陷入while循环/索引越界异常

Question

I am trying to figure out when a step occurs. So I have written a method called countSteps to do this. The problem with it is that I get stuck in it because with the while loop I keep getting new data and I don't think it ever returns back to onSensor. I also get an error called indexoutofboundsexception: invalid index 2,size 2.

So my first question is are there any other ways to implement the method I have without the while loop? Second is how can I fix the indexoutofboundsexception. public class MainActivity extends Activity implements SensorEventListener {

    private SensorManager mSensorManager;
    private Sensor mRotationVector;
    private Sensor mAccelerometer;
    private TextView mTextView4;
    private TextView mTextView5;
    private TextView mTextView6;
    private TextView mTextView7;
    private TextView mTextView8;
    float a, b, c, d, x, y, z, xyz;
    float[] retVals = new float[3];
    float avg = 10;
    float factor = (float) 1.15;
    ArrayList<Float> accelData = new ArrayList<Float>();
    public int peakCounter = 0;
    public int underAvgCounter = 0;

    public void countSteps() {
        int n = 0;
        float controlPoint = accelData.get(0);
        while (accelData.iterator().hasNext()) {
            if (accelData.get(n) != accelData.get(n + 1)) {
                if (accelData.get(n) > accelData.get(n + 1)) {
                    if (accelData.get(n) < controlPoint) {
                        n++;
                    } else {
                        if (accelData.get(n) < avg * factor) {
                            underAvgCounter++;
                        }
                        peakCounter++;
                        n++;
                    }
                } else {
                    controlPoint = accelData.get(n + 1);
                    n++;
                }
            } else {
                n++;
            }
            peakCounter -= underAvgCounter;
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView4 = (TextView) findViewById(R.id.textView4);
        mTextView5 = (TextView) findViewById(R.id.textView5);
        mTextView6 = (TextView) findViewById(R.id.textView6);
        mTextView7 = (TextView) findViewById(R.id.textView7);
        mTextView8 = (TextView) findViewById(R.id.textView8);
        mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
        mAccelerometer = mSensorManager
                .getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        accelData.add((float) 0);
    }

    protected void onResume() {
        super.onResume();
        mSensorManager.registerListener(this, mAccelerometer,
                SensorManager.SENSOR_DELAY_GAME);
    }

    protected void onPause() {
        super.onPause();
        mSensorManager.unregisterListener(this);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onSensorChanged(SensorEvent event) {
            x = event.values[0];
            y = event.values[1];
            z = event.values[2];
            xyz = (float) Math.sqrt((x * x) + (y * y) + (z * z));
            accelData.add(xyz);
            mTextView7.setText("magnitude accel " + xyz);
            countSteps();
            mTextView8.setText("steps " + peakCounter);

    }
}

Answer 1

 while (accelData.iterator().hasNext()) {

Is always true if you have at least 1 element in collection.

1. accelData.iterator() creates new iterator every time,
2. accelData.iterator().hasNext() checks for the first element to be in collection every time
3. n++ is executed until end is reached and
4. accelData.get(n + 1) throws IndexOutOfBoundsException

One of the things you can do is to use for loop.

 for (int n = 0; n < accelData.size() - 1; n++) {  // size-1 is used since you're accessing n+1 index
     if (accelData.get(n) != accelData.get(n + 1)) {
         if (accelData.get(n) > accelData.get(n + 1)) {
                if (accelData.get(n) >= controlPoint) {
                    if (accelData.get(n) < avg * factor) {
                        underAvgCounter++;
                    }
                    peakCounter++;                        
                }
        } else {
            controlPoint = accelData.get(n + 1);
        }
    } 
}
peakCounter -= underAvgCounter;

Answer 2

Iterator it = accelData.iterator();
while(it.hasNext()) {
  float elem = it.next();
  ...
}

Answer 3

With IndexOutOfBounds, the following code code snippets is an issue:

accelData.get(n) and accelData.get(n + 1)

You are doing a check for iterator().hasNext(), but it doesnt guarantee the existence of n+1 element.

Infinite loop, I think is because you never used iterator().next() to traverse to the next element. Please refer to a simple iterator code for better understanding.

And you can use any looping techniques like for, do while, while and advanced for loops in java, it doesnt matter. All that matters is if you are doing it efficiently and its readable.