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我正在尝试使用我的 android 应用程序创建一个文件。我需要写入特定类中的文件。下面列出了我目前拥有的代码。nullpointerexception当它试图写入文件时，我不断收到一个。下面列出了确切的错误。我是android新手，所以请详细说明。

有人可以告诉我我做错了什么以及如何解决这个问题吗？

//the class where i need to create and write to the file
public class DataRobot {
    Context tThis;
    FileOutputStream fOut;
    OutputStreamWriter writer;

    public DataRobot(SmartApp smartApp) extends Activity {
        tThis = (Context) smartApp;
    }
    public void onCreate(Bundle savedInstanceState) {
        try {
            //file = new File("/sdcard", "test.csv");
            fOut = openFileOutput("test.csv", MODE_WORLD_READABLE);
            writer = new OutputStreamWriter(fOut);
        } catch (NullPointerException e) {
            e.printStackTrace();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public void analyzeData(SmartDataObject temp) {
        data = temp;
        try {
                //the following line is where the error is occurring.
            writer.write(Double.toString(data.getHeartRate()));
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
     }
}   

//the error
03-26 03:47:53.924: WARN/System.err(273): java.lang.NullPointerException
03-26 03:47:53.934: WARN/System.err(273):     at cpe495.smartapp.DataRobot.analyzeData(DataRobot.java:80)
03-26 03:47:53.946: WARN/System.err(273):     at cpe495.smartapp.SmartApp$1.dataReceivedReceived(SmartApp.java:49)
03-26 03:47:53.956: WARN/System.err(273):     at cpe495.smartapp.ConnectDevice.fireDataReceivedEvent(ConnectDevice.java:79)
03-26 03:47:53.956: WARN/System.err(273):     at cpe495.smartapp.ConnectDevice.run(ConnectDevice.java:46)
03-26 03:47:53.965: WARN/System.err(273):     at java.lang.Thread.run(Thread.java:1096)
4

3 回答 3

3

看起来您正在构造函数中执行此操作。你不能那样做。您需要等到 onCreate()，即您知道对象已被初始化。

于 2011-03-26T02:12:44.260 回答
0

你应该通过内容提供者访问文件，我认为路径是错误的......看看这个文件教程

于 2011-03-26T00:16:25.223 回答
0

以下代码是我想出的并且正在工作。抱歉这么久才发帖。 

public class DataRobot {
    Context tThis; //was enabled for preferences
    private FileOutputStream fOut;
    private OutputStreamWriter writer;

    public DataRobot(SmartApp smartApp) {
        tThis = (Context) smartApp;

    }

    public void analyzeData(SmartDataObject temp) {
        try {
            fOut = tThis.openFileOutput("test.csv", tThis.MODE_APPEND);
            writer = new OutputStreamWriter(fOut);
        } catch (NullPointerException e) {
            e.printStackTrace();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            writer.write("Test");//0
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}
于 2011-04-04T23:54:08.023 回答