Java Array 列表在按钮内时不会更新

Question

I'm new to Java (and stackoverflow...), so this might be obvious but couldn't find similar when I searched for it (if I missed it please don't hesitate to just link me the solution!)

I'm struggling with trying to make my Array list update within a button. It keeps saying 'Cannot resolve symbol fileEntry' when I use it in onSensorChanged (I used to have it defined outside the button but it wouldn't update it when I clicked the starting button), and it also says 'Variable fileEntry is never used' within the button. See code below :

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    starting_button = (Button) findViewById(R.id.button_start);
    starting_button.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {

            ArrayList fileEntry = new ArrayList();

And this is where I am trying to use it:

@Override
public void onSensorChanged(SensorEvent sensorEvent) {

    fileEntry.add(sensorEvent.values[0] + "," + sensorEvent.values[1] + "," + sensorEvent.values[2] + "\r");
}

What I would like it to do is to have a new fileEntry (ie clear it) every time I click on the starting button.

I know my code is far from being perfect, sorry in advance!

Answer 1

You should declare your ArrayList as a property in your activity class like this in order to use it in your different methods :

class MainActivity extends AppCompatActivity() {
    private ArrayList fileEntry;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        fileEntry = new ArrayList();

        starting_button = (Button) findViewById(R.id.button_start);
        starting_button.setOnClickListener(new View.OnClickListener() {
    }

    @Override
    public void onClick(View v) {
        // Do something with your fileEntry
    }

    @Override
    public void onSensorChanged(SensorEvent sensorEvent) {
        fileEntry.add(sensorEvent.values[0] + "," + sensorEvent.values[1] + "," + sensorEvent.values[2] + "\r");
    }    
}

The scope of your property will now be limited to your class, and not simply your onClick method.