Question

I am supposed to be creating a recursive function within Java that prints out all possible colors from a list of colors. E.G.{r, b, g ; r, g, b ; g, r, b ; g, b, r} etc…

I believe I had it figured out and my code is below. Unfortunatly I continue to recieve a null pointer exception within the base case of the recursion function and it never runs. I’ve included a test within my application test class to show that the list of colors is in fact created. I am unsure as to what is causing my error, or where I have erred in my code.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class SequentialPrint {
    private List colors;
    private List prefix;

    public SequentialPrint(List colors) {
        this.colors = colors;

    }

    public void printAllSequences(List colors) {
        int prefixCount = 0;
        int colorCount = 0;
        List prefix = new ArrayList();
        if (colors.isEmpty() || prefixCount == colors.size()) { //Base Case
            System.out.print("All Sequences Printed");
        }
        else {
            Object color = colors.remove(0);
            prefix.add(color); //add first color from colors list.
            prefixCount++; //increases prefix counter
            while (prefixCount <= colors.size() + 1) { //prints first rotation of colors
                System.out.println(prefix);
                System.out.print(colors);
                while (colorCount < colors.size() - 1) { //rotates list and prints colors, until entire list has been rotated once.
                    Collections.rotate(colors, 1);
                    System.out.println(prefix);
                    System.out.print(colors);
                }
            }
        }

    }

}



import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * @author Cash
 *
 */
public class SequentialPrintDemonstration {
    private SequentialPrint colorSequence;
    private List colorsList;
    private List prefixList;

    /**
     * @param args
     */
    public SequentialPrintDemonstration() {

        List colorsList = new ArrayList();

        colorsList.add("blue");
        colorsList.add("green");
        colorsList.add("red");
        colorsList.add("yellow");
        colorSequence = new SequentialPrint(colorsList);
        System.out.println(colorsList);


    }

    public void execute() {
        this.colorSequence.printAllSequences(colorsList);
    }

}

Answer 1

Your class has a private data member named prefix, which you don’t initialize in the constructor:

public class SequentialPrint {
    private List colors;
    private List prefix;

    public SequentialPrint(List colors){
        this.colors = colors;  // what if the array you pass in is null?
        // why not initialize prefix here?  it's null if you don't.
    }

Then you have a method that declares a local variable List named prefix:

public void printAllSequences(List colors){
    int prefixCount = 0;
    int colorCount = 0;
    List prefix = new ArrayList();  // this one shadows the private data member

Do you mean to use the private data member here?

Why do you pass in colors? How is that related to the private data member?

I don’t understand this code after a quick glance. Do you?