Java Interview Questions and Answers

What is Java?

Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It is designed to be platform-independent, allowing developers to write code once and run it on any Java-enabled platform without recompilation.

What are the main features of Java?

Some of the main features of Java include:

Platform independence: Java programs can run on any platform with a Java Virtual Machine (JVM).

Object-oriented: Java supports object-oriented programming principles such as encapsulation, inheritance, and polymorphism.

Robustness: Java provides strong memory management, exception handling, and type checking mechanisms to ensure robust and reliable code.

Multi-threading: Java supports concurrent execution of multiple threads, allowing developers to write scalable and responsive applications.

Portability: Java programs are portable across different platforms, making them suitable for developing cross-platform applications.

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

JDK (Java Development Kit) is a software development kit that includes tools for developing and compiling Java applications, such as the Java compiler (javac) and the Java runtime environment (JRE).

JRE (Java Runtime Environment) is a runtime environment that provides the necessary libraries and components to run Java applications. It includes the JVM (Java Virtual Machine) and core Java libraries.

JVM (Java Virtual Machine) is an abstract computing machine that provides an execution environment for Java bytecode. It interprets and executes Java bytecode instructions and manages memory, threads, and other runtime aspects of Java applications.

What is the difference between abstract class and interface in Java?

An abstract class is a class that cannot be instantiated and may contain abstract methods (methods without a body) as well as concrete methods. It can also contain instance variables and constructors. Subclasses of an abstract class must implement all abstract methods or be declared abstract themselves.

An interface is a reference type in Java that defines a contract of methods that a class must implement. It can only contain constant variables and method signatures (without method bodies). A class can implement multiple interfaces but can only extend one abstract class.

What is the difference between Array List and LinkedList in Java?

Array List is implemented as a dynamic array, meaning that it stores elements in a contiguous memory location and supports random access to elements using an index. It is efficient for accessing elements by index but less efficient for inserting or removing elements from the middle of the list.

LinkedList is implemented as a doubly linked list, meaning that it stores elements as nodes with references to both the previous and next nodes. It is efficient for inserting or removing elements from the middle of the list but less efficient for random access.

These are just a few examples of Java interview questions. Depending on the role and level of the interview, questions may cover more advanced topics such as design patterns, concurrency, memory management, and performance optimization. It's important to thoroughly prepare for interviews by reviewing core Java concepts, practicing coding exercises, and gaining hands-on experience with Java programming.