Question

I’m trying to learn Java and basically my approach has been to take the procedural style I learned with python, and apply it to Java. So I never use classes and just put everything in a single class with many methods(which I just use as python functions). I think I’ve hit a problem, and need to bite the bullet and use classes, but I’m having trouble wrapping my head around how to do it.

To simplify my problem(ignore the poor design- it’s just to illustrate the point), I have a program that takes a list and within a for loop does some math on each item(in this case adds 1 to the value of the list). I only want it to do work on 2 items on the list and then stop(in this example it’s the first 2 items but in my real program it could be anywhere in the list). Here’s the working code that is similar to how I’m already doing it:

No Classes:

public class LearningClasses {
    public static void main(String[] args) {
        int[] list = new int[]{1,2,3,4,5,6,7,8,9,10};
        int[] data_list = new int[list.length];

        for (int current_location = 0; current_location<list.length;current_location++) {
            for (int i =0; i<100; i++){
                if (check_size(data_list) == false ) {
                    break;
                }
                data_list[current_location] = (list[current_location]+1);
            }
        }

        //its done now lets print the results
        for (Integer item : data_list) {
            System.out.println(item);
        }
    }

    private static boolean check_size(int[] data_list) {
        // TODO Auto-generated method stub
        int count = 0;
        for (int item : data_list) {
            if (item != 0) {
                count++;
                if (count>=2) {
                    break;
                }
            }
        }
        if (count>=2) {
            return false;
        } else {
            return true;
        }
    }
}

The problem with this code is although it works it’s inefficient because it calculates the count on every iteration of the second for loop. In my program I cannot put anything above the first for loop but I can put anything below it, so I thought instead of doing the count every time maybe I could use a class to somehow maintain state and just increment the number as oppose to recalculating every time.

With classes:

public class LearningClassesCounter {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int[] list = new int[]{1,2,3,4,5,6,7,8,9,10};
        int[] data_list = new int[list.length];
        for (int current_location = 0; current_location<list.length;current_location++) {
            //can only put commands in here. Nothing above.
            Counter checker = new Counter(data_list);
            System.out.println(checker.check_data());
            for (int i =0; i<100; i++){
                data_list[current_location] = (list[current_location]+1);
            }
        }

        //its done now lets print the results
        for (Integer item : data_list) {
            System.out.println(item);
        }
    }
}


class Counter {
    private int count;             // current value
    private boolean continue_or_not;
    private int[] data_list;

    // create a new counter with the given parameters
    public Counter(int[] data_list) {
        data_list = this.data_list;
        count = 0;
        continue_or_not = true;
    } 

    public boolean check_data() {
        // TODO Auto-generated method stub
        int count = 0;
        for (int item : data_list) {
            if (item != 0) {
                count++;
                if (count>=3) {
                    break;
                }
            }
        }

        if (count>=3) {
            return false;
        } else {
            return true;
        }
    }

    // increment the counter by 1
    public void increment() {
        count++;
    } 

    // return the current count
    public int value() {
        return count;
    } 
}

This doesn’t work because it thinks the data_list is a null pointer(I know I’m declaring it null, but if I make it private int[] data_list = data_list it doesn’t compile either). My ultimate goal is to have some kind of controls, in this case its limiting it to 2 items but I want to also add other limits like total value of al items cannot exceed X or cannot be lower than X and want to save CPU power by not having to do full calculations every time. So I think I need to be able to increment the values and then need to check that those increments haven’t exceeded thresholds.

Can anyone help me understand what I’m doing wrong? Am I only wrong with syntax; or am I designing this wrong?

Answer 1

//can only put commands in here. Nothing above.
Counter checker = new Counter(data_list);
System.out.println(checker.check_data());

When you are calling checker.check_data(), its trying to parse through the data_list, but its empty. So, it throws a NullPointerException. The data_list is empty because inside your constructor, you may need to initialize like this this.data_list = data_list instead of data_list = this.data_list (here this.data_list has no reference so NULL)

If you avoid that call, the output will be 2,3,4,5,6,7,8,9,10,11.