help with with my null error

[zhangster]

public class Partition3 
{

public static void main(String[] args) 
    {
    int number;
    Scanner in = new Scanner(System.in);
    System.out.println("Please insert the number to be partitioned");
    number = in.nextInt();
    System.out.println(number);
    generator a = new generator(number);
    
}
}
class generator
{
	public generator(int anumber)
	{
		ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
		Partition(anumber);
	}
public void print(ArrayList<Integer> x) {
    for (int j = 0; j < x.size(); j++)
    	System.out.println(x.get(j) + " " +  x.get(j + 1) + "\n" + " ");
        //System.out.println("%d%c", x[j], j + 1 == x.size() ? '\n' : ' ');
}

public boolean equal(ArrayList<Integer> a, ArrayList<Integer> b) {
    if (a.size() != b.size()) return false;
    for (int i = 0; i < a.size(); i++)
        if (a.get(i) != b.get(i)) return false;
    return true;
}

public void add(ArrayList<Integer> x) {
	Collections.sort(x);
	    for (int i = 0; i < ret.size(); i++)
        if (x.equals(ret.get(i))) return;
    ret.add(x); //fvsfv
}

public ArrayList<Integer> construct(ArrayList<Integer> a, ArrayList<Integer> b) {
    for (int i = 0; i < b.size(); i++)
        a.add(b.get(i));
    return a;
}

public ArrayList<Integer> make(int a, int b) {
    ArrayList<Integer> x = new ArrayList<Integer>();
    x.add(a);
    x.add(b);
    return x;
}

public ArrayList<Integer> subset(ArrayList<Integer> x, int first, int last) { //[first,last)
   ArrayList<Integer> m = new ArrayList<Integer>();
    for (int i = first; i < last; i++)
        m.add(x.get(i));
    return m;
}

public void partition(ArrayList<Integer> x) {   
    add(x);
    for (int i = 0; i < x.size(); i++)
        for (int j = 1; j < x.get(i); j++)
            partition(construct(make(x.get(i) - j, j), construct(subset(x, 0, i), subset(x, i + 1, x.size()))));
}

void Partition(int N) {
    ArrayList<Integer> x = new ArrayList<Integer>();
    x.add(N);
    partition(x);

    for (int i = 0; i < ret.size(); i++)
        print(ret.get(i));
}
private  ArrayList<ArrayList<Integer>> ret;
}

when i run the program, i get a

Exception in thread "main" java.lang.NullPointerException
    at generator.add(Partition3.java:41)
    at generator.partition(Partition3.java:67)
    at generator.Partition(Partition3.java:76)
    at generator.<init>(Partition3.java:23)
    at Partition3.main(Partition3.java:14)

can anybody help me with this error? thanks

edit*
the program is to parse a number entered by the user
so if you wanted to parse 4, you should get:
4
3 1
2 2
2 1 1
1 1 1 1
but not necessarily in that order

[pbrockway2]

The stack trace is telling you that the add() method is using a variable whose value is null in a way that is invalid for null values.

Use System.out.println() or some other method, to figure out which variable is the culprit. Candidates are pretty much limited to x and ret. Once you have figured out which of these is null ask yourself "where did I initialise that variable?" ie where did you give it (or thought you gave it!) a nonnull value?

And then figure out why that didn't happen.

[zhangster]

i solved it..i initialized ret again in the constructor >.>

[pbrockway2]

Great, I'm glad you sorted it out.

(Actually what you did was declare another variable in the constructor so that the instance variable ret never got initialised. But I see what you mean.)