Java – Write and read multiple objects to a file

I’m designing a handwriting app for android.

I want to write the information (class LogInfo) to the log file every time the user presses the Enter button.

After that, I want to read the stored information.

This is part of my class with a custom write method :

public class LogInfo implements Serializable {

private static final long serialVersionUID = -5777674941129067422L;

public static List<Point[][]> strokes;
public static List<byte[]> codes;

 Only write and read methods shown

private void writeObject(ObjectOutputStream stream) throws IOException
{
    stream.defaultWriteObject();
    stream.writeInt(strokes.size());
    Point[][] pointsArray = null;
    for (int i = 0; i < strokes.size(); i++)
    {
        pointsArray = ((Point[][])strokes.get(i));
        stream.writeInt(pointsArray.length);
        for (int j = 0; j < pointsArray.length; j++)
        {
            stream.writeInt(pointsArray[j].length);
            for (int k = 0; k < pointsArray[j].length; k++)
            {
                stream.writeInt(pointsArray[j][k].x);
                stream.writeInt(pointsArray[j][k].y);
                stream.writeObject(elementData[i]);
            }
        }
    }

int size = codes.size();
    stream.writeInt(size);
    for (int i = 0; i < size; i++)
    {
        stream.write(codes.get(i));
    }
}

Here is the read method:

private void readObject(java.io.ObjectInputStream stream)
    {
        stream.defaultReadObject();
        int strokesSize = stream.readInt();
        for (int i = 0; i < strokesSize; i++)
        {
            int arrayXSize = stream.readInt();
            Point[][] points = new Point[arrayXSize][];
            for (int j = 0; j < arrayXSize; j++)
            {
                int arrayYSize = stream.readInt();
                points[j] = new Point[arrayYSize];
                for (int k = 0; k < arrayYSize; k++)
                    points[j][k] = new Point(stream.readInt(), stream.readInt());
            }
            strokes.add(points);
        }

int codesSize = stream.readInt();
        for (int i = 0; i < codesSize; i++)
        {
            byte[] buffer = new byte[3];
            stream.read(buffer, 0, 3);
            codes.add(buffer);
        }
    }

Works well when I save only one object in the file. When I try to save more, the read doesn’t work (it throws a StreamCorruptedException). It reads only one object in the while loop!

In the main class, I only used two simple methods:

// WRITE TO FILE
logInfo.writeLog();

 READ FROM FILE
ArrayList<LogInfo> logInfoArrayList = logInfo.readLog();

Defined as:

public void writeLog()
{
    File file = new File (Environment.getExternalStorageDirectory().getAbsolutePath(), "data.log");
    FileOutputStream fos;
    try {
        fos = new FileOutputStream(file, true);
        fos = openFileOutput(Environment.getExternalStorageDirectory().getAbsolutePath() + "/data.log", Context.MODE_APPEND);
        ObjectOutputStream os = new ObjectOutputStream(fos);
        os.writeObject(this);
        os.close(); 
    } catch (IOException e) {
         TODO Auto-generated catch block
        e.printStackTrace();
    }
}

public ArrayList<LogInfo> readLog()
{
    ArrayList<LogInfo> logInfoArray = new ArrayList<LogInfo>();

try{
        File file = new File (Environment.getExternalStorageDirectory().getAbsolutePath(), "data.log");
        FileInputStream fis  = new FileInputStream(file);
        ObjectInputStream reader = new ObjectInputStream(fis);  

LogInfo tempLogInfo = new LogInfo();
        while((tempLogInfo = (LogInfo)reader.readObject()) != null)
            logInfoArray.add(tempLogInfo);
        reader.close();
    } catch (Exception e) {
     TODO Auto-generated catch block
     e.printStackTrace();
    }

return logInfoArray;
}

Request Update:

//We use this class to not write a header in a file that already exist
class MyObjectOutputStream extends ObjectOutputStream {

public MyObjectOutputStream(OutputStream os) throws IOException {
        super(os);
      }

@Override
    protected void writeStreamHeader() {}
}

1. You can’t append to an existing file created with ObjectOutputStream, at least not effortlessly. There is a trick about extending the ObjectOutputStream and overriding the writeStreamHeader() method so as not to write the stream header a second time, but I don’t approve of it. You really should rewrite the entire file, perhaps as a list.

2. You don’t need all this code. Just make strokes and codes non-static and non-transient, and get rid of the readObject() and writeObject() methods.