Question

This code throws a null pointer exception at line 20, where the compareTo() is called. Any advice on how to get it to work?

package exam1review;

import java.util.Random;

public class ArrayTester {

/**
 * @param args
 */
public static void main(String[] args) {

    int result, max=0;

    Integer[] myArray = new Integer[10];
    Random rand = new Random();
    for (int i = 0; i < 10; i++) {
        myArray[i] = rand.nextInt();
        System.out.println(myArray[i]);
        while (i != myArray.length - 1) {
            result = myArray[i].compareTo(myArray[i+1]);
            if (result > 0)
                max = myArray[i];
            else
                max = myArray[i+1];
        }
    }
    System.out.println(max);
}

}

Answer 1

the problem is that you’re looping over i, but your comparison tries to compare to myArray[i+1] before it’s set.

Populate the array first, then look for the max value.

Like this:

package cruft;

import java.util.Random;

public class ArrayTester {

    /**
     * @param args
     */
    public static void main(String[] args) {

        Integer[] myArray = new Integer[10];
        Random rand = new Random();
        for (int i = 0; i < myArray.length; i++) {
            myArray[i] = rand.nextInt();
        }        
        for (Integer value : myArray) {
            System.out.println(value);
        }
        System.out.println(String.format("max value: %d", findMax(myArray)));
    }

    public static int findMax(Integer [] values) {
        int max = Integer.MIN_VALUE;

        if (values != null) {
            for (Integer value : values) {
                if (value.compareTo(max) >= 1) {
                    max = value;
                }
            }
        }

        return max;
    }
}