Array method returning all null elements instead of object elements

Question

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.

You are simply reassigning a local variable. If you get into the habit of declaring your local variables as final when they are constant then this error would be flagged up by the compiler. — Boris the Spider
– Boris the Spider, Commented Apr 20, 2014 at 18:51

assylias · Accepted Answer · 2014-04-20 18:41:57Z

4

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.

answered Apr 20, 2014 at 18:41

assylias

330k84 gold badges680 silver badges806 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user2964403 Over a year ago

Yes, r is a variable. But it is not needed. It is just supposed to point out to an array element. Mouse m1 = new Mouse(); Mouse m2 = m1; m1 = null; m2 will still hold the object. Thats why i think that losing r variable is not a problem.

Boris the Spider Over a year ago

@user2964403 it is a local variable which is a reference to the array element. Reassigning the variable does not change the value it is referencing. This is Java 101.

Amit · Accepted Answer · 2014-04-20 18:48:26Z

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
}

Collectives™ on Stack Overflow

Array method returning all null elements instead of object elements

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related