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up vote 1 down vote favorite

I've got this code started as an assignment for class. We are to set up a class that will create an array of objects. We are supposed to have two constructors so the array can be initialized with a given size, or a default size of 100. Here is what I have so far

public class Set {

    private int maxObjects;
    private int sz;

    public Set(int maxObjects) {
        this.maxObjects = maxObjects;
        this.sz = maxObjects;
        Object[] a = new Object[maxObjects];
    }

    public Set() {
        this.maxObjects = 100;
        this.sz = 100;
        Object[] a = new Object[100];
    }

    public void add(Object object) {
        a[0] = Object;
    }

The problem I'm coming across is that it's not seeing a as a variable in the add() method. Also the arrays are supposed to be initialized with a set capacity, but as empty, and I'm not sure how to do that.

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4 Answers 4

up vote 1 down vote accepted

It's not seeing a as a variable in the add() method

Your problem is that a is declared in the Set constructor, so it's local to the constructor. This means that it can't be used outside of the constructor (such as from the add method).

The solution is pretty simple. Declare a with your other fields:

public class Set {

    private int maxObjects;
    private int sz;
    private Object[] a;

    public Set(int maxObjects) {
        this.maxObjects = maxObjects;
        this.sz = maxObjects;
        this.a = new Object[maxObjects];
    }

    public Set() {
        this.maxObjects = 100;
        this.sz = 100;
        this.a = new Object[100];
    }

    public void add(Object object) {
        a[0] = object;
    }
}

I recommend reading up on scopes in Java (a simple Google search will produce help there).

Also the arrays are supposed to be initialized with a set capacity, but as empty.

This is done by default. For an array of any non-primitive type, the array's elements are all initialized to null. Here, an all-null array is essentially an empty array. Calling new Object[capacity] is doing this for you.

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Thanks for the help! –  Xander Sep 26 '12 at 2:54
up vote 4 down vote

You should define a the same place you defined maxObjects and sz. 

private Object[] a;

Then place this in your constructors:

a = new Object[maxObjects];

Read about the scope of variables.

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3  
You don't need the second half of your statement, that would initialize the array with a size of 0. The array reference 'a' will be reassigned in the constructor. Assigning it in the top will just cause more work for the garbage collector. –  Scott Sep 26 '12 at 2:46
    
You are right Scott. I've changed it. –  davidXYZ Sep 26 '12 at 2:48
    
Looks promising now +1 :-) –  nIcE cOw Sep 26 '12 at 3:48
up vote 2 down vote

You are very close to having this correct. The problem you have is related to the scope of variables. Examine closer the scope of variable a.

Also you have a typo in your add method. 

a[0] = Object

should be 

a[0] = object

(with a lower case o)

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Thanks very much! –  Xander Sep 26 '12 at 2:43
up vote 1 down vote

Couple of things:

  • a should be a field
  • your default constructor should call the other constructor
  • typo

Try this:

public class Set {

    private int maxObjects;
    private int sz;
    private Object[] a;

    public Set(int maxObjects) {
        this.maxObjects = maxObjects;
        this.sz = maxObjects;
        a = new Object[maxObjects];
    }

    public Set() {
        this(100);  // fyi, this is the syntax for calling another constuctor
    }

    public void add(Object object) {
        a[0] = object;
    }
}
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