CODEWITHSUNDEEP
javaarrays
user2964403
user2964403

Reputation: 13

Array method returning all null elements instead of object elements

I have the problem with the following method. It is supposed to create an array, fill it up with objects, and return the filled array. However, for a reason unknown to me, the returned array is full of null values.

 public Rodent[] getRodents(int amount) {
        Random rng = new Random();
        Rodent[] rodentArray = new Rodent[amount];

        for (Rodent r : rodentArray) {
            switch (rng.nextInt(3)) {
                case 0:
                    r = new Mouse(); // Subclass of Rodent
                    break;
                case 1:
                    r = new Gebril(); // Subclass of Rodent
                    break;
                case 2:
                    r = new Hamster(); // Subclass of Rodent
                    break;
                default:
                    r = new Rodent(); // For safety
            }
            System.out.println(r.getName()); // Test command inside the loop. Prints the name of all object correctly.
        }

        for (Rodent r : rodentArray) { // Test loop outside of previous loop.
            System.out.println(r.getName()); // Throws null pointer exception.
        }

        return rodentArray; // Returns null filled array
    }

EDIT:

Found my answer. Thanks. Now i need to stop thinking about the enhanced for loop like a pointer.

Upvotes: 1

Views: 72

Answers (2)

Amit
Amit

Reputation: 288

You are never putting newly created rodent object into array. Here is the correct code - 

public Rodent[] getRodents(int amount) {
    Random rng = new Random();
    Rodent[] rodentArray = new Rodent[amount];

    for(int count = 0; count < rodentArray.length; count++) {
            Rodent r;
            switch (rng.nextInt(3)) {
            case 0:
                r = new Mouse(); // Subclass of Rodent
                break;
            case 1:
                r = new Gebril(); // Subclass of Rodent
                break;
            case 2:
                r = new Hamster(); // Subclass of Rodent
                break;
            default:
                r = new Rodent(); // For safety
        }
        rodentArray[count] = r;
        System.out.println(r.getName()); // Test command inside the loop. Prints the name of all object correctly.
    }

    for (Rodent r : rodentArray) { // Test loop outside of previous loop.
        System.out.println(r.getName()); // Throws null pointer exception.
    }

    return rodentArray; // Returns null filled array
}

Upvotes: 0

assylias
assylias

Reputation: 328863

r is a local variable in your loop that does not exist outside a given iteration. Your code is equivalent to the code below where it is maybe easier to see that the r is discarded at the end of each iteration:

for (int i = 0; i < rodentArray.length; i++) {
    Rodent r = rodentArray[i];
    //your switch here
}

So in your case you should probably use a standard for loop and fill the array with rodentArray[i] = new Mouse(); or with other animals as appropriate.

Upvotes: 4

Related Questions