send an array of objects from one class to another

Question

I have two classes:

FirstClass{
    private int id;
    private SecondClass[] arrayofSecond;
    private int ind;
    public FirstClass(){}
    public FirstClass(int id){
         this.id=id;
    }
    public void addElements(SecondClass e){
         arrayofSecond[ind]=e;
         ind++;
    } 
    public SecondClass[] getArraySecondClass(){
        return arrayofSecond;
    }    
}

SecondClass{
    private int id;
    private SecondClass(){}
    private SecondClass(int id){
          this.id=id;
    }
    public int getId(){
          return id;
    }
}

Now I need to get the array that has been filled in the FirtsClass in the Main program, like this:

FirstClass f1=new FirstClass(10);
SecondClass s1=new SecondClass(10);
SecondClass s2=new SeconClass(20);
f1.addElements(s1);
f1.addElements(s2);
FirstClass f=null;
SecondClass s[];
s=f.getArraySecondClass();
for (int i=0;i<s.length;i++){
    System.out.println(s[i].getId());
}

The problem is that I get an error of Null pointer exception. I have read that maybe this is not a correct way to do this by the principle of "not talk to strangers", but I do not see another way to make this.

Any help?

Sunkas · Accepted Answer · 2013-04-26 12:43:53Z

2

Why are you creating a f variable when you already have f1?

FirstClass f1=new FirstClass(10);
SecondClass s1=new SecondClass(10);
SecondClass s2=new SeconClass(20);
f1.addElements(s1);
f1.addElements(s2);

SecondClass s[];
s=f1.getArraySecondClass();
for (int i=0;i<s.length;i++){
    System.out.println(s[i].getId());
}

edited Apr 26, 2013 at 12:43

answered Apr 26, 2013 at 12:36

Sunkas

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Comments

maba · Accepted Answer · 2013-04-26 12:35:56Z

0

You'll have to initialize the arrayofSecond.

In the constructor of FirstClass you'll have to do arrayofSecond = new SecondClass[10]; or any other length you desire.

answered Apr 26, 2013 at 12:35

maba

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Comments

Achintya Jha · Accepted Answer · 2013-04-26 12:37:16Z

0

FirstClass f=null;

should be

FirstClass f= new FirstClass(10);

f is set to null so you are getting NPE.

answered Apr 26, 2013 at 12:37

Achintya Jha

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Daniel Stucki · Accepted Answer · 2013-04-26 12:38:04Z

0

You have to initiate the arrayOfSecond variable to avoid NullPointerException.

change private SecondClass[] arrayofSecond; to e.g.

    private SecondClass[] arrayofSecond = new SecondClass[10];

answered Apr 26, 2013 at 12:38

Daniel Stucki

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Comments

Danstahr · Accepted Answer · 2013-04-26 12:42:32Z

0

First, you should never allow outer classes to access your private data fields directly. So instead of

public SecondClass[] getArraySecondClass(){
    return arrayofSecond;
}

you should do something like

public SecondClass[] getArraySecondClass(){
    return (SecondClass[])arrayofSecond.clone();
}

Besides that, you need to initialize arrayOfSecond in FirstClass constructor and

f.getArraySecondClass();

isnt't the best idea when f is null.

answered Apr 26, 2013 at 12:42

Danstahr

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