Introduction to Classes and Objects in Java

Introduction to Classes, Objects, and Methods

• Java uses classes to define the structure and behavior of objects
• A class is a template for creating objects
• An object is an instance of a class
• Classes contain data (instance variables) and code (methods)
• Instance variables store data associated with an object
• Methods define actions that an object can perform
• The class keyword defines a class in Java
• The syntax of a class includes a list of instance variables and methods
• Object creation is done using the new keyword

Topics

• Class Fundamentals: Focuses on defining objects, reference variables, and assignments.
• Methods: Covers returning a value, using parameters, and method definition.
• Constructors: Includes explaining default and parameterized constructors, new operator, this keyword, and finalize().
• Wrapper Classes: Details how to convert primitive types to objects (autoboxing) and objects to primitive types (unboxing). Also covering parsing.
• I/O: Introduces command-line arguments, the Scanner, and BufferedReader classes for input/output operations.

Class Fundamentals

• A class is a template defining the structure and behavior of objects.
• Data defined within a class are called instance variables.
• Methods are blocks of code defining the actions an object can perform.
• Methods and variables are the members of a class.

A Simple Class

• Demonstrates creating a class named Box with instance variables width, height, and depth.
• Shows creating an object of type Box (e.g., Box mybox = new Box();).
• Creates a BoxDemo class containing a main method for initializing the box object's variables.
  • Assigns values to the properties within the main method.
  • Calculates and prints the box's volume.

Object Creation

• Declaring a reference to an object (e.g., Box mybox;).
• Allocating a Box object (e.g., mybox = new Box();).
• Initialization of properties in a new object.

Assigning Object Reference Variables

• Demonstrates assigning values to a reference variable and linking objects (e.g., Box b2 = b1;).
• This creates a shared reference, not a copy. Changes in one variable will affect the other.

Methods

• Classes contain instance variables and methods.
• Methods are defined with the syntax type method\_name(parameter-list){...} .

Method with parameters

• Methods can accept parameters.
• Example int square(int i){return i*i; } illustrates a method that calculates the square of an integer input.

Returning a value

• A method can return a value using the return keyword.
• Demonstrates a volume() method that returns a calculated volume from instance variables of a Box class.

The this Keyword

• this keyword refers to the current object.
• this.variable within a method accesses a particular instance variable of the class's object. Important for distinguishing between parameters and instance variables with the same name.

Garbage Collection

• When an object is no longer needed, its memory is reclaimed (garbage collected).
• The finalize() method is a way to perform actions on an object before it's garbage collected(though note that it's a method best avoided now in Java).

Wrapper Classes

• Primitive types are converted to corresponding objects using wrapper classes (e.g., Integer, Double).
• Autoboxing converts primitives to objects.
• Unboxing converts objects to primitives.

Parsing

• The parse() method extract information from strings containing numerical data (converting a string to a primitive type).

Java Command-Line Arguments

• Command-line arguments are passed to a Java program when it runs.

Java Scanner Class

• The Scanner class is used to read from various sources (console, files, etc.).
• The nextInt() method reads an integer.

Buffered Reader Class

• BufferedReader class performs buffered input, enhancing performance.
• This class is commonly used in conjunction with InputStreamReader or FileReader classes to read input from files or the standard input stream (System.in).

Reading from System.in and File

• Shows how to read input from the standard input stream using InputStreamReader.
• Shows how to read input from a file using FileReader.

Methods in Buffered Reader

• read() method reads a single character.
• readLine() method reads an entire line of characters.