Button array causes app to crash

Question

I have multiple buttons in my app and I want to set the same text for every button using a loop. So I declared an array and filled it with the buttons in this manner:

public class MainActivity extends AppCompatActivity {
    Button b1,b2,b3;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        b1=(Button)findViewById(R.id.button);
        b2=(Button)findViewById(R.id.button2);
        b3=(Button)findViewById(R.id.button3);
    }

    Button[] buArray ={b1,b2,b3};

    void b1clicked(View view) {
        for (int i =0;i<3;i++) {
            buArray[i].setText("it works");
        }
    }
}

But when i try to set text for a button from the array like this

buArray[0].setText("some text");

This causes my app to crash and force closes And gives me an error like this in the the log "NullPointerException

Where did you put this line (`buArray[0].setText("some text")`)? — Sufian Latif, Jul 02 '18 at 16:44
You might like https://stackoverflow.com/q/218384/3166697 and https://stackoverflow.com/q/23353173/3166697 — Dima Kozhevin, Jul 02 '18 at 17:02
There real answer here: subtle details often matter a lot. Therefore I tend to recommend newbies to first learn a but of pure Java before going for Android. It is complicated enough to understand in which order Java initializes fields, without the additional complexity of the Android onWhatever() methods... — GhostCat, Jul 02 '18 at 18:13

score 1 · Answer 1 · answered Jul 02 '18 at 16:47

1

Since your Button[] buArray ={b1,b2,b3}; is outside of the methods, it is initialized when the Activity object is created. At that point, b1, b2 and b3 are all null.

Move the initialization into the onCreate() method and it will work.

Button b1,b2,b3;
Button[] buArray;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    b1=(Button)findViewById(R.id.button);
    b2=(Button)findViewById(R.id.button2);
    b3=(Button)findViewById(R.id.button3);
    buArray = new Button[] {b1, b2, b3};  
}

answered Jul 02 '18 at 16:47

Ridcully

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Wow i have alooot to learn . I think i tried that too and gave me the same result ... Am not home now ...will try it later – user10021973 Jul 02 '18 at 16:55
Rest assured, the answer is correct. – GhostCat Jul 02 '18 at 18:14

Button array causes app to crash

1 Answers1