Hey everyone! Greetings from my blog. Your being here excites me. You’re probably getting ready for software development job interviews since you’re reading this. Java remains among the leaders in this area. If you want a software engineer role or a full-stack developer job, learn Java. Even in MNCs, you’ll often need to review Data Structures and Algorithms (DSA) using Java or C++. Why? Many enterprise applications depend on Java.
I’ve collected 23 common Java interview questions from my experiences and feedback. This list will help you review key Java concepts and succeed in your next interview. So, grab your notebook and let’s get started.
Basic Java Interview Questions
These initial questions often set the stage for the rest of the interview. They test your foundational knowledge and understanding of core Java principles. Let’s tackle them together.
Q1. What is Java and why is it not considered a pure object-oriented programming language?
Java is a class-based, object-oriented programming language. It focuses on simplicity and portability. Its “write once, run anywhere” philosophy, thanks to the JVM, makes it very versatile. However, Java isn’t purely object-oriented. Here’s why:
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Primitive Data Types: Java has primitive types such as int, char, and boolean. These types are not objects. Wrapper classes like Integer, Character, and Boolean exist. However, primitives break the pure OOP model.
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Static Members: You can use static methods and variables without creating a class object. This contradicts the OOP principle that everything should interact through objects.
Q2. Key components to run a Java Program.
Running a Java program involves several key components working together seamlessly:
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JDK (Java Development Kit): This is your toolbox. It contains the compiler, debugger, and other essential tools for development.
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JRE (Java Runtime Environment): This is the environment where your compiled code runs. It includes the JVM.
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JVM (Java Virtual Machine): This is the heart of Java’s portability. The JVM executes the bytecode, making your program platform-independent.
Q3. Main Features of Java.
Java’s popularity stems from a powerful combination of features:
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Its syntax is quite easy to learn, especially when you compare it to languages like C++.
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Secure: Built-in security features help protect against malicious code.
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Portable: The JVM allows Java programs to run on any platform.
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Object-Oriented: Java embraces OOP principles, promoting code organization and reusability.
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Robust: Java’s exception handling and memory management make it reliable.
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Multi-threaded: Java can handle multiple tasks concurrently.
Q4. Java String Pool.
The String Pool is a special memory area within the heap. It’s designed to save memory and improve performance. When you create a string literal, Java first checks if that string already exists in the pool. If it does, a reference to the existing string is returned, preventing duplication.
Q5. Wrapper Class.
Wrapper classes provide a way to represent primitive data types as objects. This is essential when working with collections like ArrayList, which only store objects. Autoboxing and unboxing streamline the conversion between primitives and their corresponding wrapper objects.
Q6. Scenario-based question on Collections in Java.
Imagine managing an online bookstore’s data. Java Collections are perfect for this:
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ArrayList: Store the entire catalog of books. Its dynamic nature allows the list to grow as needed.
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HashSet: Maintain a set of unique genres, automatically eliminating duplicates.
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HashMap: Map authors to their books, enabling quick lookups.
Q7. Use of this and super keywords.
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this: Refers to the current object. Use it to access instance variables or methods within the same class.
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super: Refers to the parent class. Use it to access members of the parent class.
Q8. Difference between static and instance methods.
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Static Methods: Belong to the class itself. Called using the class name.
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Instance Methods: Belong to an object of the class. Called using an object reference.
Advanced Java Interview Questions
Now, let’s delve into more complex topics that truly demonstrate your Java expertise. These questions often explore nuanced concepts and require a deeper understanding.
Q9. Constructors and their types.
Constructors are special methods that initialize objects when they are created. There are two main types:
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Default Constructor: A no-argument constructor. Java provides a default constructor if you don’t define any explicitly.
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Parameterized Constructor: Accepts arguments to initialize object fields with specific values.
Q10. StringBuffer vs. StringBuilder.
Both are used for mutable strings, but with a key difference:
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StringBuffer: Synchronized (thread-safe). Slower but safe for concurrent access.
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StringBuilder: Non-synchronized (not thread-safe). Faster but not suitable for multi-threaded environments.
Q11. Abstract Classes vs. Interfaces.
Both define methods that subclasses must implement, but they serve different purposes:
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Abstract Classes: Can have both abstract and concrete methods. Single inheritance only.
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Interfaces: Can only have abstract methods (prior to Java 8 default methods). Multiple inheritance allowed.
Q12. Method Overloading and Can we overload the Main() method?
Method overloading allows you to use the same method name in a class with different parameters. You can overload the main method, too. However, the JVM will only see the standard public static void main(String[] args) as the entry point.
Q13. Method Overriding.
Method overriding allows a subclass to create its own version of a method that the superclass already has. The method signature must match the superclass method. Static and private methods cannot be overridden.
Q14. Exception Handling in Java.
Java handles errors using try, catch, and finally blocks. This helps manage runtime problems smoothly. The try block contains code that might throw an exception. The catch block handles specific exceptions. The finally block always executes, ensuring cleanup operations.
Q15. Lifecycle of a thread.
A Java thread goes through several stages:
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New: Thread is created but not yet started.
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Runnable: Thread is ready to run.
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Running: Thread is currently executing.
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Blocked/Waiting/Sleeping: Thread is temporarily inactive.
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Terminated: Thread has completed execution.
Q16. Singleton class.
A singleton class is designed to have only one instance throughout the application. This is achieved through a private constructor and a static method that returns the single instance.
Q17. Aggregation vs. Composition in Java.
Both represent relationships between objects, but with different levels of dependency:
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Aggregation: “Has-a” relationship. Objects can exist independently.
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Composition: “Part-of” relationship. One object controls the lifecycle of the other.
Q18. Anonymous inner class.
An anonymous inner class is a class without a name, defined and instantiated all at once. It’s useful for short, one-time use implementations of interfaces or abstract classes.
Q19. Difference between Implicit and Explicit Type conversion.
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Implicit: Automatic conversion by the compiler (e.g., int to double).
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Explicit: Manual conversion by the programmer (casting, e.g., (int)doubleValue).
Q20. Purpose of Volatile Keyword.
The volatile keyword makes sure that when one thread changes a variable, all other threads see that change right away. It prevents caching of the variable’s value.
Q21. System.out, System.err, and System.in.
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System.out: Standard output stream for general messages.
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System.err: Standard error stream for error messages.
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System.in: Standard input stream for user input.
Q22. Access Specifiers in Java and their purpose.
Java’s access specifiers control the visibility of class members:
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public: Accessible from anywhere.
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private: Accessible only within the same class.
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protected: Accessible within the same package and by subclasses.
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(default/package-private): Accessible within the same package.
Q23. Final, Finally, and Finalize keywords.
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final: Keyword used to declare a constant, prevent method overriding, or prevent class inheritance.
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finally: Block in exception handling that always executes.
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finalize: Method called by the garbage collector before an object is destroyed (rarely used).
Read Also:
How I Mastered Data Modeling For Interviews