如何在Java中正确实例化二维数组

Question

I am having problems on how to instantiate bi-dimensional array of objects. I tried to demonstrate below a small sample to reproduce the error I am getting.

I have this class named Node that basically stores one character. This class is used inside the class named Test as a bi-dimensional attribute. I used some user input to establish the size of the array and instantiate it inside the instantiate() method. Then, I try to populate the map using a set method. However, the compiler gives me the following error message:

Eclipse Console Output:

Exception in thread "main" java.lang.NullPointerException at Test.populate(Main.java:44) at Main.main(Main.java:77)

My input was:

Enter height: 3 Enter width: 3

Below is the code I am using to reproduce this error:

Class Node:

class Node {
    private char content;

    Node(){
        this.content = ' ';
    } 

    Node(Node node){
        this.content = node.getContent();
    }

    //Setter
    public void setContent(char c) {
        this.content = c;
    }

    //Getter
    public char getContent() {
        return this.content;
    }
}

Class Test:

class Test {
    private Node[][] map;
    private int height, width;

    public void instantiate(){
        Scanner reader = new Scanner(System.in);
        System.out.println("Enter height: ");
        this.height = reader.nextInt();
        System.out.println("Enter width: ");
        this.width = reader.nextInt();
        map = new Node[height][width];
        reader.close();
    }

    public void populate(){
        for(int i=0;i<height;i++)
            for(int j=0;j<width;j++){
                if((i+j) %2 == 0)
                    map[i][j].setContent('a');
                else
                    map[i][j].setContent('b');
            }
        /*
         * a b a b
         * b a b a  ...
         * a b a b
         * b a b a
         *  . . .
         */
    }

    public void print(){
        for(int i=0;i<height;i++){
            for(int j=0;j<width;j++){
                System.out.print(map[i][j].getContent());
            }
        System.out.println();
        }   
    }

    public Node[][] getMap(){
        return this.map;
    }

}

Main method:

public class Main {

    public static void main(String[] args) {
        Test testing = new Test();
        testing.instantiate();
        testing.populate();
        testing.print();
    }
}

The code can also be seen here: http://pastebin.com/agFMmB38
I am still getting used to Java (coming from C++), so they have some differences that I couldn't figure it out yet.

Any help would be greatly appreciated. Thank you!

Answer 1

map[i][j] needs to be set to a new object, like map[i][j] = new Node('a') (well, if you had a constructor in Node which worked like that: it would be written Node(char a) { this.content = a; } ).

You cannot do map[i][j].setContent('a') because it is not a preexisting Node object.

Answer 2

You need to init an Node object before use setContent(char c) method.

public void populate() {
    for(int i=0;i<height;i++) {
        for(int j=0;j<width;j++) {
            map[i][j] = new Node();
            if((i+j) %2 == 0)
                map[i][j].setContent('a');
            else
                map[i][j].setContent('b');
        }
    }
}

Then use method getContent() to retrieve value what you want.

public void print() {
    for(int i=0;i<height;i++) {
        for(int j=0;j<width;j++) {
            System.out.print(map[i][j].getContent());
        }
        System.out.println();
    }   
}