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Java interview questions

Updated: Mar 4, 2023


What is Java?

  • Java is a high-level programming language that is class-based, object-oriented, and designed to have as few implementation dependencies as possible. It was developed by James Gosling at Sun Microsystems in the mid-1990s and is now owned by Oracle Corporation.


What is the difference between JDK, JRE, and JVM?

  • JDK (Java Development Kit) is a software development kit that includes the necessary tools to develop, test, and run Java applications. It includes the JRE (Java Runtime Environment) and other tools such as the Java compiler and debugger.

  • JRE (Java Runtime Environment) is a set of libraries and tools that allow Java applications to run on a computer. It contains the Java Virtual Machine (JVM) and the Java class libraries.

  • JVM (Java Virtual Machine) is a virtual machine that runs Java bytecode. It is responsible for interpreting the bytecode into machine code that can be executed by the computer.

  • What is the difference between private and protected in Java?

  • Private: A private method or variable can only be accessed within the same class in which it is declared. It cannot be accessed from outside the class.

  • Protected: A protected method or variable can be accessed within the same class and any subclasses of that class. It cannot be accessed from outside the class or package.


What is an object in Java?

  • An object in Java is an instance of a class. It represents a real-world entity and contains data and behavior. An object is created from a class and can be used to call methods and access variables of that class.


What is inheritance in Java?

  • Inheritance is a mechanism in Java that allows a class to inherit properties and behavior from a parent class. The class that inherits from another class is called a subclass and the class that is being inherited from is called the superclass. Inheritance allows for code reuse and promotes abstraction.



What is an interface in Java?

  • An interface in Java is a blueprint for classes. It defines a set of methods that a class must implement, but it does not provide any implementation for those methods. Interfaces are used to define common behavior for a set of related classes and promote abstraction.


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

  • Abstract Class: An abstract class is a class that cannot be instantiated on its own. It provides a base for subclasses to inherit from and can contain both abstract and concrete methods.

  • Interface: An interface defines a set of methods that a class must implement, but it does not provide any implementation for those methods. Interfaces are used to define common behavior for a set of related classes and promote abstraction.


What is a constructor in Java?

  • A constructor in Java is a special method that is called when an object is created from a class. It is used to initialize the object's state and must have the same name as the class. A constructor can take arguments and can be overloaded.


What is the difference between == and .equals in Java?

  • == operator: The == operator compares the memory addresses of two objects to determine if they are the same .equals method: The .equals method compares the values of two objects to determine if they are equal. The .equals method is defined in the Object class and can be overridden in subclasses to provide a more specific implementation


What is the difference between a stack and a queue?

  • Stack: A stack is a linear data structure that follows the Last-In-First-Out (LIFO) principle. The last element to be added to the stack will be the first one to be removed.

  • Queue: A queue is a linear data structure that follows the First-In-First-Out (FIFO) principle. The first element to be added to the queue will be the first one to be removed.

What is a HashMap in Java?

  • A HashMap is a class in Java that implements the Map interface and stores key-value pairs. It uses a hash table to implement the map, which allows for efficient operations like add, remove, and search.


What is a method overloading in Java?

  • Method overloading in Java is a feature that allows a class to have two or more methods with the same name, but with different parameters. When an overloaded method is called, Java uses the type and number of arguments to determine which version of the method to execute.


What is an anonymous inner class in Java?

  • An anonymous inner class in Java is a class that is declared and instantiated at the same time, without giving it a name. Anonymous inner classes are typically used as a convenient way to create instances of a class for one-time use, such as in event listeners.


What is a final variable in Java?

  • A final variable in Java is a variable that cannot be changed once it is assigned a value. Final variables can be either class-level (static) or instance-level.



What is the difference between a for loop and a for-each loop in Java?

  • For Loop: A for loop is a control structure that allows you to execute a set of statements a specified number of times. It is typically used to iterate over an array or a list.

  • For-Each Loop: A for-each loop is a control structure that allows you to iterate over an array or a list in a more concise and readable way. It is a variation of the for loop that was introduced in Java 5.


What is the difference between a while loop and a do-while loop in Java?

  • While Loop: A while loop is a control structure that allows you to execute a set of statements as long as a specified condition is true. The condition is evaluated before each iteration of the loop.

  • Do-While Loop: A do-while loop is a control structure that is similar to a while loop, but with one key difference. The condition is evaluated after the first iteration of the loop, which means that the loop will always be executed at least once.


What is a static method in Java?

  • A static method in Java is a method that belongs to the class and not to any particular instance of the class. Static methods can be called without creating an instance of the class and can only access static variables.


What is a Thread in Java?

  • A Thread in Java is a concurrent unit of execution that can run independently of other threads. It is used to implement multitasking in Java and allows multiple tasks to be performed simultaneously within a single program.


What is a synchronized method in Java?

  • A synchronized method in Java is a method that is used to control access to a shared resource by multiple threads. When a synchronized method is executed, it locks the object that it is associated with, ensuring that only one thread can access


What is a constructor in Java?

  • A constructor in Java is a special type of method that is used to initialize an object. It has the same name as the class and is called when an object of the class is created. A constructor does not have a return type, not even void.


What is the difference between a constructor and a method in Java?

  • Constructor: A constructor is a special type of method that is used to initialize an object. It has the same name as the class and is called when an object of the class is created.

  • Method: A method is a block of code that performs a specific task. It can be called from other parts of the program and can return a value to the caller.




What is a superclass in Java?

  • A superclass in Java is a class that is higher in the inheritance hierarchy than another class. A subclass can inherit properties and methods from its superclass.


What is a subclass in Java?

  • A subclass in Java is a class that inherits properties and methods from its superclass. A subclass can also add new properties and methods or override existing ones.


What is inheritance in Java?

  • Inheritance in Java is a mechanism that allows a subclass to inherit properties and methods from its superclass. This allows for code reuse and can make it easier to maintain and extend a program.


What is an abstract class in Java?

  • An abstract class in Java is a class that cannot be instantiated on its own. It provides a common interface for its subclasses, but requires the subclasses to provide specific implementations of the abstract methods.


What is an interface in Java?

  • An interface in Java is a blueprint for classes. It defines a set of methods that a class must implement, but does not provide any implementation for those methods. Interfaces are used to define contracts that classes must follow.


What is a package in Java?

  • A package in Java is a collection of related classes and interfaces. Packages are used to organize code in a Java program and to prevent naming conflicts between classes with the same name.


What is a singleton class in Java?

  • A singleton class in Java is a class that can only have one instance. The singleton pattern is used to ensure that a class has only one instance, while also providing a global point of access to that instance.


What is a serialization in Java?

  • Serialization in Java is the process of converting an object's state to a byte stream, so it can be persisted to a file or transmitted over the network. Deserialization is the reverse process of converting a byte stream back into an object.


What is a volatile variable in Java?

  • A volatile variable in Java is a variable that can be changed by multiple threads. The Java Memory Model ensures that changes to a volatile variable are visible to all threads, even if the variable is cached in a local register.




What is a JavaBean in Java?

  • A JavaBean in Java is a Java class that follows certain naming and design conventions. JavaBeans are used to encapsulate data and behavior in a reusable component, and can be easily manipulated by tools like JavaServer Faces (JSF) and Enterprise JavaBeans (EJB).


What is a JAR file in Java?

  • A JAR file in Java is a compressed archive file that contains Java class files and other resources. JAR files can be used


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