Java Classes and Objects

Java Classes

• A class does not occupy memory until it is instantiated, meaning that an object of the class is created.
• A class groups variables of different data types and methods.
• Components of a class include modifiers, the class keyword, class name, superclass, interfaces, and the class body.
• Data members, methods, and constructors are part of a Java class.
• A class can extend one class and implement multiple interfaces.

Modifiers

• Modifiers define the accessibility levels of a class:
  • private: Accessible only within the class where defined.
  • default or package-private: Accessible only to classes in the same package.
  • protected: Accessible only to classes that subclass the class directly within the current or different package.
  • public: Accessible from any class.

Java Objects

• Objects are instances of a class that represent real-life entities.
• A Java program typically creates and interacts with multiple objects.
• Objects have state, behavior, and identity.

State

• State is represented by the attributes of an object, which reflect its properties.

Behavior

• Behavior is represented by the methods of an object, which define its interactions with other objects.

Identity

• Identity distinguishes an object from other objects and enables it to interact with them.

Declaring an Object

• Creating an object of a class, also known as instantiation, allows you to use the attributes and behavior defined by that class.

Initializing a Java Object

• When a new object is created, constructors ensure that the instance variables are initialized with correct values.

Difference between Java Class and Objects

• A class is a blueprint, while an object is a concrete instance of that blueprint.
• A single class can have multiple objects, each with its own unique state, while sharing the same behavior.

Java Variables

• Variables are data containers that store values during program execution.
• Each variable has a data type that specifies the type and quantity of value it can hold.
• Variables are assigned a memory location name.

Types of Variables in Java

• Local Variables:
  • Defined within a block, method, or constructor.
  • Created at declaration and destroyed after exiting the block or method.
  • Scope is limited to the block where declared.
  • Initialization is mandatory before use within the scope.
• Instance Variables:
  • Declared in a class outside of methods, constructors, or blocks.
  • Created when an object of the class is created and destroyed when the object is destroyed.
  • Allow access modifiers.
  • Default values are assigned if not initialized.
  • Can only be accessed through object creation.
  • Initialized via constructors or instance blocks.
• Static Variables:
  • Also known as class variables.
  • Declared similarly to instance variables but with the static keyword.
  • Belong to the class rather than individual objects.
  • Accessed directly using the class name.
  • Only one copy of a static variable exists for the class.

Unary Operators

• Unary operators operate on a single operand.
• ++ increment operator:
  • Pre-increment: ++a: Increment then use.
  • Post-increment: a++: Use then increment.
• -- decrement operator:
  • Pre-decrement: --a: Decrement then use.
  • Post-decrement: a--: Use then decrement.

Assignment Operator

• Assigns a value to a variable.
• Uses the = operator.
• Has right-to-left associativity.

Relational Operators

• Check for relations like equality, greater than, and less than.
• Return boolean values based on the comparison.
• Used in loops and conditional statements.
• Operators include:
  • == (Equal to)
  • != (Not Equal to)
  • > (Greater than)
  • < (Less than)
  • >= (Greater than or equal to)
  • <= (Less than or equal to)

Logical Operators

• Perform logical operations like "AND" and "OR," similar to logic gates in digital electronics.
• Evaluate conditions in a short-circuiting manner.
• Operators include:
  • && (Logical AND): True if both operands are true.
  • || (Logical OR): True if at least one operand is true.
  • ! (Logical NOT): True if the operand is false and vice versa.

Ternary Operator

• A shorthand version of the if-else statement.
• Has three operands:
  • Condition
  • Expression if true
  • Expression if false
• Format: condition ? if true : if false

Input From User in Java

• Java I/O streams enable user input and output operations.
• Two common methods:
  • BufferedReader Class: Reads a sequence of characters.
  • Scanner Class: Provides a more versatile interface for reading different data types.

Using BufferedReader Class for String Input

• BufferedReader is a simple class used for reading character sequences.
• Provides functions like read(), read(char[] cbuf), and readLine() for reading characters and lines.