Most important Java interview questions with answer Part -1

Mayank Gupta
3 min readJul 8, 2023

What is Java?

  • Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It is known for its platform independence and is widely used for building a variety of applications.

What are the main features of Java?

  • Some key features of Java include platform independence, object-oriented programming, automatic memory management (garbage collection), robust exception handling, and a rich standard library.

What is the difference between JDK, JRE, and JVM?

  • JDK (Java Development Kit) is a software development kit that provides tools for developing, compiling, and running Java applications.
  • JRE (Java Runtime Environment) is a runtime environment that includes the necessary libraries and components to run Java applications.
  • JVM (Java Virtual Machine) is the virtual machine that executes Java bytecode. It is responsible for converting bytecode into machine-specific instructions.

What is the difference between an object and a class?

  • A class is a blueprint or template that defines the structure and behaviour of objects.
  • An object is an instance of a class that represents a specific entity in memory and can have its own state and behaviour.

What is the difference between an abstract class and an interface?

  • An abstract class can have both abstract and non-abstract methods and can provide default implementations.
  • An interface can only have abstract methods and cannot provide any implementation. It defines a contract that classes implementing the interface must follow.

What is the difference between checked and unchecked exceptions?

  • Checked exceptions are checked at compile-time and must be handled by the calling code using try-catch blocks or declaring them in the method signature using the “throws” keyword.
  • Unchecked exceptions (RuntimeExceptions) are not checked at compile-time and do not require explicit handling.

What is the difference between the “equals” method and the “==” operator?

  • The “equals” method is used to compare the contents of two objects for equality.
  • The “==” operator is used to compare the memory addresses of two objects to check if they refer to the same instance.

What is the difference between ArrayList and LinkedList?

  • ArrayList is implemented as a resizable array and provides fast access to elements using their index. Insertion and deletion operations are slower.
  • LinkedList is implemented as a doubly linked list and provides fast insertion and deletion operations. Accessing elements by index is slower.

What is the difference between final, finally, and finalize?

  • “final” is a keyword used to declare a constant variable, a method that cannot be overridden, or a class that cannot be subclassed.
  • “finally” is a block that follows a try-catch block and is executed regardless of whether an exception is thrown or caught.
  • “finalize” is a method defined in the Object class that is called by the garbage collector before an object is destroyed.

Explain the concept of multithreading in Java.

  • Multithreading allows concurrent execution of multiple threads within a single program.
  • Threads are independent units of execution that share the same memory space but run concurrently.
  • Multithreading can improve performance by utilizing CPU resources effectively and handling tasks in parallel.

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Mayank Gupta

QA Automation Lead | Web Automation | Mobile Automation | API Automation l Performance | Web Security | IOT | Blockchain