Input & output in java

I don't write code that needs to read from files/streams very often, so I am not quite familiar with input/output methods in Java. Especially when I try to review the readN questions on LeetCode, I realize I need to brush up some of the knowledge in this area.

Scanner
The Scanner class can parse primitive types and strings using regular expressions. A Scanner breaks its input into tokens using a delimiter pattern, which by default matches white space. The resulting tokens may then be converted into values of different types using various next methods.

A Scanner can read text from any object which implements the Readable interface. If an invocation of the underlying readable's Readable.read() method throws an IOException then the Scanner assumes the end of the input has been reached. The most recent IOException thrown by the underlying readable can be retrieved via ioException() method.

ByteArrayInputStream
This class reads an input byte array (when initialized).
The read() method reads the next byte of data from this input stream.

Source: http://www.codatlas.com/github.com/openjdk-mirror/jdk7u-jdk/master/src/share/classes/java/io/ByteArrayInputStream.java?keyword=ByteArrayInputStream&line=143

read(byte b[], int off, int len) method, on the other hand, read byte array b to the stream.

Source: http://www.codatlas.com/github.com/openjdk-mirror/jdk7u-jdk/master/src/share/classes/java/io/ByteArrayInputStream.java?keyword=ByteArrayInputStream&line=176

FileReader & FileInputStream
FileReader: reading character files.
read() method uses StreamDecoder's read() method, which reads bytes from the file, and then convert them to chars.
FileInputStream: reading streams of raw bytes.
Both can be used to instantiate a Scanner class.



BufferedReader
Reads text from a character-input stream, buffering characters so as to provide for the efficient reading of characters, arrays, and lines.
In general, each read request made of a Reader causes a corresponding read request to be made of the underlying character or byte stream. It is therefore advisable to wrap a BufferedReader around any Reader whose read() operations may be costly, such as FileReaders and InputStreamReaders.

BufferedReader in = new BufferedReader(new FileReader(fileName));

The above example will buffer the input from the specified file. Without buffering, each invocation of read() or readLine() could cause bytes to be read from file, converted into characters, and then returned, which can be very inefficient. 

read() method will read a single character, as an integer in the unicode range. The private method fill() will fill the buff array from input stream if necessary.

Source: http://www.codatlas.com/github.com/openjdk-mirror/jdk7u-jdk/master/src/share/classes/java/io/BufferedReader.java?keyword=bufferedreader&line=170

Private method read1() is the actual implementation of read4() in the LeetCode problem. char[] cbuf is the destination buffer. It will fill the inner buffer array from the input stream, copy it to the destination array, and output the number of characters read.

Source: http://www.codatlas.com/github.com/openjdk-mirror/jdk7u-jdk/master/src/share/classes/java/io/BufferedReader.java?keyword=bufferedreader&line=195

Ok, here comes the actual read(char cbuf[], int off, int len) method which, if you throw away the int off (the offset of the starting point), is pretty much the readN() implementation.

This method iteratively calling the read1() method, until len character is read. 

Source: http://www.codatlas.com/github.com/openjdk-mirror/jdk7u-jdk/master/src/share/classes/java/io/BufferedReader.java?keyword=bufferedreader&line=269

readLine(), which is the most common method used. Fill the buffer array (char[] cb), and return a String. Reading is terminated by a line feed('\n') or a carriage return ('\r').