Question

package survival;

public class World {
    private int width;
    private int height;

    private int distancePenalty = 2;
    private int turnPenalty = 4;
    private double hungerPenalty = 0.1;

    private Plant[] plants;
    private Herbivore[] herbivores;
    private Carnivore[] carnivores;

    private int[] newestHerbivore;
    private int[] newestCarnivore;

    public World(int plantNumber, int herbivoreNumber, int carnivoreNumber, int width, int height) {
        this.width = width;
        this.height = height;

        plants = new Plant[plantNumber];
        for (int i = 0; i < plantNumber; i++) {
            plants[i] = new Plant(Math.random() * width, Math.random() * height);
        }        
        herbivores = new Herbivore[herbivoreNumber];
        for (int i = 0; i < herbivoreNumber; i++) {
            herbivores[i] = new Herbivore(Math.random() * width, Math.random() * height);

            //////////// This line causes java.lang.NullPointerException
            newestHerbivore[i] = 1;
        }
        carnivores = new Carnivore[carnivoreNumber];
        for (int i = 0; i < carnivoreNumber; i++) {

            //////////// This line causes java.lang.NullPointerException
            newestCarnivore[i] = 1;
        }        
    }
}

Why is this line:

newestHerbivore[i] = 1;

an element causing an exception?

Answer 1

You need to initialize the array before you assign to it:

herbivores = new Herbivore[herbivoreNumber];
newestHerbivore = new int[herbivoreNumber];

BTW: arrays are cumbersome and inflexible. In Java, you should use Collections instead. In most cases, ArrayLists will do fine as a replacement for array code.