BufferedReader

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layout: default title: BufferedReader

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One of the most common computer file types is the text file: a file whose character-string records are separated by newlines (or similar record separator.)

• Lines vary in length from zero to any number of characters.

• Lines may be completely unstructured (plain old text), or delimited into fields by say, , characters (comma-separated values or CSV files), tabs (TSV), :, or some other pattern.

The BufferedReader class's readLine method allows us to easily read a text file one line at a time, retrieving each line regardless of length as a String rather than a char array.

BufferedReader needs a source of input: it just knows how to read lines, it doesn't care where they come from.

FileReader

A FileReader wraps a stream of bytes coming from a File; it doesn't know how to read lines, just how to retrieve chars from the file.

• It has a constructor that takes a filename as a String.

• Because the filename may be incorrect, this constructor throws FileNotFoundException, which must be either declared or caught by the caller.

  try {
    FileReader fr = new FileReader(fileName);
  }
  catch (FileNotFoundException e) {
    System.err.println("Invalid filename: " + e.getMessage());
  }
  

  You can pass a FileReader object to the BufferedReader constructor to provide its source of input.

BufferedReader br = new BufferedReader(fr);

Using BufferedReader.readLine

The readLine method: * Reads characters up to and including the line terminator - \n, \r, or \r\n. * Discards the line terminator. * Returns the line as a String. * Leaves the input stream at the position it stopped, awaiting the next read operation.

When there is no more data to read, readLine returns null, which tells us we're at the end of the file.

Like most java.io operations, readLine throws java.io.IOException which we must declare or catch.

try {
  FileReader fr = new FileReader(fileName);
  BufferedReader br = new BufferedReader(fr);
  String line;
  while ( (line = br.readLine()) != null) {
    System.out.println("Read the line: " + line);
  }
  br.close();
}
catch (FileNotFoundException e) {
  System.err.println("Invalid filename: " + e.getMessage());
  return;
}
catch (IOException e) {
  System.err.println("Problem while reading "+fileName+": "+e.getMessage());
  return;
}

Practice Exercise

Notice this line:

while ( (line = br.readLine()) != null) { /*...*/ }

Because comparison, !=, has higher precedence than assignment, =, we override precedence by putting the assigment in parentheses, then comparing the resulting value to null.

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