• The JVM is a key component of the Java runtime environment (JRE). It executes Java bytecode, enabling platform independence. In other words, JVM allows Java programs to run on any machine or operating system that has the JVM installed. It abstracts the underlying operating system, making Java programs write once, run anywhere.

• JDK (Java Development Kit) is a complete package that includes tools for developing and running Java programs, including the JRE and JVM.
• JRE (Java Runtime Environment) contains the necessary libraries and JVM to run Java applications, but it doesn’t include development tools like compilers.
• JVM (Java Virtual Machine) is responsible for executing Java bytecode on a specific platform.

Java packages are a way of grouping related classes and interfaces to prevent naming conflicts and manage the code structure efficiently. Using packages makes the code more modular, reusable, and easier to maintain. For example, java.util provides utility classes like ArrayList, HashMap, and Date.

Java achieves platform independence through bytecode. When Java source code is compiled, it is converted into bytecode by the Java compiler. This bytecode can be executed on any machine with a JVM, regardless of the underlying hardware or operating system. This is known as the “write once, run anywhere” principle.

The main() method serves as the entry point for any standalone Java application. It must be declared as public static void main(String[] args) in order for the JVM to identify and invoke it when the program starts. Without a main method, the JVM will not know where to begin execution.

The **this** keyword in Java refers to the current object instance. It is used to differentiate between instance variables and local variables when they have the same name. It can also be used to call other constructors within the same class or pass the current object as a parameter.

Inheritance is a mechanism in Java where one class can acquire properties and behaviors (methods) from another class. The class that inherits is called a subclass (child class), and the class it inherits from is called a superclass (parent class). Inheritance promotes code reusability and hierarchical classification. Java supports single inheritance but not multiple inheritance (with classes), though this can be achieved using interfaces.

A Java interface is a blueprint of a class that contains abstract methods (without implementation). Interfaces are used to define a contract that a class must follow, encouraging polymorphism and decoupling. A class can implement multiple interfaces, allowing a form of multiple inheritance. For example, the Comparable interface in Java is used to define the natural ordering of objects.

Method overloading occurs when multiple methods have the same name but differ in parameters (number, type, or order). It allows methods to be defined with different input types within the same class.

Method overriding, on the other hand, happens when a subclass provides a specific implementation for a method that is already defined in its superclass. Overriding allows for runtime polymorphism.

Garbage collection in Java is an automatic memory management feature that reclaims memory occupied by objects that are no longer referenced or used by the program. This helps prevent memory leaks and ensures efficient memory utilization. The Java garbage collector operates in the background and is part of the JVM.

Exceptions are unexpected events or errors that occur during the execution of a program. In Java, exceptions are handled using try-catch blocks. The code that may throw an exception is placed inside the try block, and the potential handling of the exception occurs in the catch block. Java also supports a finally block, which executes regardless of whether an exception occurs or not.

Checked exceptions are exceptions that are checked at compile time. If a method is known to throw a checked exception, it must either handle it using a try-catch block or declare it in the method signature using the throws keyword. Examples include IOException and SQLException.

Unchecked exceptions, also known as runtime exceptions, are not checked at compile time and occur due to logical errors such as dividing by zero or accessing an invalid array index.

Multithreading in Java allows multiple threads to run concurrently, enabling parallel execution of tasks. Java supports multithreading through the Thread class and the Runnable interface. Multithreading is particularly useful for tasks such as network communication, file handling, and large computations. Synchronization mechanisms are used to manage shared resources between threads to avoid conflicts.

A singleton class is a design pattern in Java where only one instance of the class can be created throughout the program’s lifetime. This is useful when managing a shared resource or providing a global point of access to a resource. To implement a singleton class, the constructor is made private, and a static method is provided to return the instance of the class.