

[Stefan Jaeschke]

Hi,
Thank you for your help and please excuse my stupid question.

Unfortunatly I found no other way to solve my problem: I want to create two (or more) arrays with different lengths.

I could create the arrays inside the main, but that look much like old school non OO-Programming, right? Otherwise I could create a new class just for the array length:

class BadStyle {
static int x;
}
 
class MyNumbers {
 double a[] = new double[BadStyle.x];
 void fillArray () {
 for (int i = 0; i < BadStyle.x; i++)
 a[i] = Math.random();
 }
 void showMe() {
 System.out.println(a[1]);
 }
 }
 
 public class MyArray {
 public static void main (String [] args) {
 BadStyle.x = 100;
 MyNumbers z = new MyNumbers();
 z.fillArray();
 z.showMe();
 BadStyle.x = 1000;
 MyNumbers y = new MyNumbers();
 y.fillArray();
 y.showMe();
 
 System.out.println(z.a.length);
 System.out.println(y.a.length);
 }
}

I just can not believe that this is real OO ...

So - do you have any ideas? I hope there is an easy way I just can not see.

Thanks!

Bye Stefan

[John Cebedo]

your code confused me, why do you have the BadStyle class?

why not simply do:


public class MyNumbers {
private double[] a;

public MyNumbers(int x) {
a = new double[x];
for (int i = 0; i < a.length; i++)
a[i] = Math.random();
}

public void showMe() {...}
}

so then if you want an array of 100 numbers, you go:
MyNumbers n = new MyNumbers(100);
n.showMe();

[John Cebedo]

sorry for the bad formatting above...

public class MyNumbers {
   private double[] a;
 
   public MyNumbers(int x) {
      a = new double[x];
      for (int i = 0; i < a.length; i++)
         a[i] = Math.random();
   }
 
   public void showMe() {...}
}

[Burkhard Hassel]

Hi cowboys,

nice idea from John with the constructor taking the array length.
You would have two instances of the MyNumbers class, each with its own array of doubles, when you have to have two arrays, one with 100 and one with 1000 elements.

Another possibility is not to make the method void, but give a double [] as a return type for fillArray.

public class MyArray {
public static void main(String[] args) {
MyNumbers z = new MyNumbers();
int sizeArray = 100;
z.a = z.fillArray(sizeArray); // hundred
z.showMe(); // should be ok
sizeArray = 1000;
z.a = z.fillArray(sizeArray); // thousand
z.showMe();
}
}
 
class MyNumbers {
double[] a;
 
[B]double[][/B] fillArray(int howMany) {
double[] a = new double[howMany];
for (int i = 0; i < howMany; i++)
a[i] = Math.random();
return a;
}
 
void showMe() {
System.out.println(a[1]);
System.out.println("Array length = "
+ a.length);
}
}

BTW, you can but shouldn't code like
double a[] = new double[100];
Use
double [] a = new double[100];
instead, only for better readability.




Yours,
Bu.