Java array not staying initialized

Question

I've got an array in an array and want to initialize it with a for each loop.

// class variable  
Tree[][] trees;

// in constructor

this.trees = new Tree[length][with];

// initialize
for (Tree[] tree : this.trees){
    for(Tree tree2 : tree){
    tree2 = new Tree();
    System.out.println(tree2);
    }
}

for (Tree[] tree : this.trees) {
    for (Tree tree2 : tree) {
    System.out.println(tree2);
    }
}

What happens is that the first println prints initialized trees, so they got initialized. I thought everything is ok. But when I try to use these trees, I get a nullpointerexception. So I tried to loop through the arrays again, and the second println gives me null for every tree. How can this be? What am I missing here? Thank you!

Edit: Oh I'm very sorry, It wasn't the main but the constructor method the loops are placed.

you can't use this in the main method, because main is static. Also, this code tree2 = new Tree(); has no effect at all, because tree2 is just a local variable in the loop. There is no way to initialize objects the way you want with a for each loop. — jlordo
– jlordo, Commented Jan 24, 2013 at 9:53

Veger · Accepted Answer · 2013-01-24 09:56:08Z

3

tree2 is a local variable (available in the scope the loop), you assign the newly created Tree instance to that variable not to your array. Next you print the contents of that variable so it seems to work...

Instead you need to explicitly store the instance into trees, like this:

for (int l = 0; l < length; l++){
  for(int h = 0; h < height; h++) {
    trees[l][h] = new Tree();
    System.out.println(trees[l][h]);
  }
}

As you can see, it stores the instance in the array and uses the array to print the value.

answered Jan 24, 2013 at 9:56

Veger

38k11 gold badges110 silver badges118 bronze badges

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

Comments

jlordo · Accepted Answer · 2013-01-24 09:57:17Z

2

As per my comment above, this:

for (Tree[] tree : this.trees){
    for(Tree tree2 : tree){
    tree2 = new Tree(); // changes tha variable tree2, does not touch the array.
    System.out.println(tree2);
    }
}

has no effect. You need

for (int i = 0; i < length; i++) {
    for (int j = 0; j < width; j++) {
        trees[i][j] = new Tree(); // obviously changes the array content.
    }
}

answered Jan 24, 2013 at 9:57

jlordo

37.9k7 gold badges63 silver badges84 bronze badges

Comments

AlexR · Accepted Answer · 2013-01-24 09:58:04Z

1

Your problem is in the first loop:

tree2 = new Tree();

This line indeed creates instance of Tree but it stores it in local variable tree2 instead of in element of array this.trees.

You should iterate over array using inexed for loop:

for (int i = 0; i < trees.length; i++) {
    for (int j = 0; j < trees[i].length; j++) {
        threes[i][j] = new Tree();
    }
}

The difference between lines threes[i][j] = new Tree(); and tree = new Tree(); is that first stores instance in element of array and the second stores it in single variable.

Java references are not C pointers. The assignment of objects is assignment of reference by value.

answered Jan 24, 2013 at 9:58

AlexR

116k16 gold badges137 silver badges216 bronze badges

Comments

codeMan · Accepted Answer · 2013-01-28 03:49:29Z

1

in this for loop

for(Tree tree2 : tree)
{
    tree2 = new Tree();

you are doing the assignment to the local reference variable tree2, where as tree[i][j] doesn't get assigned any value.

edited Jan 28, 2013 at 3:49

answered Jan 24, 2013 at 9:58

codeMan

5,7603 gold badges29 silver badges53 bronze badges

Collectives™ on Stack Overflow

Java array not staying initialized

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related