Java File and Stream Handling

Introduction to Files and Streams

• Data stored in variables and arrays is temporary and lost when a program terminates or a local variable goes out of scope.
• Computers use files for long-term retention of data, which exists beyond the duration of program execution.
• Files are stored on secondary storage devices such as hard disks, optical disks, flash drives, and magnetic tapes.

Files and Streams

• Java views each file as a sequential stream of bytes.
• A file stream can be used to input and output data as bytes or characters.
• Streams that input and output bytes are known as byte-based streams, representing data in its binary format.
• Streams that input and output characters are known as character-based streams, representing data as a sequence of characters.
• Files created using byte-based streams are referred to as binary files, while files created using character-based streams are referred to as text files.
• A Java program opens a file by creating an object and associating a stream of bytes or characters with it.
• Java creates three stream objects when a program begins executing: System.in, System.out, and System.err.

Class File

• Class File provides information about files and directories.
• Class File provides four constructors to specify the name of a file or directory to associate with the File object.
• The constructor with a String argument specifies the name of a file or directory, which can contain path information.
• The constructor with two String arguments specifies an absolute or relative path and the file or directory to associate with the File object.
• The constructor with File and String arguments uses an existing File object that specifies the parent directory of the file or directory specified by the String argument.
• The fourth constructor uses a URI object to locate the file.

Sequential-Access Text Files

• Sequential-access files store records in order by the record-key field.
• Text files are human-readable files.
• Java imposes no structure on a file, and notions such as records do not exist as part of the Java language.
• You must structure files to meet the requirements of your applications.

Creating a Sequential-Access Text File

• Formatter outputs formatted Strings to the specified stream.
• The constructor with one String argument receives the name of the file, including its path.
• If a path is not specified, the JVM assumes that the file is in the directory from which the program was executed.
• If the file does not exist, it will be created. If an existing file is opened, its contents are truncated.
• A SecurityException occurs if the user does not have permission to write data to the file.
• A FileNotFoundException occurs if the file does not exist and a new file cannot be created.

Reading Data from a Sequential-Access Text File

• Scanner method hasNext determines whether the end-of-file key combination has been entered.
• A NoSuchElementException occurs if the data being read by a Scanner method is in the wrong format or if there is no more data to input.
• Formatter method format works like System.out.printf.
• A FormatterClosedException occurs if the Formatter is closed when you attempt to output.
• Formatter method close closes the file.