Top Java Interview Questions and Answers

General Java and Object Oriented Programming:

  1. History of Java?
  2. What is Java?
  3. What is JVM?
  4. Why is Java called the Platform Independent Programming Language?
  5. What is the difference between JDK, JRE and JVM?
  6. What does the static keyword mean?
  7. Can you override private or static method in Java?
  8. Can you access the non-static variable in static context?
  9. What are the Data Types supported by Java?
  10. What are the concepts of OOPs?
  11. What is the unique feature of Java?
  12. What is an instance and a local variable?
  13. What are functional or SAM interfaces?
  14. Difference between == and equals() method in java?
  15. What is a Lambda expression and its syntax?
  16. What is the difference between Collection and Streams in Java?
  17. What are the advantages of design patterns in Java?
  18. Why String is called an immutable object?
  19. What is an Optional class in Java?
  20. What is Autoboxing and Unboxing?
  21. What is Function Overriding and Overloading in Java?
  22. Does Java support multiple inheritance?
  23. What is the difference between an Interface and an Abstract class?
  24. What are pass by reference and pass by value?
  25. What is the purpose of a Volatile Variable?
  26. What is the purpose of a Transient variable?
  27. What is Local Variable and Instance Variable?
  28. What are the different access modifiers available in Java?
  29. What are wrapper classes?
  30. What is singleton class and how can we make a class singleton?
Java Threads
  1. What is the difference between processes and threads?
  2. Explain different ways of creating a thread. Which one would you prefer and why?
  3. What is the difference between a method and block that are synchronized?
  4. What are the differences between wait and sleep method in Java?

1) History of Java?

  • James Gosling and Sun Microsystems invented the Java programming language in 1991.
  • He had a single motto while creating the language: “Write once; run anywhere.”
  • The language was initially called Oak because of the oak tree outside Gosling’s office.
  • Later, the name changed to Green, then to Java Coffee, a type of coffee from Indonesia, and eventually shortened to Java in 1995.
  • Gosling designed Java with a C/C++-style syntax that system and application programmers would find familiar.
  • Sun Microsystems released the first public implementation as Java 1.0 in 1996.
  • It promised write once, run anywhere (WORA) functionality, providing no-cost run-times on popular platforms.

2) What is Java?

  • Java is a widely-used programming language for coding web applications.
  • It has been a popular choice among developers for over two decades, with millions of Java applications in use today.
  • Java is a multi-platform, object-oriented, and network-centric language that can be used as a platform in itself.
  • It is a fast, secure, reliable programming language for coding everything from mobile apps and enterprise software to big data applications and server-side technologies.
  • Java is free to download for personal and development use.
  • It is used for:
    • Mobile applications (specially Android apps)
    • Desktop applications
    • Web applications
    • Web servers and application servers
    • Games
    • Database connection
    • And much, much more!

3) What is JVM?

  • The JVM (Java Virtual Machine) is only one part of Java software that is involved in running an application.
  • It acts as a run-time engine to run Java applications. JVM is the one that actually calls the main method present in a Java code. It is a part of JRE(Java Runtime Environment).
  • Java applications are called WORA (Write Once Run Anywhere). This means a programmer can develop Java code on one system and can expect it to run on any other Java-enabled system without any adjustment. This is all possible because of JVM.

4) Why is Java called the Platform Independent Programming Language?

  • Java is platform-independent means the Java code that has been compiled generates compiled code or the byte code, and this byte code can run on all of the operating systems provided they have JVM installed in it.
  • This means you can write a Java program on one platform (such as Windows) and then run it on a different platform (such as macOS or Linux) without making any changes to the code.
  • The JVM acts as an interpreter for the Java bytecode, translating it into instructions that the host machine can understand and execute. This means that the same Java program can run on any device that has a JVM, making it a truly “write once, run anywhere” language.

5) What is JDK, JRE and JVM?

  • JDK is for development purpose whereas JRE is for running the java programs.
  • JDK and JRE both contain JVM so that we can run our java program.
  • JVM is the heart of java programming language and provides platform independence.

6) What does the static keyword mean?

  • The static keyword is mainly used for memory management in Java.
  • The static keyword can be used with variables, methods, code blocks, and nested classes.
  • In Java, it is used to share the same variable or method of a given class.
  • If we want to access class members without creating an instance of the class, we need to declare the class members static.
  • Static methods are also called class methods. It is because a static method belongs to the class rather than the object of a class.

7) Can you override private or static method in Java?

  • No, we cannot override private or static methods in Java.
    • Private methods are not visible to any other class in Java.
    • Static methods cannot be overridden in Java. Because static methods are associated with the class itself, but not with the instance of a class. Therefore, when a subclass inherits a static method from its parent class, it cannot modify the behavior of the static method in any way.

8) Can you access the non-static variable in static context?

  • Non-static members cannot be accessed from a static context.
    • Non-static variables and methods depend on class instances, as they store and manipulate data specific to individual objects.
    • There has to be a class instance that references these members.

9) What are the Data Types supported by Java?

  • There are two types of data types in Java:
    • Primitive data types: The primitive data types include boolean, char, byte, short, int, long, float, and double.
    • Non-primitive data types: The non-primitive data types include Classes, Interfaces, and Arrays.
      • Example:
      • Primitive data types:
        • byte: Byte data type is an 8-bit signed two's complement integer.
        • short: Short data type is a 16-bit signed two's complement integer.
        • int: Int data type is a 32-bit signed two's complement integer.
        • long: Long data type is a 64-bit signed two's complement integer.
        • float: Float data type is a single-precision 32-bit IEEE 754 floating point.
        • double: double data type is a double-precision 64-bit IEEE 754 floating point.
        • boolean: boolean data type represents one bit of information.
        • char: char data type is a single 16-bit Unicode character.

10) What are the concepts of OOPs?

  • Object-Oriented Programming System (OOPs) is a paradigm that provides many concepts such as:
    • Inheritance
    • Data binding
    • Polymorphism
    • Encapsulation
    • Abstraction
    • Message Passing

11) What is the unique feature of Java?

  • The main unique feature of Java is its platform independence.
    • Unlike many other programming languages like C and C++, when Java is compiled, it is not compiled into platform-specific machine code, but rather into platform-independent byte code.
    • This byte code is distributed over the web and interpreted by the JVM (Java Virtual Machine) on whichever platform it is being run.
    • Java is guaranteed to be Write Once, Run Anywhere.

12) What is an instance and a local variable?

  • Instance variables: Instance variables are variables that are declared in a class but outside a method, constructor, or any block. They are created when an object of the class is instantiated and destroyed when the object is destroyed.
  • Local variables: Local variables are variables that are declared inside a method, constructor, or any block. They are created when the method, constructor, or block is entered and destroyed when it is exited.

13) What are functional or SAM interfaces?

  • A functional interface is an interface that has just one abstract method, and thus represents a single function contract. It is also called a SAM (Single Abstract Method) interface.
  • Examples:
    • Runnable: The Runnable interface is a SAM interface with a single method void run().
    • Callable: The Callable interface is a SAM interface with a single method V call().

14) Difference between == and equals() method in java?

  • == operator: In Java, the == operator compares the memory reference (or address) of two objects, not the contents of the objects. If two object references point to the same memory address, then the == operator will return true.
  • equals() method: The equals() method is defined in the Object class and is used to compare the actual contents of the objects.
    • For example, two String objects may have the same value, but different memory references. In this case, the == operator would return false, but the equals() method would return true.

15) What is a Lambda expression and its syntax?

  • Lambda expression: A Lambda expression is a short block of code which takes in parameters and returns a value. Lambda expressions are similar to methods, but they do not need a name and they can be implemented right in the body of a method.
  • Syntax: (argument-list) -> {body}
    • Example:
    • (int a, int b) -> { return a + b; }

16) What is the difference between Collection and Streams in Java?

  • Collection:
    • Collections are in-memory data structures that hold all the values the data structure currently contains.
    • Collections store data for processing.
    • Examples of Collection are List, Set, Map, etc.
  • Stream:
    • Streams are a new abstraction introduced in Java 8 that represents a sequence of elements from a source that supports aggregate operations.
    • Streams process data on demand.
    • Streams do not store data; they perform a series of computational steps on the data.

17) What are the advantages of design patterns in Java?

  • Advantages:
    • They are reusable in multiple projects.
    • They provide the solutions to the commonly occurring problems in software design.
    • They lead to more flexible, scalable, and robust code.
    • They improve the readability of the code.
    • They help in faster software development.

18) Why String is called an immutable object?

  • String is called an immutable object because its value cannot be changed once it has been created.
    • Example: If we create a String with a value of "Hello", then that value cannot be changed. Any operation that appears to modify the value will actually create a new String object with the new value, and leave the original String unchanged.
    • Reasons for immutability:
      • Security: String is widely used as a parameter for many Java classes, e.g., for opening network connections, loading classes, and so on. If the string is mutable, it could cause serious security threats.
      • Synchronization: The immutability of strings makes them inherently thread-safe. Multiple threads can access a single "immutable" string instance without any extra synchronization.

19) What is an Optional class in Java?

  • Optional: The Optional class in Java is a container object that may or may not contain a non-null value.
  • Optional is a public final class and is used to deal with NullPointerException in Java application.
    • Example:
      • Optional gender = Optional.of("MALE");
      • Optional emptyGender = Optional.empty();

20) What is Autoboxing and Unboxing?

  • Autoboxing: Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes.
    • Example: converting an int to an Integer, a double to a Double, and so on.
  • Unboxing: Unboxing is the reverse process of converting an object of a wrapper type to its corresponding primitive value.
    • Example: converting an Integer to an int, a Double to a double, and so on.

21) What is Function Overriding and Overloading in Java?

  • Method Overriding:
    • When a method in a subclass has the same name, same parameters, and same return type as a method in its superclass, then the method in the subclass is said to override the method in the superclass.
  • Method Overloading:
    • When a class has multiple methods with the same name but different parameters, then these methods are said to be overloaded.
    • It can be achieved by changing the number of arguments or changing the data type of arguments.

22) What is a constructor in Java?

  • Constructor: A constructor is a special method that is used to initialize objects. The constructor is called when an object of a class is created.
  • Rules for creating a constructor:
    • The constructor name must be the same as its class name.
    • A constructor must not have an explicit return type.
    • A Java constructor cannot be abstract, static, final, and synchronized.

23) What is Singleton class in Java?

  • Singleton class: A Singleton class in Java is a class that can have only one object (an instance of the class) at a time.
  • Example: a database connection pool, where we want to limit the number of database connections being created.
    • To create a Singleton class:
      • Make the constructor of the class private.
      • Create a static method that returns the instance of the class.

24) What is the difference between ArrayList and Vector in Java?

  • ArrayList:
    • ArrayList is not synchronized.
    • ArrayList is fast as it is non-synchronized.
    • ArrayList increases its size by 50% of the array size.
    • ArrayList does not define the increment size.
  • Vector:
    • Vector is synchronized.
    • Vector is slow as it is thread-safe.
    • Vector increases its size by doubling the array size.
    • Vector defines the increment size.

25) What is a Thread in Java?

  • A thread in Java is the path followed when executing a program.
  • All Java programs have at least one thread, known as the main thread, which is created by the Java Virtual Machine (JVM).
  • Threads are also called lightweight processes.

26) What is a package in Java?

  • A package in Java is a namespace that organizes a set of related classes and interfaces.
  • Packages are used to group related classes and interfaces together. They provide access protection and namespace management.
  • Packages can be categorized into two types:
    • Built-in packages: java, lang, util, etc.
    • User-defined packages

27) Explain the 'this' keyword in Java.?

  • The 'this' keyword in Java is a reference variable that refers to the current object.
  • Uses of 'this' keyword:
    • It can be used to refer to the instance variable of the current class.
    • It can be used to invoke or initiate the current class constructor.
    • It can be passed as an argument in the method call.
    • It can be passed as an argument in the constructor call.
    • It can be used to return the current class instance.

28) What is the final keyword in Java?

  • The final keyword in Java is used to restrict the user.
    • The final keyword can be used in many contexts. Final can be:
      • Variable: Once a final variable is initialized, it cannot be changed.
      • Method: A final method cannot be overridden.
      • Class: A final class cannot be inherited.

29) What is Exception Handling in Java?

  • Exception Handling in Java is a powerful mechanism that handles runtime errors and maintains the normal flow of the application.
    • Exception Handling in Java is managed via five keywords: try, catch, throw, throws, and finally.
      • try: The code that might throw an exception is placed inside a try block.
      • catch: This block catches the exception thrown by the try block and handles it.
      • throw: This keyword is used to explicitly throw an exception.
      • throws: This keyword is used to declare an exception in the method signature.
      • finally: This block contains code that is always executed, whether an exception is handled or not.

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

  • Abstract Class:
    • An abstract class can have abstract methods (methods without body) as well as concrete methods (methods with body).
    • Abstract class can have member variables.
    • It can have constructors.
    • It supports multiple inheritance through interfaces.
  • Interface:
    • An interface can only have abstract methods (methods without body) until Java 8. From Java 8, it can have default and static methods with body.
    • Interface cannot have member variables.
    • It cannot have constructors.
    • It supports multiple inheritance by default.

 

Java Threads

31) What is the difference between processes and threads?

  • A process is an independent program that runs in its own memory space, whereas a thread is a smaller unit of a process that runs within the memory space of the process.
  • Processes are isolated from each other, whereas threads share the same memory space.
  • Processes are more heavyweight and require more resources to create and manage, whereas threads are lightweight and have lower overhead.

32) Explain different ways of creating a thread. Which one would you prefer and why?

  • There are two main ways to create a thread in Java:
    • By extending the Thread class: Subclass Thread and override its run() method.
    • By implementing the Runnable interface: Implement the run() method and pass an instance of the implementing class to a Thread object.
  • Preferred method: Implementing Runnable interface is generally preferred because it allows the class to extend another class if needed and separates the task from the thread itself, promoting better design and code reuse.

33) What is the difference between a method and a block that is synchronized?

  • Synchronized Method: Locks the object for which the method is called. Only one thread can execute the synchronized method at a time.
  • Synchronized Block: Locks a specific object or block of code within a method. This provides more granular control over the synchronization, potentially improving performance by reducing the scope of the lock.

34) What are the differences between the wait and sleep methods in Java?

  • wait() is used for inter-thread communication. It releases the lock on the object and allows other threads to acquire the lock.
  • sleep() is used to pause the execution of the current thread for a specified period. It does not release the lock held by the thread.
  • wait() must be called within a synchronized block or method, while sleep() can be called from any context.
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Top Interview Microservices Question

  1. What does Microservices mean? Explain the term Microservices?
  2. What Do You Mean by End-To-End Testing of Microservices?
  3. Why Would You Opt for Microservices Architecture?
  4. What are the advantages of using Microservices?
  5. Why Would You Need Reports and Dashboards in Microservices?
  6. Why Do People Hesitate to Use Microservices?
  7. Explain the major components of a Microservices Architecture?
  8. When designing Microservices, what are the best practices to follow?
  9. How is Microservice architecture different from Monolithic architecture?
  10. How is Microservice architecture different from Monolithic architecture?
  11. What is the meaning of RESTful?
  12. What is the meaning of OAuth? And why is it used?
  13. What are some challenges faced while using Microservices?
  14. What is the use of containers in Microservices?
  15. Explain end-to-end Microservices testing?
  16. In what kind of application we should use microservices?
  17. What is Docker used for?
  18. What is a "Client certificate"?
  19. How do independent Microservices communicate with each other?
  20. What are some common Microservices design principles?
  21. Give the major difference between Cohesion and Coupling?
  22. What is difference between API Gateway and Load Balancer in Microservices?
  23. How will you achieve asynchronous communication between Microservices?
  24. What is CQRS Pattern in Microservices? What problem does it solve?

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