Help! Object Arrays...

Question

Hi,
I'm a novice programmer trying to learn Java. I'm trying to solve a problem in a book I'm learning from - I can use arrays of primitive types with no problem, but I'm having some trouble with arrays of objects. To illustrate what I'm trying to do I'll write a very simple piece of code...I know this is cheesy but it's is only an example:
class Cat
     public String name;
     public int age;
     public static void main( String[] args )
          Cat fluffy = new Cat();
          fluffy.name = "fluffy";
          fluffy.age = 2;          
}In the above example, I've created a class called Cat. It has two fields, 'name' and 'age'. In the main() method I create the object 'fluffy' and set his age to '2'.
That's fine and dandy, but lets say I own more than one cat and I dont want to explicitly create each one like that, so I'll create an array that references objects of the type 'Cat':
class Cat
     public String name;
     public int age;
     public static void main( String[] args )
          Cat[] myPets = new Cat[3];
          myPets[0].name = "fluffy";
          myPets[0].age = 2;
          myPets[1].name = "garfield";
          myPets[1].age = 4;
          myPets[2].name = "heathcliff";
          myPets[2].age = 3;          
}In the main() method I create an array called myPets that references three Cat objects. Afterwards, I try to set the fields of each Cat object in the array (the 'name' and 'age'). This is where I'm having problems...when this code is run it produces a NullPointerException. So my question is...how do I set objects' individual fields when the objects are in an array?

Answer 1

You are having an array of Cats. Which means that the array can act as container for cats, and it contains three fields.
Cat pets[] = new Cat[3];
What you need to do now, is to put cats into the container. Only if there are cats in there, you can give those cats name. This means :
//make a new cat
Cat cat1 = new Cat();
// put cat into array
pets[0] = cat1;
// name cat within the array
pets[0].name = "Garfield";
pets[0].age = 3;
Hope this helps,
rh