在android上创建一个文件

Question

我正在尝试用我的android应用程序创建一个文件。我需要在一个特定的类中写入该文件。下面列出了我目前为它编写的代码。当nullpointerexception试图将写入文件时，我一直得到它。准确的错误列在下面。我是android的新手，所以请详细说明。

谁能告诉我我做错了什么，我如何解决这个问题？

//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)

Answer 1

下面的代码是我计算出来的，并且正在起作用。很抱歉花了这么长时间才发出去。

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();
        }
    }
}

Answer 2

看起来，您正在构造函数中执行此操作。你不能这么做。您需要等到onCreate()，即当您知道对象已经初始化时。

Answer 3

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