Empty arraylist when pass to another class

Question

I have made an arraylist in one class but when I pass it to another class it is empty. There are elements in one class but in another class it is empty.

Here is my code in one class.

System.out.println(mot);
Paint p=new Paint(mot, start );

Here is my code in another class

ArrayList<String> motif;
ArrayList<Integer> number;
NewJFrame j=new NewJFrame();

public Paint(ArrayList<String> motif,ArrayList<Integer> number) {
   initComponents();
   motif=new ArrayList();
   number=new ArrayList<>();
   j=new NewJFrame();
}

public void paint(Graphics g) {
   System.out.print(j.mot);
}

Please tell me how to solve my problem of empty arraylist?

What exactly is the problem? Where do you use motif or number? What is j.mot or just mot? — Elliott Frisch
– Elliott Frisch, Commented Dec 5, 2013 at 7:53
motif=new ArrayList(); will make the passed arraylist motif to be new instance of arraylist, and thus since its new , its empty. — Bnrdo
– Bnrdo, Commented Dec 5, 2013 at 7:56
I have not use motif or number. Mot is string arraylist in another class (NewJFrame). When I call it in another class i-e with j.mot it gives me empty arraylist. — Sarah
– Sarah, Commented Dec 5, 2013 at 7:59

sadhu · Accepted Answer · 2013-12-05 07:56:12Z

2

change these lines of code:

motif=new ArrayList();
number=new ArrayList<>();

to

this.motif = motif;
this.number = number;

answered Dec 5, 2013 at 7:56

sadhu

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Comments

Johan Nordli · Accepted Answer · 2013-12-05 07:55:39Z

1

This two lines make new instance of listes, try to change them:

motif=new ArrayList();
number=new ArrayList<>();

This would be better:

this.motif = motif;
this.number = number;

answered Dec 5, 2013 at 7:55

Johan Nordli

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Comments

Infinite Recursion · Accepted Answer · 2013-12-05 07:58:13Z

1

Try addAll:

 public Paint(ArrayList<String> pMotif,ArrayList<Integer> pNumber) {
    initComponents();
    motif=new ArrayList();
    motif.addAll(pMotif); //use addAll 
    number=new ArrayList<>();
    motif.addAll(pNumber); //use addAll

answered Dec 5, 2013 at 7:58

Infinite Recursion

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Comments

SQB · Accepted Answer · 2013-12-05 08:04:53Z

1

You're overwriting the copies of motif and number you got with new (empty) versions in these lines

motif=new ArrayList();
number=new ArrayList<>();

If you want to set your class members to an empty ArrayList, refer to them with this:

this.motif = new ArrayList();
this.number = new ArrayList<>();

If you want to set them to the ArrayLists passed in, do this:

this.motif = motif;
this.number = number;

Another option would be copying the values from the ArrayLists passed in:

this.motif = new ArrayList(motif);
this.number = new ArrayList<>(number);

edited Dec 5, 2013 at 8:04

answered Dec 5, 2013 at 7:58

SQB

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Comments

KP_JavaDev · Accepted Answer · 2013-12-05 08:07:08Z

1

you are passing the value to another class, so you should tell your compiler to use current object i.e this object.....

you are assigning the values using Constructor So, alwayz use either "this" or "super" ( which is first statement in Constructor )

use

this.motif = motif; this.number = number;

answered Dec 5, 2013 at 8:07

KP_JavaDev

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