Android :: Android Writing Logs To Text File

I'm Trying to Write Logs to Custom Log.txt File on Android File using this code of Mine but then this method creates file but contains nothing. Basically I want to read previous contents of the file and then append my data with the existing content. The Code is as follows :
public static void write(String str)
InputStream fileInputStream = null;
FileOutputStream fileOutpurStream = null;
try { FileInputStream = new FileInputStream(file);
fileOutpurStream = new FileOutputStream(file);
if(file.exists())
{int ch = 0;
int current = 0;
StringBuffer buffer = new StringBuffer();
while((ch = fileInputStream.read()) != -1)
{buffer.append((char) ch);
current++;
}byte data[]=new byte[(int)file.length()];
fileInputStream.read(data);
fileOutpurStream.write(data);
fileOutpurStream.write(str.getBytes(),0,str.getBytes().length);
fileOutpurStream.flush();
} else
{file.createNewFile();
fileOutpurStream.write(str.getBytes(),0,str.getBytes().length);
fileOutpurStream.flush();
}catch(Exception e)
{e.printStackTrace();
}finally
{try
{fileInputStream.close();
fileOutpurStream.flush();
fileOutpurStream.close();
fileOutpurStream = null;
fileInputStream = null;
}catch (IOException e)
{// TODO Auto-generated catch block
e.printStackTrace();}}