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Questions

OOP is a programming paradigm based on the concept of "objects," which can contain data and code. The four pillars of OOP are encapsulation, inheritance, polymorphism, and abstraction.

An abstract class can have both abstract and concrete methods, while an interface only allows abstract methods. A class can implement multiple interfaces, but it can extend only one abstract class.

static is used to create class-level variables and methods. They are associated with the class rather than with instances of the class. Static methods can be called without creating an instance of the class.

JVM is a virtual machine that allows Java bytecode to be executed on different platforms. It provides an abstraction layer between Java applications and the underlying hardware and operating system.

Garbage collection is the process of automatically reclaiming memory occupied by objects that are no longer reachable. Java's garbage collector identifies and frees up memory occupied by objects with no active references.

Java has four access modifiers: public, protected, default (no modifier), and private. They control the visibility of classes, methods, and fields.

Checked exceptions are checked at compile time, and the programmer is required to handle them using try-catch blocks or declare them in the method signature. Unchecked exceptions, also known as runtime exceptions, are not checked at compile time and are typically caused by programming bugs.

The finally block is used to ensure that a certain code segment is always executed, whether an exception is thrown or not. It is often used for cleanup activities, such as closing resources.

ArrayList is implemented as a dynamic array, providing fast random access but slower insertion and deletion. LinkedList is implemented as a doubly-linked list, offering faster insertion and deletion but slower random access.

HashMap is a data structure that stores key-value pairs. It uses hashing to map keys to indices in an array, allowing for fast retrieval of values. Collisions are handled using linked lists.

Synchronization is the process of controlling the access of multiple threads to shared resources. It is important to prevent race conditions and ensure data consistency. The synchronized keyword is often used to achieve synchronization.

In Java, a class implementing the Runnable interface can be used to create a thread, while the Thread class itself can be subclassed. Implementing Runnable is generally preferred as it allows better separation of concerns.