Frequently Asked Java Interview Questions

Java is a class-based, general-purpose, object-oriented programming language and one of the most widely used programming languages in the world.

If you are looking for a new job as a Java developer, you are likely to encounter Java questions in technical interviews. However, there's no need to worry. In this content, we have listed frequently asked Java Interview Questions.

By studying these questions, you can easily achieve your dream job. So, let's review Java interview questions. 👇🔎

 

Most Popular Java Interview Questions and Answers

 

1. What is Java?

Java is an extensive, object-oriented, platform-independent programming language. Java can be used for web applications, mobile applications, desktop applications, games, or any other software.

Java allows code to run on multiple platforms by writing it once. It is executed by a machine called the Java Virtual Machine (JVM), which enables Java code to run on any platform. Java also includes many open-source libraries and tools.

 

2. What are the features of the Java programming language?

• Object-oriented: Java is an object-oriented programming language.
• Platform-independent: Java is written once and can run on different platforms. It supports many different operating systems.
• Automatic memory management: Java automatically handles memory management, eliminating the need for the programmer to manage memory.
• Secure: Java is highly secure. It ensures the security of the code.
• Open source: Java is an open-source programming language. Anyone can use Java. It allows users to modify or use the code.
• Multithreading: Java supports multiple threads and is used to perform multiple operations simultaneously.
• Dynamic: Java connects dynamically.
• Extensive: Java is an extensive programming language. It can be used in many different fields and to develop different types of software.
• Large user community: Java has a large user ecosystem. It also has a large user community. With this community, Java developers can help each other.

 

3. What is the Java Virtual Machine?

✔️ Java Virtual Machine (JVM) is a software necessary for running Java programs. It interprets code called bytecode and produces the desired output.

✔️ It enables the Java code to be understood by the operating system.

✔️ JVM also handles memory management, security, and debugging.

 

4. How does Java provide high performance?

Java provides high performance by using the JVM, Multi-Threading, and JIT compilers.

The JVM runs code called Java bytecode and enables the Java code to be understood by the operating system.

Multi-Threading in Java offers support for multiple threads. This allows multiple processes to be executed simultaneously. The JIT compiler is used to improve the performance of Java programs. Additionally, Java provides optimized libraries that speed up commonly used operations. Thus, Java provides high performance.

 

5. Why are pointers not used in Java?

Pointers are a common source of memory management issues in programming languages. Pointers are used to access data in memory, and during this access, misconfigurations can lead to memory-related problems.

 

6. What is a JIT compiler?

👉 A JIT compiler is a compiler that enables faster compilation of code.

 

7. What are Java IDEs?

A Java IDE is software that allows Java developers to easily write and debug Java programs. It includes various programming tools and helpful features like code completion and syntax highlighting. Codenvy, Eclipse and NetBeans are some of the most popular Java IDEs.

 

8. What is a Classloader?

The Java ClassLoader is a system used by the Java Virtual Machine (JVM). This system is used to load and run the classes required by Java programs.

In Java, there are three class loaders: Bootstrap ClassLoader, Extension ClassLoader , and System/Application ClassLoader.

 

9. What are the differences between JVM, JRE, and JDK?

JVM, JRE, and JDK are the three fundamental concepts of Java. JVM runs Java code, JRE provides the environment necessary to run Java programs, and JDK provides the tools necessary for developing Java programs. To elaborate further:

 

• 🎯 JVM stands for Java Virtual Machine, which makes Java code platform-independent and ensures that it runs on all operating systems and platforms.
• 🎯 JRE stands for Java Runtime Environment, which is software necessary for running Java programs. JRE includes all the necessary files related to Java, including the JVM.
• 🎯 JDK stands for Java Development Kit, which is software necessary for developing Java programs. JDK includes all the tools, libraries, compilers, and debuggers required for developing Java programs.

 

10. Is Java a platform-independent language?

Yes, Java is a platform-independent language. Java is compiled by the compiler and then converted to bytecode. Therefore, it is not dependent on any hardware or software. The bytecode is also platform-independent.

Java codes are run by the JVM. The JVM converts Java bytecode into a format understandable by the operating system and runs it. This allows Java codes to run on different operating systems and platforms.

 

11. Explain Typecasting and provide a code example.

Typecasting refers to the process of assigning/converting a variable of one data type to another data type. There are two types of data types: implicit and explicit.

Typecasting is usually performed by the programmer. For example, in Java, to convert an int value to a float value, the (int) expression is used.

 

The code is shown below 👇:

12. What is an object?

After a class is defined, a new data type is created. In Java, a variable belonging to this new data type created is called an object.

Two important features of a Java object are behavior and state. The JVM creates an object as soon as it encounters the new keyword.

 

13. Define classes in Java.

A class is a collection of objects that have similar data types. In Java, classes are a commonly used concept in programming languages. In fact, a class defines the design and behavior of an object.

Classes contain data and methods. In Java, the keyword "class" is used to define a class.

 

For example 👇:

 

14. What are static methods and variables?

A class has two elements that declare variables and methods. In Java, static methods and variables are methods and variables that are placed when the class is loaded, not when the objects of the class are created.

 

15. What are constructors in Java?

Java constructors also known as constructors, refer to the code blocks used to initialize objects. Constructors must have the same name as the class. They are automatically created when an object or class is created. They work with the " new" keyword.

When defining a constructor, it is important to follow these rules:

• ✅ The name of the constructor must be the same as the name of the class.
• ✅ The constructor must not have a return type.
• ✅ If multiple constructors are defined in a class, they must have different numbers and types of parameters.

 

 

16. What does singleton mean in Java?

A singleton is a class that can have only one instance created in a JVM at any given time. It is also referred to as a design pattern that ensures a class has only one instance and provides a single point of access to that instance throughout the application.

This pattern ensures that only one instance of a class is created and that this instance can be accessed from a single point throughout the application.

In Java, to create an object, the “new” keyword is used. The constructor, also known as the constructor call, is invoked. In this case, the object of this class is created as shown below 👇:

 

 

17. What are equals() and == in Java? What is the difference between them?

In Java, equals() and == operators are used to compare the equality of objects.

 

1️⃣ Equals() operator is used to check the equality of two objects as defined by business logic. The equals() operator is defined in the Object class.

The String class and some other classes override the "equals()" method for content comparison.

 

2️⃣ In Java's default implementation, the == operator can also be used to compare two objects. The "==" operator is used to check if two variables are pointing to the same memory location.

 

18. What is the super keyword in Java? When is it used?

In Java, the super keyword is a reference variable that refers directly to the superclass object. When you create an instance of a subclass, you also create an instance of the superclass referred to by the super keyword.

The uses of the Java super keyword are as follows:

1️⃣ It is used when you want to refer to a superclass instance variable.

2️⃣ The super keyword is used to call a superclass method.

3️⃣ Super() is used to call the superclass constructor.

 

Output:

Black

White

 

19. What are HashSet, TreeSet, and LinkedHashSet in Java?

HashSet, LinkedHashSet, and TreeSet are Set types in Java.

 

1️⃣ HashSet is a class that implements the Set interface and stores elements in a structure that does not allow duplicates. The HashSet interface orders its data irregularly. It provides performance savings.

In this case, the output will be: [null, Java, MySQL, Spring, Oracle, Angular]

 

2️⃣ LinkedHashSet maintains the order of the elements based on the order they were added. It is a subclass of HashSet. If the order of the data does not matter, LinkedHashSet will be unnecessarily costly.

 

3️⃣ TreeSet is a class that implements the NavigableSet interface and stores elements in a sorted manner. This class organizes elements based on partial sorting or natural sorting order.

 

20. What is the difference between HashSet and TreeSet?

 

21. When to use HashSet, LinkedHashSet, and TreeSet?

• 💻 Use HashSet: When there is no need to maintain the order of elements.
• 💻 Use LinkedHashSet: When you need a group of unique elements and the order of insertion is also important.
• 💻 Use TreeSet: When you need a group of unique elements/objects and the elements need to be sorted according to some comparator.

 

22. What is Java reflection?

Reflection is a runtime API used to inspect and modify the behavior of methods, classes, and interfaces.

Java Reflection is a powerful tool that allows the analysis of classes, interfaces, fields, and methods at runtime.

 

23. What is synchronization in Java?

✅ Java allows the execution of multiple threads. 

✅ Synchronization ensures that threads execute one after another.
✅ It prevents memory errors caused by accessing shared memory. 

 

An example of synchronization code:

 

24. What is Break in Java?

In Java, the break statement is used within loops to terminate the loop prematurely, controlling the flow of iterative structures.

It is used in constructs like "for, while, do while" loops and switch statements to exit the switch or loop immediately when executed.

In the following example, the loop is stopped when i=3.

 

25. What is Continue in Java?

In Java, the "continue" statement is a control statement used within loops or switch-case structures to skip a particular iteration and continue with the next iteration of the loop. It can only be used with loop statements.

 

Code example: 

 

26. What is an infinite loop in Java?

An infinite loop in Java is a loop or condition that never terminates because the loop condition always remains true. Essentially, the loop repeats indefinitely.

 

📍 Example:

 

27. What is the Java String Pool?

Image source: Edureka

 

The Java String Pool refers to a pool of strings stored in the heap memory. When a new object is created, the String pool first checks if the object already exists in the pool. If the object is in the pool, the same reference variable is returned. Otherwise, a new object is created in the String pool, and the corresponding reference is returned.

String objects can be created in two ways:

 

1- Using double quotes:

2- Using the new keyword:

 

28. What is OOP (Object-Oriented Programming) in Java?

Object-Oriented Programming or OOP is a programming model or approach where programs are organized around objects instead of logic and functions.

OOP focuses on objects instead of logic. This approach allows the development of large and complex codebases in a more organized and understandable manner.

 

29. What are the principles of OOP in Java?

1. Inheritance: Inheritance is when a class inherits the properties of another class.
2. Encapsulation: In Java, encapsulation is the bundling of data and code into a single unit.
3. Abstraction: Abstraction is the methodology of hiding implementation details from the user and providing only the necessary functionality.
4. Polymorphism: Polymorphism is the ability of a variable, function, or object to take on multiple forms.

 

 

30. What are access modifiers in Java?

Access modifiers control the access level of classes, methods, and variables, allowing or restricting access from other classes or packages.

There are 4 different access modifiers in Java: 

1️⃣ public: Can be accessed from anywhere.

2️⃣ protected: Can be accessed by classes, packages, and subclasses.

3️⃣ private: Can only be accessed within the class itself.

4️⃣ default(no access modifier): Can be accessed within the same package without using an access modifier.

 

31. What are this() and super() in Java? What are their differences?

These are used to call constructors within a class. Both methods must be written as the first line inside the constructor method. Otherwise, it will result in an error.

 

👉 "this()": This method can be used to call other constructor methods within the same class and invoke methods.

👉 "super()": It can be used to call superclass constructors and methods from a subclass.

 

32. What are overloading and overriding in Java?

👉 Overloading: In Java, overloading refers to having multiple methods with the same name within the same class. These methods differ in their parameters despite having the same name. This enhances code readability.

👉 Overriding: In Java, overriding refers to redefining a method in a subclass that already exists in the superclass. This allows the functionality of the superclass method to be changed and prevents code duplication.