Java interview question quiz Quiz

Test your understanding of important Java programming concepts and commonly asked interview questions on OOP, Java platform, bytecode, constructors, multithreading, and more. This quiz helps reinforce your Java knowledge with practical scenarios and technical reasoning.

1. Java Language Definition

   Which statement best describes Java as a programming language?

   1. Java is a low-level, hardware-dependent scripting language.
   2. Java is only used for mobile application development.
   3. Java is a high-level, object-oriented programming language designed for platform independence.
   4. Java is a markup language for webpage formatting only.

   Explanation: Java is high-level and object-oriented, offering platform independence through its bytecode and JVM. It is not low-level or hardware-dependent, nor is it limited to mobile or web markup purposes. The other options confuse Java's scope and nature.

2. OOP Pillars

   What are the four main pillars of object-oriented programming (OOP) in Java?

   1. Encapsulation, Inheritance, Polymorphism, Abstraction
   2. Abstraction, Enumeration, Aggregation, Overloading
   3. Inheritance, Compilation, Networking, Abstraction
   4. Polymorphism, Encryption, Serialization, Compilation

   Explanation: Encapsulation, Inheritance, Polymorphism, and Abstraction are the core OOP principles in Java. The other options either include unrelated or incorrect concepts such as Networking, Serialization, or Aggregation, which do not constitute OOP pillars.

3. JDK vs JRE

   How does the Java Development Kit (JDK) differ from the Java Runtime Environment (JRE)?

   1. JRE includes compilers and debuggers, but JDK does not.
   2. JDK is only required for running Java programs, JRE for developing them.
   3. JDK and JRE are exactly the same with no practical difference.
   4. JDK contains development tools, while JRE only provides the environment to run Java applications.

   Explanation: JDK has the necessary tools, such as compilers and debuggers, for creating Java programs; JRE only allows running Java applications. The second statement reverses their roles, and the third claims they're identical, which is false. JRE doesn’t include development tools as the fourth option suggests.

4. Java Platform Components

   Which of the following is NOT a core component of the Java platform?

   1. Database Management System
   2. Java Runtime Environment
   3. Java Virtual Machine
   4. Java Development Kit

   Explanation: Java platform consists mainly of JDK, JVM, and JRE. A Database Management System is separate and not inherently a Java platform component. The other three are direct parts of Java’s runtime and development environment.

5. String Reversal Algorithm

   Given the example, which approach reverses a string without using built-in methods in Java?

   1. Iterate from end of the string and append each character to a new string.
   2. Replace all vowels and print the string.
   3. Return the substring of the original string.
   4. Sort the characters in ascending order.

   Explanation: Reversing is done by iterating from the end and building a new string. Sorting rearranges content but doesn't reverse, substring extracts, and vowel replacement changes specific characters rather than reversing the entire string.

6. Prime Number Logic

   To check if a number is prime in Java, what is a required step as seen in the discussed logic?

   1. Sort all digits of the number.
   2. Add one to the number repeatedly.
   3. Ensure the number is greater than 1 and check divisibility up to its square root.
   4. Multiply the number by two.

   Explanation: A prime check involves verifying the number is over 1 and not divisible by any number up to its square root. Multiplying, adding, or sorting digits does not check for primality and misrepresents the standard approach.

7. Fibonacci Iterative Series

   What pattern is followed in the iterative approach to generating a Fibonacci series in Java?

   1. Multiply every number by three.
   2. Sort the range and print all numbers.
   3. Repeat printing the starting value only.
   4. Start with two numbers, repeatedly sum the previous two to generate the next one.

   Explanation: The Fibonacci sequence is constructed by summing the previous two numbers. Multiplying or sorting numbers, or repeating the start value, do not produce the Fibonacci series.

8. Java Platform Independence

   Why does compiled Java code run on multiple operating systems without modification?

   1. Each Java installation rewrites the code for the local device.
   2. Java programs are always interpreted from source code.
   3. Java is converted to embed operating system commands directly.
   4. Java code is compiled into platform-independent bytecode executed by the JVM.

   Explanation: Java’s compiled bytecode is interpreted by the JVM, enabling cross-platform execution. Embedding OS commands or source interpretation are not how Java achieves this. Java does not rewrite code per device at runtime.

9. Java vs C++

   Which is a correct difference between Java and C++?

   1. C++ is platform independent, Java is not.
   2. Java lacks garbage collection, while C++ has automatic collection.
   3. Java does not support pointers, but C++ does.
   4. Java enables operator overloading, C++ does not.

   Explanation: Java restricts direct pointer manipulation for safety; C++ uses them. Java does not support operator overloading, but C++ does. C++ is platform-dependent and does not have built-in garbage collection; Java includes it.

10. Java Bytecode

    When Java source code is compiled, what is the output and why is it important?

    1. Only a compressed archive of the code files.
    2. A document file readable by users.
    3. A .class file containing bytecode that can be executed on any JVM.
    4. A .exe file specific to the current operating system.

    Explanation: The compilation creates a .class file with platform-independent bytecode readable by any JVM. .exe files are operating system specific and not used by Java. Documentation files and archives are not produced by default compilation.

11. this() vs super()

    What is the main difference between this() and super() in Java constructors?

    1. super() is used for returning values, while this() is not.
    2. this() calls another constructor in the same class, while super() calls the superclass constructor.
    3. this() changes the parent class, while super() changes the current class.
    4. Both this() and super() can only be used in methods.

    Explanation: this() is to invoke another constructor of the current class, while super() invokes the parent class constructor. Neither is used solely for returning values or changing class definitions. These calls are only valid in constructors, not arbitrary methods.

12. JIT Compiler Role

    What best describes the function of the Java JIT (Just-In-Time) compiler?

    1. It converts bytecode to native machine code at runtime to improve performance.
    2. It interprets Java source code word by word.
    3. It only checks syntax during initial compilation.
    4. It compresses Java files for deployment.

    Explanation: The JIT compiler dynamically compiles bytecode into native instructions, enhancing runtime efficiency. It does not interpret source code directly, only check syntax, or compress files. Its role is performance optimization during execution.

13. Overloading vs Overriding

    Which scenario illustrates method overriding in Java?

    1. Two unrelated classes have methods with similar names.
    2. A subclass redefines a superclass method with the same name and parameters.
    3. A class has multiple methods with the same name but different parameter lists.
    4. A method is called within its own body recursively.

    Explanation: Overriding means a subclass implements a method already present in its superclass, matching the name and parameters. Overloading involves different parameter lists, not overriding. Recursion and similar method names in unrelated classes do not fit either definition.

14. Multithreading Meaning

    What does multithreading allow a Java program to do?

    1. Run multiple threads concurrently within a single program.
    2. Print multiple copies of the same string.
    3. Save files to several folders at once.
    4. Compile code in several languages simultaneously.

    Explanation: Multithreading enables concurrent execution of independent tasks (threads) in the same program, improving efficiency and responsiveness. It does not directly compile other languages, print duplicates, or control file storage actions.

15. Order of Constructor Calls

    In Java, what is required when both this() and super() exist in a constructor?

    1. Either this() or super() must be the first statement in the constructor body.
    2. Neither this() nor super() can be used in constructors.
    3. super() must always be called after this().
    4. Both this() and super() can be called anywhere in the constructor.

    Explanation: Java rules dictate that calls to this() or super() must occur as the constructor’s very first statement. Using them elsewhere or calling both together is not allowed. Ignoring both or using neither would default to the no-argument superclass constructor call.

16. Java Method Overloading

    Which of the following statements about method overloading in Java is correct?

    1. Method overloading allows multiple methods with the same name but different parameter types or counts in one class.
    2. Method overloading requires that two classes have identically named methods with the same parameters.
    3. Method overloading is only possible with static methods.
    4. Method overloading means redefining a method from the parent class in the child class.

    Explanation: Method overloading is limited to methods with the same name, differing in parameters, within a single class. It does not involve inheritance (that would be overriding) nor is it exclusive to static methods. Overloading doesn't mean methods in different classes with the same name.