CODEWITHSUNDEEP
javaarraysobject
Little
Little

Reputation: 3477

send an array of objects from one class to another

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?

Upvotes: 0

Views: 109

Answers (5)

Sunkas
Sunkas

Reputation: 9590

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());
}

Upvotes: 2

Danstahr
Danstahr

Reputation: 4319

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.

Upvotes: 0

Daniel Stucki
Daniel Stucki

Reputation: 203

You have to initiate the arrayOfSecond variable to avoid NullPointerException.

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

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

Upvotes: 0

Achintya Jha
Achintya Jha

Reputation: 12843

FirstClass f=null;

should be 

FirstClass f= new FirstClass(10);

f is set to null so you are getting NPE.

Upvotes: 0

maba
maba

Reputation: 48065

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.

Upvotes: 0

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