java - Android App 中的 NullPointerException

Question

我正在尝试创建一个应用程序，它将使用 AndroidPlot 库绘制存储在 raw 中的 WAV 文件的波形。然后，首先我要做的是获取波形的字节数组。我猜它是以 2 个字节为对存储的，我尝试获取它们中的每一个的振幅，并将其存储在一个 int 数组中，该数组稍后将转换为一个 Number 数组。我现在只想尝试字节数组的前 100 个样本。这是我到目前为止的代码:

    @Override
    protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);

    Number[] n = new Number[51];
    int[] array = new int[51];

    try {
        byte [] payload = IOUtils.toByteArray(this.getResources().openRawResource(R.raw.radio));
        for (int i = 0; i <= 100; i+=2) {
            // convert byte pair to int
           int j=i/2; 
           int audioSample = (int) ((payload[i+1] & 0xff) << 8) | (payload[i] & 0xff)/ 32767;
            array[j]=audioSample;
            n[j] = (Number)array[j];
            j++;
        }
    } catch (NotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

     XYSeries series1 = new SimpleXYSeries(
                Arrays.asList(n),          // SimpleXYSeries takes a List so turn our array into a List
                SimpleXYSeries.ArrayFormat.Y_VALS_ONLY, // Y_VALS_ONLY means use the element index as the x value
                "Series1");

    LineAndPointFormatter series1Format = new LineAndPointFormatter(Color.RED, Color.GREEN, Color.BLUE, null);


    // add a new series' to the xyplot:
    plot.addSeries(series1, series1Format);

    // reduce the number of range labels
    plot.setTicksPerRangeLabel(3);
    plot.getGraphWidget().setDomainLabelOrientation(-45);


}

当我尝试编译代码时出现 NullPointerException 错误。我猜是 tha 数组出了问题。请你帮助我好吗？我对 Java 很陌生

02-06 18:45:30.946: E/AndroidRuntime(2642): FATAL EXCEPTION: main
02-06 18:45:30.946: E/AndroidRuntime(2642): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.representaaudio/com.example.representaaudio.MainActivity}: java.lang.NullPointerException
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.os.Handler.dispatchMessage(Handler.java:99)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.os.Looper.loop(Looper.java:137)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.app.ActivityThread.main(ActivityThread.java:5039)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at java.lang.reflect.Method.invokeNative(Native Method)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at java.lang.reflect.Method.invoke(Method.java:511)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at dalvik.system.NativeStart.main(Native Method)
02-06 18:45:30.946: E/AndroidRuntime(2642): Caused by: java.lang.NullPointerException
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.util.TypedValue.applyDimension(TypedValue.java:327)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at com.androidplot.util.PixelUtils.dpToPix(PixelUtils.java:103)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at com.androidplot.xy.LineAndPointFormatter.a(LineAndPointFormatter.java:101)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at com.androidplot.xy.LineAndPointFormatter.<init>(LineAndPointFormatter.java:63)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at com.androidplot.xy.LineAndPointFormatter.<init>(LineAndPointFormatter.java:74)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at com.example.representaaudio.MainActivity.onCreate(MainActivity.java:76)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.app.Activity.performCreate(Activity.java:5104)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
02-06 18:45:30.946: E/AndroidRuntime(2642):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
02-06 18:45:30.946: E/AndroidRuntime(2642):     ... 11 more

Answer 1

如果你正在使用 Plotstuff pixelUtils，你应该先调用 init 方法

 /**
 * Recalculates scale value etc.  Should be called when an application starts or
 * whenever the screen is rotated.
 */
public static void init(Context ctx) {
    //DisplayMetrics dm = ctx.getResources().getDisplayMetrics();
    //SCALE = dm.density;
    //X_PIX = dm.widthPixels;
    //Y_PIX = dm.heightPixels;
    metrics = ctx.getResources().getDisplayMetrics();

}