Top 250 Java Interview Questions and Answers

A precondition is a condition that must be true before a function or method can be executed.

• Preconditions are used to ensure that the inputs to a function or method are valid.

• They help prevent errors and unexpected behavior by checking if certain conditions are met.

• If a precondition is not satisfied, an exception or error may be thrown.

• For example, a precondition for a function that calculates the square root of a number could be that the number must be positive.

Dagger is required for dependency injection in Android development.

• Dagger helps in managing dependencies and reduces boilerplate code.

• It provides compile-time safety and improves code readability.

• Dagger also helps in testing and modularizing the codebase.

• It is widely used in Android development for building scalable and maintainable apps.

User defined exceptions can be defined by creating a new class that extends the Exception class.

• Create a new class that extends the Exception class

• Add constructors to the class to initialize the exception

• Throw the exception using the 'throw' keyword

Yes, I can write code for printing a star in Java.

• Create an array of strings to represent the star

• Use a loop to iterate through each row of the array

• Use another loop to iterate through each column of the array

• Print the value at each row and column position

• Example: String[] star = {" * ", " *** ", "*****", " *** ", " * "};

• Example: for (String row : star) { System.out.println(row); }

Checked exceptions are checked at compile-time while unchecked exceptions are not.

• Checked exceptions are checked at compile-time and must be handled or declared in the method signature.

• Unchecked exceptions are not checked at compile-time and do not need to be handled or declared.

• Checked exceptions are usually used for recoverable errors while unchecked exceptions are used for unrecoverable errors.

• Examples of checked exceptions include IOException and SQLException while exampl...read more

A super class is a class that is inherited by other classes, providing them with common attributes and behaviors.

• Super class is also known as a base class or parent class.

• It serves as a blueprint for creating derived classes.

• Derived classes inherit the properties and methods of the super class.

• Super class can be extended or overridden by the derived classes.

• Super class promotes code reusability and modularity.

• Example: In a vehicle hierarchy, 'Vehicle' can be a super class wit...read more

The @primary annotation is used to mark a primary key in a database table.

• It is used in database design to indicate the primary key of a table

• It is often used in conjunction with other annotations such as @Entity and @Id

• It can be used to specify the name of the primary key column

• Example: @Entity @Table(name = "users") public class User { @Id @GeneratedValue @Column(name = "user_id") private Long id; }

Yes, I have knowledge of IT skills like Java, Python, C++, etc.

• I have experience in recruiting IT professionals with expertise in Java, Python, C++, etc.

• I am familiar with the technical requirements and job responsibilities of IT roles that require these skills.

• I keep myself updated with the latest trends and advancements in the IT industry.

• I have successfully placed candidates with proficiency in these skills in the past.

• I am comfortable discussing technical aspects of the j...read more

Extends is a keyword in object-oriented programming that allows a class to inherit properties and methods from another class.

• Extends is used in inheritance to create a subclass that is a modified version of the parent class.

• The subclass inherits all the properties and methods of the parent class and can also have its own unique properties and methods.

• For example, a class Animal can be extended by a subclass Dog, which inherits all the properties and methods of Animal and can ...read more

Python is a dynamically typed language with simpler syntax and better readability than Java.

• Python has a simpler syntax than Java

• Python is dynamically typed while Java is statically typed

• Python has better readability than Java

• Python has a larger standard library than Java

• Java is faster than Python in terms of execution speed

James Gosling is considered the father of Java programming language.

• James Gosling is a Canadian computer scientist who created the Java programming language.

• He developed Java while working at Sun Microsystems in the mid-1990s.

• Java was initially designed for interactive television, but it quickly gained popularity for its platform independence and object-oriented features.

• Gosling's team also developed the original Java compiler and virtual machine.

• Java has since become one of ...read more

Java core refers to the fundamental components of the Java programming language.

• Includes basic syntax, data types, control structures, and object-oriented programming concepts

• Provides the foundation for building Java applications

• Examples include classes like String, Integer, and Boolean

Java is a high-level programming language used for developing applications and software.

• Java is object-oriented and platform-independent.

• It is used for developing desktop, web, and mobile applications.

• Java is widely used in enterprise applications, such as banking and finance.

• It is also used in developing Android applications.

• Java is known for its security features and robustness.

Can you explain how to implement inheritance in Java?

• Inheritance allows a class to inherit properties and methods from another class

• Use the 'extends' keyword to create a subclass that inherits from a superclass

• The subclass can override methods from the superclass or add new methods

• Access modifiers can be used to control the visibility of inherited members

Bean not found exception occurs when a requested bean is not present in the container.

• Occurs during runtime when a bean is not defined in the application context

• Can be caused by typos in bean names or incorrect configuration

• Can be resolved by defining the missing bean or correcting the configuration

Static block is a block of code that is executed when a class is loaded into memory.

• Static block is declared using the 'static' keyword.

• It is executed only once when the class is loaded.

• It is used to initialize static variables or perform any other static initialization.

• It can throw exceptions which need to be handled.

• Multiple static blocks can be defined in a class and they are executed in the order they are defined.

I would rate myself 8 out of 10 in Java.

• I have a strong understanding of core Java concepts.

• I am proficient in object-oriented programming and can write clean and efficient code.

• I have experience in using Java frameworks and libraries like Spring and Hibernate.

• I am familiar with Java development tools and IDEs like Eclipse and IntelliJ.

• I have worked on various Java projects and have a good understanding of Java best practices.

Java is a versatile and widely-used programming language known for its platform independence and extensive libraries.

• Java is platform independent, meaning it can run on any operating system.

• It has a large and active community, providing support and resources.

• Java offers extensive libraries and frameworks for various purposes, making development faster and easier.

• It is widely used in enterprise applications, Android development, and big data processing.

• Java's object-oriented n...read more

stuff() replaces a specified number of characters with another string, while substring() extracts a portion of a string.

• stuff() is used to replace a specified number of characters in a string with another string.

• substring() is used to extract a portion of a string based on the starting and ending index.

• stuff() can be used to mask sensitive data in a string, such as credit card numbers.

• substring() can be used to extract a specific part of a string, such as the first name in a ...read more

Java Program is a set of instructions written in the Java programming language that can be executed by a computer.

• Java Program is a collection of classes and methods that define the behavior of a software application.

• It is written in the Java programming language and can be executed on any platform that has a Java Virtual Machine (JVM).

• Java Programs are compiled into bytecode, which is then interpreted and executed by the JVM.

• They can perform various tasks such as data manipu...read more

Public and instance classes are two types of classes in object-oriented programming.

• Public classes can be accessed from anywhere in the program, while instance classes can only be accessed within their own instance.

• Public classes are often used for utility classes or classes that need to be accessed globally, while instance classes are used for encapsulation and data hiding.

• Example of public class: Math class in Java. Example of instance class: Person class with private insta...read more

public static use for accessing methods and variables without creating an object

• Allows access to methods and variables without creating an object

• Useful for utility classes where objects are not needed

• Can be used to create global variables or constants

• Example: Math class in Java has only static methods and constants

Yes, Java is safe for me.

• Java is a secure programming language with built-in security features.

• It has a strong security model that prevents unauthorized access to sensitive data.

• Java also has a robust community that regularly updates and patches security vulnerabilities.

• However, like any technology, it is important to stay up-to-date with the latest security best practices.

• For example, using secure coding practices and keeping software up-to-date with security patches.

Static methods cannot be overridden in Java.

• Static methods belong to the class itself, not to any instance of the class.

• They cannot be overridden as they are resolved at compile-time based on the class they are called on.

• However, it is possible to hide a static method in a subclass by declaring a method with the same name and signature.

• This is known as method hiding.

Abstract class can have implementation while interface cannot. A class can implement multiple interfaces but can only inherit from one abstract class.

• Abstract class can have constructors while interface cannot.

• Abstract class can have non-abstract methods while interface can only have abstract methods.

• Abstract class can have instance variables while interface cannot.

• A class implementing an interface must implement all its methods while a class inheriting from an abstract class...read more

A declare block becomes mandatory when a variable needs to be declared before it can be used.

• A declare block is used in programming languages to define variables and their types.

• It becomes mandatory when a variable needs to be declared before it can be used in the code.

• This ensures that the variable is properly initialized and can be accessed correctly.

• For example, in C programming, a declare block is mandatory at the beginning of a function to declare local variables.

Java Core Java 8 is a major release of the Java programming language that introduced new features and enhancements.

• Java Core Java 8 introduced lambda expressions, which allow for functional programming.

• It also introduced the Stream API, which provides a more concise and functional way to work with collections.

• The Optional class was introduced to handle null values more effectively.

• Java Core Java 8 also included the new Date and Time API, which improved upon the previous java....read more

Java is a popular programming language known for its versatility and compatibility with various platforms.

• Java is an object-oriented language

• It is used for developing desktop, web, and mobile applications

• Java Virtual Machine (JVM) enables cross-platform compatibility

Abstract classes are classes that cannot be instantiated and can have both abstract and non-abstract methods. Interfaces are contracts that define the methods that a class must implement.

• Abstract classes can have constructors and instance variables, while interfaces cannot.

• A class can implement multiple interfaces, but can only inherit from one abstract class.

• Abstract classes are used when there is a need for a common base class for multiple related classes, while interfaces ...read more

int is a primitive data type, Int is a class in Kotlin, and Integer is a class in Java.

• int is a primitive data type in Java and Kotlin

• Int is a class in Kotlin that represents a nullable integer

• Integer is a class in Java that wraps an int value and provides utility methods

Abstract classes are classes that cannot be instantiated and can have both abstract and non-abstract methods. Interfaces are contracts that define the methods that a class must implement.

• Abstract classes can have constructors while interfaces cannot

• A class can implement multiple interfaces but can only inherit from one abstract class

• Abstract classes can have instance variables while interfaces cannot

• Interfaces can have default methods while abstract classes cannot

• Abstract cla...read more

Inner classes are used for encapsulation and code organization. Static inner classes are used for utility classes.

• Inner classes are used to group related classes together and improve encapsulation.

• Non-static inner classes have access to the outer class's fields and methods.

• Static inner classes are used for utility classes that don't need access to the outer class's fields and methods.

• Inner classes can also be used for anonymous classes and event listeners.

Servlet is an interface in Java EE used to handle HTTP requests and responses.

• Servlet interface is implemented by classes like HttpServlet

• It has methods like init(), service(), and destroy()

• Servlets are used to create dynamic web pages and web applications

Abstract class can have implementation while interface only has method signatures.

• Abstract class can have constructors while interface cannot.

• A class can implement multiple interfaces but can only extend one abstract class.

• Abstract class can have non-abstract methods while interface only has abstract methods.

• Abstract class is used for code reusability while interface is used for polymorphism.

• Example of abstract class: Animal with abstract method 'makeSound' and non-abstract m...read more

String is immutable, StringBuffer and StringBuilder are mutable. StringBuffer is thread-safe while StringBuilder is not.

• String is a final class and its value cannot be changed once created.

• StringBuffer is synchronized and can be accessed by multiple threads safely.

• StringBuilder is not synchronized and is faster than StringBuffer.

• StringBuffer and StringBuilder both can be modified and their length can be increased or decreased.

• Example: String name = "John"; StringBuffer sb = n...read more

Yes, we can create private constructors in Java.

• Private constructors are used to restrict the creation of objects of a class.

• They can only be accessed within the class.

• They are commonly used in Singleton design pattern.

• Private constructors can also be used in utility classes where all methods are static.

Yes, constructor can be overridden.

• Constructor can be overridden in Java using method overloading.

• The subclass constructor can call the superclass constructor using super() keyword.

• Overriding constructor can be used to provide different ways of initializing an object.

Protected modifier limits access to members within the same package or subclasses.

• Protected members can be accessed within the same package or subclasses.

• It provides a level of encapsulation and security.

• Example: protected int age; can be accessed within the same package or subclasses.

Abstract class is a class that cannot be instantiated, while interface is a contract that a class must implement.

• Abstract class can have both abstract and non-abstract methods, while interface can only have abstract methods.

• A class can implement multiple interfaces, but can only inherit from one abstract class.

• Abstract class can have constructors, while interface cannot.

• Abstract class can have instance variables, while interface cannot.

• Abstract class is used to provide a comm...read more

Java is a high-level programming language used for developing applications and software.

• Java is platform-independent and can run on any operating system.

• It is object-oriented and has a vast library of pre-built classes and functions.

• Java is used for developing web applications, mobile applications, desktop applications, and games.

• It is also used for developing enterprise-level applications and software.

• Java is widely used in the development of Android applications.

• Some popula...read more

A private constructor is a constructor that can only be accessed within the class and not from outside the class.

• Private constructors are used to prevent the creation of objects of a class from outside the class.

• They are also used to implement the Singleton design pattern.

• Private constructors can be used to create utility classes that contain only static methods and cannot be instantiated.

• Private constructors can also be used to enforce a certain initialization order of stati...read more

Java is a high-level programming language used for developing applications and software.

• Java is an object-oriented language

• It is platform-independent

• Java code is compiled into bytecode

• Java Virtual Machine (JVM) is used to run Java programs

• Java is used for developing web applications, mobile applications, desktop applications, and more

Finally code cannot be stopped once executed.

• Finally block is always executed, even if an exception is thrown or caught.

• There is no way to stop the execution of finally block.

• To prevent certain code from executing in finally block, use conditional statements.

To check Java version on server, use command line interface and run java -version command.

• Open command prompt or terminal

• Type java -version and press enter

• The installed Java version will be displayed

StringBuilder is a class in .NET that allows for efficient manipulation of strings.

• StringBuilder is mutable, meaning it can be modified without creating a new object.

• It is useful for concatenating large amounts of text.

• It has methods for inserting, replacing, and removing characters.

• Example: StringBuilder sb = new StringBuilder(); sb.Append("Hello"); sb.Append("World");

• Example: sb.Insert(5, " there"); sb.Replace("World", "Universe"); sb.Remove(11, 5);

Action class methods are used to perform keyboard and mouse actions in Selenium.

• Action class is part of Selenium's WebDriver API

• Methods include click(), doubleClick(), contextClick(), dragAndDrop(), etc.

• Used to simulate user interactions with web elements

• Can be used for testing complex user interactions like drag and drop, hover, etc.

A marker interface is an interface with no methods used to mark a class as having a particular property or behavior.

• Marker interfaces are used to provide metadata about a class.

• They are often used in frameworks and libraries to indicate that a class should be treated in a special way.

• Examples include Serializable, Cloneable, and Remote interfaces in Java.

• Marker interfaces can also be used to enforce design patterns, such as the Decorator pattern.