Top 13 Java Io Interview Questions You Must Prepare 06.Dec.2023

"println()" is a method of PrintStream class. "out" is a static object of PrintStream class defined in "System" class. System is a class from java.lang package used to interact with the underlying operating system by the programmer.

These two are general purpose classes used by the programmer very often to copy file to file. These classes work well with files containing less data of a few thousand bytes as by performance these are very poor. For larger data, it is preferred to use BufferedInputStream (or Buffered Reader) and BufferedOutputStream (or BufferedWriter).

There are four piped streams:

  • PipedInputStream
  • PipedOutputStream
  • PipedReader 
  • PipedWriter

These streams are very useful to pass data between two running threads (say, processes).

Filter streams are a category of IO streams whose responsibility is to add extra functionality (advantage) to the existing streams like giving line numbers in the destination file that do not exist int the source file or increasing performance of copying etc.

It is a special class from package which is neither a input stream nor a output stream (because it can do both). It is directly a subclass of Object class. Generally, a stream does only one purpose of either reading or writing; but RandomAccessFile can do both reading from a file and writing to a file. All the methods of DataInputStream and DataOutStream exist in RandomAccessFile.

Functionally both are same but belong to two different categories – byte streams and character streams. println() method exists in both classes.

There are four filter streams in package – two in byte streams side and two in character streams side. They are FilterInputStream, FilterOutputStream, FilterReader and FilterWriter. These classes are abstract classes and you cannot create of objects of these classes.

It is a stream of data that flows from source to destination. Good example is file copying. Two streams are involved – input stream and output stream. An input stream reads from the file and stores the data in the process (generally in a temporary variable). The output stream reads from the process and writes to the destination file.

BufferedInputStream and BufferedOutputStream on byte streams side and BufferedReader and BufferedWriter on character streams side.

It is a non-stream (not used for file operations) class used to know the properties of a file like when it was created (or modified), has read and write permissions, size etc.

There are four classes:

  • LineNumberInputStream (the extra functionality is it adds line numbers in the destination file),
  • DataInputStream (contains special methods like readInt(), readDouble() and readLine() etc that can read an int, a double and a string at a time),
  • BufferedInputStream (gives buffering effect that increases the performance to the peak) 
  • PushbackInputStream (pushes the required character back to the system).

All the byte stream classes can be divided into two categories (input stream classes and output stream classes) and all character streams classes into two (reader classes and writer classes). There are four abstract classes from which all these streams are derived. The super most class of all byte stream classes is and for all output stream classes, Similarly for all reader classes is and for all writer classes is

It is very useful to copy multiple source files into one destination file with very less code.