Question

我正在尝试使用 afreechart 绘制折线图，我已经使用SharedPreferences保存了x和y点。但是，当我尝试从共享偏好中获取一些信息时，我得到了一个错误，我怀疑它与静态和非静态方法无法协同工作，但我无法弄明白。

所以这里是我添加x和y点的类：

public class DrawView extends DemoView{

static int round;
static int score;

public DrawView(Context context) {
    super(context);

    final AFreeChart chart = createChart();

    setChart(chart);
}

private static XYSeriesCollection createDataset() {
    XYSeries xyS1 = new XYSeries("Progress", true, false);

     //Here I try to get the info from SharedPreferences via another class

    Stat t = new Stat();
    round =t.getround();
    for(int i = 0; i < round; i++){
        score = t.getscore(i);
        xyS1.add(i, score);
    }


    XYSeriesCollection xySC = new XYSeriesCollection();
    xySC.addSeries(xyS1);


    return xySC;
}


/**
 * Creates a sample chart.
 * @param dataset the dataset.
 * @return A sample chart.
 */
private static AFreeChart createChart() {
    XYDataset dataset = createDataset();
    AFreeChart chart = ChartFactory.createXYLineChart(
            "Statistics",
            "Rounds",
            "Points",
            dataset,
            PlotOrientation.VERTICAL,
            false,
            true,
            false);
    XYPlot plot = (XYPlot) chart.getPlot();
    XYLineAndShapeRenderer renderer = new XYLineAndShapeRenderer();
    renderer.setSeriesLinesVisible(0, true);
    plot.setRenderer(renderer);
    NumberAxis domainAxis = (NumberAxis) plot.getDomainAxis();
    domainAxis.setUpperMargin(0.2);

    // add some annotations...
    XYTextAnnotation annotation = null;
    Font font = new Font("SansSerif", Typeface.NORMAL, 12);
    annotation = new XYTextAnnotation("Progress", 96, 57);
    annotation.setFont(font);
    annotation.setTextAnchor(TextAnchor.HALF_ASCENT_LEFT);
    plot.addAnnotation(annotation);


    return chart;
}
}

以下是我尝试从SharePreferences获取信息的课程：

public class Stat extends Activity {

public static String filenamestat = "Stat";
SharedPreferences someStat;
/**
 * Called when the activity is starting.
 * @param savedInstanceState
 */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    DrawView mView = new DrawView(this);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(mView);

}

public int getround(){
    //Here is the error 
    someStat = getSharedPreferences(filenamestat, 0);
    String roundtaker = someStat.getString("round", "0");
    int round = Integer.parseInt(roundtaker);
    return round;
}


public int getscore(int a){
    String scoretaker = Integer.toString(a);
    String stringscore = someStat.getString(scoretaker, "0");
    int score = Integer.parseInt(stringscore);
    return score;
}        
}

LogCat

07-29 20:09:47.422: E/AndroidRuntime(414): FATAL EXCEPTION: main
07-29 20:09:47.422: E/AndroidRuntime(414): java.lang.RuntimeException: Unable to start activity ComponentInfo{at com.the.package/at com.the.package.Stat}: java.lang.NullPointerException
07-29 20:09:47.422: E/AndroidRuntime(414):  at         android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
07-29 20:09:47.422: E/AndroidRuntime(414):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
07-29 20:09:47.422: E/AndroidRuntime(414):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
07-29 20:09:47.422: E/AndroidRuntime(414):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
07-29 20:09:47.422: E/AndroidRuntime(414):  at android.os.Handler.dispatchMessage(Handler.java:99)
07-29 20:09:47.422: E/AndroidRuntime(414):  at android.os.Looper.loop(Looper.java:123)
07-29 20:09:47.422: E/AndroidRuntime(414):  at android.app.ActivityThread.main(ActivityThread.java:4627)
07-29 20:09:47.422: E/AndroidRuntime(414):  at java.lang.reflect.Method.invokeNative(Native Method)
07-29 20:09:47.422: E/AndroidRuntime(414):  at java.lang.reflect.Method.invoke(Method.java:521)
07-29 20:09:47.422: E/AndroidRuntime(414):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-29 20:09:47.422: E/AndroidRuntime(414):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-29 20:09:47.422: E/AndroidRuntime(414):  at dalvik.system.NativeStart.main(Native Method)
07-29 20:09:47.422: E/AndroidRuntime(414): Caused by: java.lang.NullPointerException
07-29 20:09:47.422: E/AndroidRuntime(414):  at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:146)
07-29 20:09:47.422: E/AndroidRuntime(414):  at at com.the.package.Stat.getround(Stat.java:33)
07-29 20:09:47.422: E/AndroidRuntime(414):  at at com.the.package.DrawView.createDataset(DrawView.java:50)
07-29 20:09:47.422: E/AndroidRuntime(414):  at at com.the.package.DrawView.createChart(DrawView.java:71)
07-29 20:09:47.422: E/AndroidRuntime(414):  at com.the.package.DrawView.<init>(DrawView.java:38)
07-29 20:09:47.422: E/AndroidRuntime(414):  at com.the.package.Stat.onCreate(Stat.java:26)

Answer 1

为什么不在你认为不起作用的代码之前打破，并且通过查看你的成员变量等单步执行。看看你是否能发现错误。你有一个调试器，所以我建议你应该使用它。：）

Answer 2

我的猜测是没有名称stat的共享首选项，因此someStat始终为null

Answer 3

您的Stat课程延伸Activity，但您将其创建为new Stat()。你不能为Activity。

这样做

相反，您可以使用

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());

Answer 4

请检查您的Manifest.xml文件中是否有所有活动的条目。如果你在帖子中显示它会很有帮助。

Answer 5

您正在尝试从共享首选项中获取值，但您没有在共享首选项中添加任何值。这就是 NullPointerException 的原因。

请检查您的方法getround（）

SharedPreferences NullPointer异常，静态和非静态方法

5 个答案: