Java is a widely used object-oriented programming language. It was developed by Sun Microsystems (now owned by Oracle) and released in 1995. Java is known for its platform independence, robustness, and versatility. It allows developers to write code once and run it on any platform that supports Java, making it suitable for a wide range of applications, including desktop, web, mobile, and enterprise development.

Java offers several key features, including:

• Platform Independence: Java programs can run on any platform with a Java Virtual Machine (JVM), making them highly portable.
• Object-Oriented Programming (OOP): Java follows the principles of OOP, including encapsulation, inheritance, and polymorphism, facilitating modular and reusable code.
• Automatic Memory Management: Java uses automatic garbage collection to manage memory, freeing developers from manual memory management.
• Security: Java provides built-in security features, such as bytecode verification and a robust security model, making it a popular choice for developing secure applications.
• Multi-threading: Java supports multi-threading, allowing concurrent execution of multiple threads within a program, enabling efficient utilization of resources and concurrent programming.

- JDK (Java Development Kit): JDK is a software development kit that provides tools, libraries, and documentation required for Java development. It includes the Java compiler, debugger, and other development utilities. - JRE (Java Runtime Environment): JRE is a runtime environment that provides the necessary components to run Java applications. It includes the JVM, libraries, and other files required to execute Java programs. - JVM (Java Virtual Machine): JVM is the runtime instance that executes Java bytecode. It provides the environment for Java programs to run, including memory management, bytecode execution, and exception handling. JVM is responsible for ensuring platform independence by converting bytecode into machine-specific instructions at runtime.

In Java, variables can be classified into the following types:

• Local Variables: Variables declared within a method or a block of code. They have limited scope and are accessible only within the block where they are declared.
• Instance Variables: Also known as non-static variables, these variables are declared within a class but outside any method. Each instance of the class has its own copy of instance variables.
• Static Variables: Also known as class variables, these variables are associated with the class rather than specific instances. They are shared among all instances of the class.
• Parameters: Variables passed to methods or constructors to hold values passed during method or constructor invocation.

- Class: A class is a blueprint or template that defines the structure, behavior, and state of objects. It serves as a template for creating objects and encapsulates the data and methods that operate on that data. - Object: An object is an instance of a class. It represents a real-world entity and has its own state and behavior defined by the class. Objects are created from a class using the "new" keyword and can interact with each other through method invocations.

- == Operator: The "==" operator is used to compare primitive data types or check if two object references point to the same memory location. It compares the values of the operands. - equals() Method: The equals() method is a method defined in the Object class and overridden by many other classes. It is used to compare the content or values of objects for equality. The default implementation of equals() in the Object class compares object references. However, it is often overridden in specific classes to provide custom comparison logic based on object properties.

Access modifiers in Java determine the accessibility or visibility of classes, variables, methods, and constructors. There are four access modifiers in Java:

• Public: Public access modifier allows unrestricted access to the class, variable, method, or constructor from any other class or package.
• Private: Private access modifier restricts access to the class, variable, method, or constructor within the same class only.
• Protected: Protected access modifier allows access to the class, variable, method, or constructor within the same class, derived classes (subclasses), and same package.
• Default (No Modifier): Default access modifier allows access to the class, variable, method, or constructor within the same package only. It is also known as package-private access.

Method overloading is a feature in Java that allows a class to have multiple methods with the same name but different parameters. The methods must differ in the number, order, or types of parameters. Java determines which overloaded method to invoke based on the arguments provided during the method call. Method overloading enables developers to create more concise and readable code by providing multiple ways to perform a similar operation.

Exception handling in Java refers to the mechanism of handling and responding to exceptional or error conditions that may occur during program execution. Exceptions represent abnormal or unexpected situations that disrupt the normal flow of the program. Java provides a robust exception handling mechanism using the try-catch-finally block. Developers can catch and handle exceptions, allowing the program to gracefully recover from errors and prevent abrupt termination.

- Checked Exceptions: Checked exceptions are exceptions that are checked at compile-time. Methods that can throw checked exceptions must declare them using the "throws" keyword. The calling code must handle or propagate checked exceptions. Examples of checked exceptions in Java include IOException and SQLException. - Unchecked Exceptions: Unchecked exceptions are exceptions that are not checked at compile-time. They are also known as runtime exceptions. Methods are not required to declare unchecked exceptions. Unchecked exceptions usually indicate programming errors or unexpected conditions. Examples of unchecked exceptions in Java include NullPointerException and ArrayIndexOutOfBoundsException.