java arraylist null pointer exception when adding

Question

Could someone please help me understand why i'm getting a null pointer when adding to my array list. Im trying to make it so that it will change the text on a button when it is clicked, however my ArrayList does not seem to be adding the things into it?

public class Game {

private GUI gui;
private ArrayList<String> pairs;
boolean clicked; 

public Game() {
    gui = new GUI(this);
    clicked = false; 
    ArrayList<String> pairs = new ArrayList<String>();

}

public void addPairs() {
    pairs.add("dog"); // where i get the null pointer
    pairs.add("dog");
}

Elliott Frisch · Accepted Answer · 2016-03-31 14:28:50Z

3

You are shadowing the class field pairs in your Game constructor. Change

ArrayList<String> pairs = new ArrayList<String>();

to

pairs = new ArrayList<String>();

or with the diamond operator like

pairs = new ArrayList<>();

answered Mar 31, 2016 at 14:28

Elliott Frisch

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Andrew · Accepted Answer · 2016-03-31 14:38:57Z

2

You should initialize the class member pairs

this.pairs = new ArrayList<>(); // this may be omitted

instead of creating a new local list

ArrayList<String> pairs = new ArrayList<String>();

edited Mar 31, 2016 at 14:38

answered Mar 31, 2016 at 14:29

Andrew

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