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• Package

  Used to group related classes. Think of it as a folder in a file directory. Used to avoid name conflicts and write better maintainable code.
• Creating Packages and importing


  1. Directory structure on your computer is related to the package name

  2. Compile with -d . to put class files in current directory

  3. Add package directory to classpath so classes can be accessed from anywhere
• CLASSPATH is an environment variable used by the Application ClassLoader to locate and load the .class files
• Setting CLASSPATH


  1. Through command prompt: set classpath=%classpath%;e:\vdemo\bindu

  2. Through environment variables: System->Advanced System Settings->Advanced->Environment variables, Variable name: classpath, Value: e:\vdemo\bindu
• Interface


  Similar to a class but has static constants and abstract methods. Cannot be instantiated. Used to achieve abstraction and multiple inheritance.
• Interfaces


  • There can be only abstract methods, no method body
  • Since Java 8, can have default and static methods
  • Since Java 9, can have private methods
• Reasons to use Java interface


  • Achieve abstraction
  • Support functionality of multiple inheritance
  • Achieve loose coupling
• Defining interface, implementing interface, applying interface


  1. Interface defines abstract methods

  2. Classes implement the interface and provide implementation for the abstract methods

  3. Interface reference can be used to call the implemented methods at runtime (polymorphism)
• Multiple Inheritance using interfaces

  1. A class can implement multiple interfaces

  2. Each interface defines abstract methods that the class must implement
• Nested interface


  An interface declared within another interface or class. Used to group related interfaces for easier maintenance. Must be referred by the outer interface or class, cannot be accessed directly.
• Variables in interface


  Public, static, and final by default. No access modifier allowed except public. Must be initialized in the interface itself. Implementing class cannot modify the variables.
• Stream


  A sequence of data, composed of bytes. Java has 3 standard streams: System.out, System.in, System.err.
• Java I/O Classes


  • Byte Streams (InputStream, OutputStream)
  • Character Streams (Reader, Writer)
• Reading and writing Files (Byte Streams)


  1. FileInputStream: read bytes from file

  2. FileOutputStream: write bytes to file

  3. BufferedInputStream: optimizes performance by buffering data in memory

  4. DataInputStream/DataOutputStream: read/write primitive data types directly
• Serialization


  Process of writing the state of an object to a byte stream to save it in persistent storage. The class must implement Serializable interface.
• Transient and static attributes are not serialized
• Serializable


  A class that can be converted to a stream of bytes and then reconstructed
• If a class is Serializable, all of its sub classes are also serializable
• Transient attributes


  Attributes that will not be serialized
• Static attributes


  Attributes that will not be serialized
• Writing object to a File


  1. Create FileOutputStream

  2. Create ObjectOutputStream

  3. Write object using writeObject()

  4. Close ObjectOutputStream
• Student class


  Implements Serializable interface
• Student class has instance variables rno and name
• Student class has a display() method to print the roll number and name
• Retrieving and displaying objects from a file


  1. Create FileInputStream

  2. Create ObjectInputStream

  3. Read objects using readObject()

  4. Call display() method on each object

  5. Close ObjectInputStream
• Character Streams


  Built on top of byte streams to facilitate handling of character (text) data
• Reader


  Abstract base class for reading (input) related classes in character streams
• Writer


  Abstract base class for writing (output) related classes in character streams
• FileReader


  Helps to read data from a file as characters (text)
• BufferedReader


  Used for performance optimization, has readLine() method to read a line at a time from the input buffer
• Reading from a file using FileReader and BufferedReader


  1. Create FileReader

  2. Create BufferedReader

  3. Read lines using readLine()

  4. Print each line

  5. Close BufferedReader
• FileWriter


  Helps to write data to a file as characters (text)
• Writing to a file using FileWriter


  1. Create FileWriter

  2. Write characters using write()

  3. Close FileWriter
• File class


  Abstract representation of file and directory pathnames, not used to actually read or write data
• File class


  • Used for making new empty files, searching for files, deleting files, and making directories
  • Provides metadata information about a file
• RandomAccessFile


  Can open a file for both reading and writing, has methods to position the file pointer at a required position
• Console class


  Provides methods to accept input from the command line and write formatted output to the monitor
• Reading a string from the console


  1. Get Console object

  2. Use readLine() method to read string
• Reading a password from the console


  1. Get Console object

  2. Use readPassword() method to read password
• Writing formatted output to the console


  1. Get Console object

  2. Use format() or printf() method to write formatted output