Java Backend Interview Questions
Top Interview Questions for Java Backend Developers
Java Backend Interview Questions are a set of questions designed to evaluate a candidate's knowledge, skills, and experience in Java programming language and its usage for developing backend applications. These questions cover a wide range of topics such as core concepts of Java, data structures and algorithms, database management, web development frameworks, server-side technologies, and problem-solving abilities. The aim of these questions is to assess a candidate's ability to handle complex backend tasks, troubleshoot and optimize code, and work with various tools and technologies to create efficient and scalable applications. Employers use these questions to evaluate a candidate's technical expertise and suitability for backend development roles.
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1 - What is the difference between HashMap and HashTable?
HashMap and HashTable are both data structures used for storing key-value pairs. However, HashMap allows one null key and multiple null values, while HashTable does not allow null keys or values. HashMap is not synchronized, making it faster for single-threaded operations, whereas HashTable is synchronized, making it safe for multi-threaded operations.
2) How does garbage collection work in Java?
Garbage collection is the process of automatically freeing up memory occupied by objects that are no longer referenced in a program. Java uses a mark-and-sweep algorithm for garbage collection. This involves marking all live objects and sweeping away the unreferenced ones, freeing up memory for new objects. The garbage collector runs periodically, and objects that are no longer referenced are removed from memory.
3) What is the difference between an interface and an abstract class?
An interface is a completely abstract class, meaning it cannot have any implementation of its own. All the methods declared in an interface are public and must be implemented by the class that implements it. Abstract classes, on the other hand, can have both abstract and non-abstract methods and can also contain implementation. A class can implement multiple interfaces, but can only extend one abstract class.
4) What are the pillars of Object-Oriented Programming (OOP)?
The four pillars of OOP are abstraction, encapsulation, inheritance, and polymorphism. Abstraction allows you to hide the implementation details and only show the necessary information to the user. Encapsulation enables you to group related data and functions together, protecting them from external access. Inheritance allows for the creation of new classes based on existing ones, inheriting their properties and behaviors. Polymorphism allows for different implementations of the same method, depending on the object it is called on.
5) What is the difference between a shallow copy and a deep copy?
A shallow copy creates a new object with the same values as the original object. However, if the original object has any references to other objects, the references are shared between the original and the shallow copy. On the other hand, a deep copy creates a complete replica of the original object, including all its references. This means that changes made to the original object will not affect the deep copy, making it more independent.
6) What is the difference between a class and an object?
A class is a blueprint or template for creating objects, while an object is an instance of a class. A class defines the properties and behaviors shared by all its objects. On the other hand, an object holds the data and performs operations based on the class it belongs to.
7) How to handle exceptions in Java?
Exceptions are runtime errors that can occur during the execution of a program. To handle exceptions, you can use the try-catch block. Code in the try block is monitored, and if an exception occurs, the catch block is executed, allowing you to handle the exception gracefully. You can also use the finally block to execute code that should be executed regardless of whether an exception occurs or not.
8) What is the difference between a static and a non-static method?
A non-static method belongs to an object of a class and can only be called on an instance of that class. A static method, on the other hand, belongs to the class itself and can be called without creating an instance of the class. Static methods are typically used for utility functions or for operations that do not require access to instance variables.
9) What is the role of a constructor in Java?
A constructor is a special method used for initializing objects in Java. It is called when an object of a class is created using the new keyword. A constructor can be used to set initial values for the object's properties or to perform any other necessary operations before the object is ready for use. Every class has at least one constructor, and if not explicitly defined, a default constructor is automatically created.
10) What is the difference between a thread and a process?
A process is a program that is currently being executed, while a thread is a separate execution unit within a process. A process can contain multiple threads, each performing a different task. Threads within the same process share memory and resources, making them lightweight and efficient compared to separate processes.
11) What is a Java Virtual Machine (JVM)?
JVM is a virtual machine that executes Java bytecode. It is responsible for interpreting and executing the code, making it platform-independent. It also provides memory management, garbage collection, and security for Java programs.
12) What is the difference between a stack and a heap?
A stack is a data structure that stores data in a last-in-first-out (LIFO) order. It is used to store local variables and method calls. On the other hand, a heap is a data structure that stores data in a random order and is used for dynamic memory allocation, mainly for objects.
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13) What is a singleton class in Java?
A singleton class is a class that allows for the creation of only one instance of itself. This is achieved by making the constructor private, preventing other classes from directly creating instances of it. The single instance can be accessed through a static method within the class, ensuring that only one object of that class exists in the entire application.
14) What is the difference between pre-emptive and non-preemptive scheduling?
In pre-emptive scheduling, a process can be interrupted and moved out of the CPU by the operating system to allow for the execution of a higher priority process. In non-preemptive scheduling, a process that is currently using the CPU will continue to do so until it releases the CPU voluntarily.
15) What is serialization and deserialization in Java?
Serialization is the process of converting an object into a stream of bytes, mainly used for network communication or for storing the object's state. Deserialization is the reverse process of converting the byte stream back into an object. Java provides the Serializable interface and the ObjectOutputStream and ObjectInputStream classes for serialization and deserialization, respectively.
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