Java, Array of Objects

Question

This is a trivial question, but my Java is rusty and it's got me stumped; I am getting a null-pointer exception. It may be obvious what I am trying to do based on the code below - but I will explain...

I need an array of objects and I don't want to create another file. For this trivial project, I do not want getters and setters. I have seen an example similar to below that uses a linked list based on a class that is located inside of another class. But, I am more proficient with arrays than linked lists, so I want to use arrays.

public class Ztest {

    Stuff[] st = new Stuff[2];

    public Ztest(){

    }

    class Stuff{
        public String x;
        public boolean y;
        public Stuff(){}
    }

    public static void main(String args[]){
        Ztest test = new Ztest();

        test.st[0].x = "hello";
        test.st[0].y = true;        
        test.st[1].x = "world";
        test.st[1].y = false;       

        System.out.println(test.st[0].x);
        System.out.println(test.st[0].y);
        System.out.println(test.st[1].x);
        System.out.println(test.st[1].y);       
    }
}

all you told what you did not want. tell us some what you want. — Nishant
– Nishant, Commented Apr 1, 2011 at 12:46
You might want to look at your array again, you forgot to initialise the array correctly. This would NPE. — Peter Lawrey
– Peter Lawrey, Commented Apr 1, 2011 at 12:50
Nishant, Joey & Joey: You are right (I goofed). I just edited the question with the null pointer exception I was getting. I'm new around here... would it be good practice to copy/paste the entire working script below the solution I picked as correct into a comment... or is that a bad idea. (I'm thinking it could be useful to someone). — John R
– John R, Commented Apr 1, 2011 at 12:57

Joey · Accepted Answer · 2011-04-01 12:48:55Z

3

You need to assign a value to st[0] and st[1] first:

test.st[0] = new Stuff();
test.st[1] = new Stuff();

answered Apr 1, 2011 at 12:48

Joey

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4 Comments

Logan Over a year ago

Can you help me here..I want to know why we need to initialize all the components separately when we have already created the array..

Joey Over a year ago

Logan: Creating the array does nothing more than reserve space for objects. It does not create the objects themselves. This is different from primitive types (int, double, etc.) which have some kind of value (which is them initialized to 0). But every object you want to exist you have to create explicitly.

Logan Over a year ago

Thanks for the explanation Joey. I have another doubt. Now when we have already allocated memory for the array, and are creating objects corresponding to each entry in the array, whether the newly created object use the memory associated with the array only or new memory will be allocated to them and then the new objects will be referenced from their corresponding entries in the array?

Joey Over a year ago

The array only holds references, the actual memory used for the objects is elsewhere.

Jeff Foster · Accepted Answer · 2011-04-01 12:46:43Z

2

Java allocates null for object values in new arrays. You'll need something like test.st[0] = new Stuff() before using it.

answered Apr 1, 2011 at 12:46

Jeff Foster

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Comments

Thomas · Accepted Answer · 2011-04-01 12:47:03Z

2

You need test.st[0]=new Stuff(); etc. since Stuff[] st = new Stuff[2]; creates an array but the elements (references) are still null.

In terms of C/C++ this would be Stuff** st = new Stuff*[2];, i.e. the st is an array of pointers to Stuff instances, whereas the pointers still point to nothing yet.

answered Apr 1, 2011 at 12:47

Thomas

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Comments

Joseph Ottinger · Accepted Answer · 2011-04-01 12:46:37Z

1

You need to put an instance of Stuff into test.st[0] and test.st[1].

answered Apr 1, 2011 at 12:46

Joseph Ottinger

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Comments

Peter Lawrey · Accepted Answer · 2011-04-01 12:52:02Z

0

You can try this if you want to use a list.

static class Stuff {
    public String x;
    public boolean y;

    // generated by my IDE.
    Stuff(String x, boolean y) {
        this.x = x;
        this.y = y;
    }

    // generated by my IDE.
    public String toString() {
        return "Stuff{" + "x='" + x + '\'' + ", y=" + y + '}';
    }
}

public static void main(String args[]) {
    List<Stuff> list = new ArrayList<Stuff>();

    list.add(new Stuff("hello", true));
    list.add(new Stuff("world", false));

    System.out.println(list);
}

prints

[Stuff{x='hello', y=true}, Stuff{x='world', y=false}]

answered Apr 1, 2011 at 12:52

Peter Lawrey

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