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basic_io.md

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Basic I/O

I/O Stream

An I/O Stream represents an input source or an output destination.

Byte Streams

  • Perform input and output of 8-bit bytes
  • All byte stream classes are descended from InputStream and OutputStream.
  • ALWAYS close streams by invoking close() in finally block
  • ALL other stream types are built on byte streams.

FileInputStream & FileOutputStream

  • Input and output int (holds a byte value) with read().
  • Input and output byte array.
  • Return -1 when reach the end of file.

Character Streams

  • In addition to byte stream, character stream automatically translates to and from the local character set.
  • Java uses Unicode convention to store characters.
  • Usually, 8-bit superset of ASCII
  • All character stream classes are descended from Reader and Writer.

FileReader & FileWriter

  • Input and output int (hold a character value) with read().
  • Input and output character array or String.
  • Return -1 when reach the end of file.

Buffered Streams

  • Read data from buffer, then native input API is called only when the buffer is empty.
  • Write data to buffer, then native output API is called only when the buffer is full
  • Buffered stream classes:
  1. BufferedInputStream -> FileInputStream
  2. BufferedOutputStream -> FileOutputStream
  3. BufferedReader -> FileReader
  4. BufferedWriter -> FileWriter

(created by wrapping unbuffered streams)

Flushing Buffered Streams

Flushing the buffer: write out without waiting for it to fill.

flush() is valid on any output stream, but has no effect unless the stream is buffered.

Scanner API

  • Formatted input -> tokens -> corresponding data type
  • Default delimiter white space including blanks, tabs, and line terminators.
  • Set delimiter: 'useDelimiter(regex)'.
  • hasNext() checks EOF
  • next() returns String
  • Set locale: useLocale(Locale.<location>).

Formatting

Stream objects that implement formatting are instances of either PrintWriter (character stream) or PrintStream (byte stream).

note: The only PrintStream objects we are likely to need are System.out and System.err.

  • print() & println(): standard way
  • format(): based on a format string.

Standard Streams

  1. Standard Input: System.in
  2. Standard Error: System.err
  3. Standard Output: System.out
  • All standard streams are byte streams.
  • System.out & System.err -> PrintStream which utilizes an internal character stream object to emulate many of the features of character streams.
  • System.in has no character stream feature. But we can wrap it in InputStreamReader.

The Console

  • Useful for secure password entry.
  • Provides both input and output streams that are true character streams.
  • Invoking System.console() to retrieve Console object if it is available.
  • readPassword() returns char array.

Data Streams

  • Support binary I/O of primitive data type values as well as String values.
  • DataInputStream -> DataInput.
  • DataOutputStream -> DataOutput.
  • ReturnType read[Type]/write[Type] ().
  • Throws EOFException when reach of EOF.
  • Don't use floating point numbers to represent monetary values because of bad decimal fractions. We should use Object Stream & BigDecimal.

Object Streams

  • Support I/O of objects.
  • ObjectInputStream -> ObjectInput -> DataInput
  • ObjectOutputStream -> ObjectOutput -> DataOutput
  • Complex object writes all its referenced objects to stream.
  • Two objects -> single object on the same stream => Two objects -> single object when read back.

File I/O

Path Class

  • Reflects underlying platform.
  • Creation:
  1. Paths.get(<path>) <=> FileSystems.getDefault().getPath(<path>).
  2. Paths.get(<dir1>, <dir2>, <fileName>)
  • Path stores name elements as a sequence: /home/user/foo => [home, user, foo].
  1. getName(index)
  2. getNameCount: number of elements in the path
  3. subpath(beginIndex, endIndex)
  4. getParent(): elements[0:n-2]
  5. getRoot(): if it is relative path, it returns null.
  • Converting a Path:
    1. toUri()
    2. toAbsolutePath()
    3. toRealPath()
    4. if true is passed to this method and the file system supports symbolic links => resolves symbolic links.
    5. relative path => absolute path
    6. remove redundant elements.
  • Joining Two Paths: resolve(<relativePath>).
  • Creating a path between two paths: p1.relativize(p2).
  • Comparing two paths:
  1. equals()
  2. startsWith(<path>)
  3. endsWith(<path>)

Checking a File or Directory

  • Checking existence:
  1. verified to exist: exists(Path, LinkOption...)
  2. verified to not exist: notExists(Path, LinkOption...)
  3. status is unknown.
  • Checking Accessibility:
  1. isReadable(Path)
  2. isWritable(Path)
  3. isExecutable(Path)
  • Checking whether two paths locate the same file.

Deleting a File or Directory

  • If deleting symbolic link, only the link is deleted.
  • delete(Path): throws an exception if the deletion fails.
  • deleteIfExists(Path): no exception is thrown.

Copying a File or Directory

copy(sourcePath, targetPath, CopyOptions...)

  • copy(InputStream, Path, CopyOptions...): copy all bytes from an input stream to a file.
  • copy(Path, OutputStream): copy all bytes from a file to an output stream.

Moving a File or Directory

move(sourcePath, targetPath, CopyOptions...)

Managing Metadata