Baquara / javaInterviewQuestions

In a Java class, a static block is a set of instructions that is run only once when a class is loaded into memory. A static block is also called a static initialization block. This is because it is an option for initializing or setting up the class at run-time. In Java, a static method is a method that belongs to a class rather than an instance of a class. The method is accessible to every instance of a class, but methods defined in an instance are only able to be accessed by that object of a class.

Multithreading is a Java feature that allows concurrent execution of two or more parts of a program for maximum utilization of CPU. Each part of such program is called a thread. So, threads are light-weight processes within a process.

Abstract classses and interfaces

Abstract Classes

An abstract class is a class that cannot be instantiated. Abstract classes can contain both abstract and concrete methods. An abstract method is a method that is declared, but not implemented in the code. A concrete method is a method that is declared and implemented in the code. Abstract classes must be subclassed in order to be used.

Abstract classes are used to provide a common base class for a group of classes. They are also used to provide default implementations of methods that can be overridden by subclasses.

Abstract classes are classes that cannot be instantiated, but can be subclassed.

Abstract classes are typically used to define a base class from which other classes can inherit. For example, an abstract class might define a basic shape, with subclasses defining specific shapes like circles, squares, and triangles.

Final method is a method that is marked as final, i.e. it cannot be overridden anymore. Just like final class cannot be inherited anymore.

Abstract method, on the other hand, is an empty method that is ought to be overridden by the inherited class. Without overriding, you will quickly get compilation error.

Interfaces

An interface is a type that can be implemented by a class. Interfaces define a set of methods that must be implemented by any class that implements the interface.

Interfaces are used to define a contract that must be followed by any class that implements the interface. They are also used to provide a level of abstraction between classes.

An interface is a contract between a class and its subclasses that specify what methods must be implemented by the subclasses.

Interfaces are typically used to define a set of methods that must be implemented by a class. For example, an interface might define a set of methods for sorting data. A class that implements the interface would then need to provide implementations for those methods.

Java 8 has introduced the concept of default methods which allow the interfaces to have methods with implementation without affecting the classes that implement the interface.

Java Access Modifiers

Java has four access modifiers: public, protected, private, and default.

Public members are accessible from anywhere. Protected members are accessible from within the same package, as well as from subclasses. Private members are only accessible from within the same class. Default members are only accessible from within the same package.

Protected means that the member can only be accessed by classes in the same package or subclasses in a different package. Default means that the member can only be accessed by classes in the same package.

OOP Paradigms

There are three major OOP paradigms:

1. Encapsulation: This is the most basic principle of OOP. It involves keeping data and code together in an object, and hiding the implementation details from the outside world. This allows for code reuse and makes code more maintainable.

2. Inheritance: This is a powerful feature of OOP that allows one object to inherit the properties of another. This can be used to create hierarchical class structures, and to reuse code.

3. Polymorphism: This is the ability of an object to take on different forms. This can be achieved through overloading or overriding methods, or by using interfaces.

#REST

GET - Used to request data from a specified resource

POST - Used to send data to a specified resource to be processed

PUT - Used to send data to a specified resource to be updated

DELETE - Used to request that a specified resource be deleted

Annotations

There are four main annotations in Spring: @Component, @Repository, @Service, and @Controller. These annotations are used to mark classes as Spring beans, which are then managed by the Spring framework.

@Component - is the most general annotation and can be used for any type of class.

@Repository - is used for classes that perform data access operations.

@Service - is used for classes that provide business services.

@Controller - is used for classes that are part of the Spring MVC framework.

@Configuration - is used by Spring to designate a class as a configuration class. This class can be used to define beans and resolve dependencies.

Each of these annotations has a corresponding XML element that can be used for configuring Spring beans.

Sr. No.	Key	@Bean	@Component
1	Auto detection	It is used to explicitly declare a single bean, rather than letting Spring do it automatically.	If any class is annotated with @Component it will be automatically detect by using classpath scan.
2	Spring Container	Bean can be created even class is outside the spring container	We can’t create bean if class is outside spring container
3	Class/Method Level Annotation	It is a method level annotation	It is a class level annotation
4	@Configuration	It works only when class is also annotated with @Configuration	It works without@Configuration annotation
5	Use Case	We should use @bean, if you want specific implementation based on dynamic condition.	We can’t write specific implementation based on dynamic condition

Spring bean

A Spring bean is a Java object that is managed by the Spring IoC container (Inversion of Control is a principle in software engineering which transfers the control of objects or portions of a program to a container or framework). It is created and initialized during the container startup, and can be injected into other beans that are managed by the container.

For example, the following bean definition will create a bean with the name "myBean" and the class "MyBean":

<bean id="myBean" class="MyBean">

The following example shows how to inject a Spring bean into another bean:

<bean id="myDependency" class="MyDependency">

<bean id="myBean" class="MyBean">

<property name="myDependency" ref="myDependency"/>

</bean>

Dependency Injection

Dependency injection is a technique for passing dependencies to a class through its constructor or setter methods. This allows the dependencies to be injected at runtime, rather than being hard-coded into the class.

Spring uses dependency injection to provide dependencies to its beans. When a Spring bean is created, its dependencies are injected into it. This allows the beans to be decoupled from their dependencies, and makes it easy to change the dependencies without changing the beans.

In case of Field-Based DI, we can inject the dependencies by marking them with an @Autowired annotation:

public class Store { @Autowired private Item item; }

Spring Security

Spring Security is a framework for providing security to Java applications. It offers a number of features, including authentication, authorization, and access control. Spring Security can be used to secure web applications, REST services, and even individual methods.

Configuring Spring Security is typically done using XML or Java configuration. Spring Security can be configured to use a number of different authentication providers, including LDAP, database, and Active Directory.

Spring MVC vs Spring Boot

Spring MVC is a framework for building web applications. It handles all of the request and response processing, and provides a number of features for building maintainable and extensible web applications.

Spring Boot is a framework for building production-ready applications. It provides a number of features, including an embedded Tomcat server and auto-configuration. Spring Boot is designed to be used in conjunction with an application server, such as WildFly or Tomcat.

Design Patterns

There are four main design patterns in Java: Singleton, Factory, Decorator, and Observer.

The Singleton pattern is used to ensure that only one instance of a class is created. The Factory pattern is used to create objects without exposing the logic for creating them. The Decorator pattern is used to add new functionality to existing objects. The Observer pattern is used to notify objects of changes to another object.

On the Singleton pattern, in a multi-threaded environment, only one instance of the class created, and it is shared among all threads (it gets the instance from the first thread it is created). Since the instance of the other threads are not used, the memory occupied by those instances can be reclaimed by the garbage collector. Garbage collection is a process of reclaiming the memory occupied by objects that are no longer needed by the application.

Singleton:

public class Coin {

private static final int ADD_MORE_COIN = 10;
private int coin;
private static Coin instance = new Coin(); // eagerly loads the singleton

private Coin() {
    // private to prevent anyone else from instantiating
}

public static Coin getInstance() {
    return instance;
}

public int getCoin() {
    return coin;
}

public void addMoreCoin() {
    coin += ADD_MORE_COIN;
}

public void deductCoin() {
    coin--;
}

}

The Factory Pattern:

public class Factory {

   // An abstract factory has multiple methods that return
   // different abstract products

   public abstract Product1 createProduct1();

   public abstract Product2 createProduct2();

}

The Observer Pattern:

public class Main {

public static void main(String[] args) {
    
    // create subject
    MyTopic topic = new MyTopic();

    // create observers
    Observer obj1 = new MyTopicSubscriber("Obj1");
    Observer obj2 = new MyTopicSubscriber("Obj2");
    Observer obj3 = new MyTopicSubscriber("Obj3");

    // register observers to the subject
    topic.register(obj1);
    topic.register(obj2);
    topic.register(obj3);

    // attach observer to subject
    obj1.setSubject(topic);
    obj2.setSubject(topic);
    obj3.setSubject(topic);

    // check if any update is available
    obj1.update();

    // now send message to subject
    topic.postMessage("New Message");
}

}

The Decorator pattern:

public class DecoratorPattern {

    public static void main(String[] args) {
        Car sportsCar = new SportsCar(new BasicCar());
        sportsCar.assemble();
        System.out.println("\n*****");

        Car sportsLuxuryCar = new SportsCar(new LuxuryCar(new BasicCar()));
        sportsLuxuryCar.assemble();
    }

}

Java 8 vs Java 11

There are a number of significant changes between Java 8 and Java 11. Some of the most notable changes include:

• The removal of the Java EE and CORBA modules
• The addition of a new module, java.se.ee, which includes all the Java EE and CORBA modules
• The introduction of a new release cycle, where a new major release occurs every six months
• The introduction of a new tool, jlink, which can create a custom runtime image containing only the modules required for a particular application
• The addition of support for HTTP/2
• The addition of a new default garbage collector, Garbage First (G1)
• The addition of a new JVM, GraalVM, which can run applications written in a variety of languages
• The addition of support for the Java Platform Module System (JPMS)

What Java 8 originally introduced?

Write the most important additions from Java 8.

• Lambda expressions
• Stream API
• Date and time API
• Default methods
• Optional class

Write an example of Java Lambda expressions.

(x, y) -> x + y

Write an example of Java Stream API

list.stream() .filter(x -> x % 2 == 0) .map(x -> x * x) .forEach(System.out::println);

This code filters a list of numbers, keeping only the even numbers. It then squares each of the remaining numbers, and finally prints them out.

What Java 8 default method does?

Java 8 default methods are methods in interfaces that have a default implementation. This allows for new methods to be added to interfaces without breaking existing code that implements the interface.

What is an Optional class?

The Optional class is a container class that represents a value that may or may not be present. It is used to avoid NullPointerException errors.

Write an example of the Date and time API.

LocalDateTime dt = LocalDateTime.now(); System.out.println(dt);

This code creates a LocalDateTime object that represents the current date and time.

Write what is the Java Stream API.

The Java Stream API is a set of tools that allow developers to manipulate collections of data in a declarative way. This can be used to perform operations such as filtering, mapping, and reducing.

Write an example of an Optional class.

Optional str = Optional.of("Hello"); System.out.println(str.isPresent());

This code creates an Optional object that contains the string "Hello". The isPresent() method is used to check if the Optional object contains a value.

Write a code using the default method.

public interface MyInterface { default void doSomething() { System.out.println("Doing something"); } }

public class MyClass implements MyInterface { public static void main(String[] args) { MyClass myClass = new MyClass(); myClass.doSomething(); } }

This code implements an interface that has a default method. The MyClass class then calls the doSomething() method.

what is event loop in js?

two sum leetcode

can constructor be private?

Yes, a constructor can be private, so when we declare a private contructor, we cannot create an object out of the class; We can use this private constructor when using the Singleton pattern.

design a singleton

    public class Test{

    private Test(){


    }


    }

1. OOP: inheritance, polymorphism, abstraction

Inheritance is when a class extends a (parent) class, polymorphism is when you can use methods from a parent class in different ways (by overloading or overriding)

2. Spring and Hibernate

Hibernate is Spring's ORM.

3. Singleton, observer, etc

What is CI/CD and DevOps?

Consider a program to remove duplicate characters

You can remove duplicate characters by using set.

Questions on collection

The Collection in Java is a framework that provides an architecture to store and manipulate the group of objects. Java Collections can achieve all the operations that you perform on a data such as searching, sorting, insertion, manipulation, and deletion. Java Collection means a single unit of objects.

throw vs throws

The throws keyword is used to declare which exceptions can be thrown from a method, while the throw keyword is used to explicitly throw an exception within a method or block of code.

types of interface

1. Functional Interface: Functional Interface is an interface that has only pure one abstract method. It can have any number of static and default methods and also even public methods of java.lang.Object classes

2. Marker Interface: An interface that does not contain any methods, fields, Abstract Methods, and any Constants is Called a Marker interface. Also, if an interface is empty, then it is known as Marker Interface. The Serializable and the Cloneable interfaces are examples of Marker interfaces.

3. Serializable Interface:

It is a marker interface in Java that is defined in the java.io package. If we want to make the class serializable, we must implement the Serializable interface. If a class implements the Serializable interface, we can serialize or deserialize the state of an object of that class. Serialization is a mechanism in which our object state is ready from memory and written into a file or from the databases. Deserialization- is the opposite of serialization means that object state reading from a file or database and written back into memory is called deserialization of an object.

How to delete all duplicate elements in a list and keep the list order unchanged.

What is the purpose of static methods and variables?

In Java, static keyword is mainly used for memory management. It can be used with variables, methods, blocks and nested classes. It is a keyword which is used to share the same variable or method of a given class. Basically, static is used for a constant variable or a method that is same for every instance of a class

What Is Factory Pattern?

What design pattern do you know?

(1)oops concept

(2)Singleton

(3) Spring Framework

(4)Spring beans

(5)collection framework

Explain java collections, you can say as much as you want.

And concept of constructors and so on

How to sort data

What is Spring MVC

1)what is Singleton? and how can you implement

2)Hash Map?

Java HashMap is a hash table based implementation of Java's Map interface. A Map, as you might know, is a collection of key-value pairs. It maps keys to values. Following are few key points to note about HashMaps in Java - A HashMap cannot contain duplicate keys.

3)Interface vs Abstract class?

4)Exception Handling?

Heap Memory

The Java heap is the area of memory used to store objects instantiated by applications running on the JVM. When the JVM is started, heap memory is created and any objects in the heap can be shared between threads as long as the application is running.

what is the difference between supervised learning and unsupervised learning

Java Core, Collection, Multithreading, Design Pattern, etc

Spring Framework concepts like IoC container, Annotations

Servlet concept, Database query concept, etc

Find all pairs in array that sum up to given number

Types of stlc, defect life cycle, debugging in JAVA, oops concept, applying different types of waits in selenium, type of models in SDLC.

Hashmap, ArrayList vs LinkedList, static, singleton

ArrayList internally uses a dynamic array to store its elements. LinkedList uses Doubly Linked List to store its elements. ArrayList is slow as array manipulation is slower. LinkedList is faster being node based as not much bit shifting required. ArrayList provides constant time for search operation, so it is better to use ArrayList if searching is more frequent operation than add and remove operation. The LinkedList provides constant time for add and remove operations. So it is better to use LinkedList for manipulation.

In java, what is the difference between list and arraylist?

ArrayList class is used to create a dynamic array that contains objects. List interface creates a collection of elements that are stored in a sequence and they are identified and accessed using the index. ArrayList creates an array of objects where the array can grow dynamically. List is always faster than ArrayList.

difference between comparator and comparable Comparable interface is used to sort the objects with natural ordering. Comparator in Java is used to sort attributes of different objects. Comparable interface compares “this” reference with the object specified. Comparator in Java compares two different class objects provided.

Data Science: Case study , random forest, decision tree, logistic algorithm

1. What are lambda expressions? Why are they used?

2. What is a functional interface?

A functional interface is an interface that contains only one abstract method. They can have only one functionality to exhibit. From Java 8 onwards, lambda expressions can be used to represent the instance of a functional interface. A functional interface can have any number of default methods. Runnable, ActionListener, Comparable are some of the examples of functional interfaces.

3. What are the features of Java 7?

4) How is I18N implemented in spring boot?

Internationalization or I18N is a process that makes your application adaptable to different languages and regions without engineering changes on the source code. You can display messages, currencies, date, time etc. according to the specific region or language, likewise you can say internationalization is a readiness of localization.

5. How do you externalize properties in spring boot?

6. What will happen if you override equals but not hashcode?

7. Have you used Kafka?

8. Have you used ESB?

How to implement Singleton?

Final vs finally vs finalize

HashMap structure

What is HashCode?

A hashcode is an integer value associated with every object in Java, facilitating the hashing in hash tables. The equals() and hashcode() are the two important methods provided by the Object class for comparing objects. Since the Object class is the parent class for all Java objects, hence all objects inherit the default implementation of these two methods.

Hashcode vs equals

The hashcode() method returns the same hash value when called on two objects, which are equal according to the equals() method. And if the objects are unequal, it usually returns different hash values. If two objects are the same as per the equals(Object) method, then if we call the hashCode() method on each of the two objects, it must provide the same integer result.

Do you know JSP Hibernate etc?

Basic Java, OOPs, LinkedList, HashMaps, etc.

overload vs override

what is an interface

Difference between Synchronized block and method?

A synchronized method provides a lock corresponding to object-level or Class level ( i.e class level means static method ), whereas, synchronized block provides a lock on any object depending on the parameter

What is the difference between abstraction class and interface.

What is static keyword.

1. What you mean by Inheritance?

2. What you mean by Abstract method?

3. Can we have a class without public?

4. Can we have a private class? How will you access the properties of that class?

5. How will you handle the exception without using try-catch block?

What is encapsulation?

What is arraylist?

Tell me the difference between method and constructor.

What's the difference between abstract class and interface?

What is exception?

What's the diff between override and overload?

what is different among private, public and protect?

what is synchronized in Java?

Synchronization in java is the capability to control the access of multiple threads to any shared resource. In the Multithreading concept, multiple threads try to access the shared resources at a time to produce inconsistent results. The synchronization is necessary for reliable communication between threads

What's the different between Object and Class?

What is the difference between errors and exceptions?

What is the difference between HashMap and HashTable?

One of the major differences between HashMap and Hashtable is that HashMap is non-synchronized whereas Hashtable is synchronized, which means Hashtable is thread-safe and can be shared between multiple threads but HashMap can not be shared between multiple threads without proper synchronization. HashMap allows one null key and multiple null values whereas Hashtable doesn't allow any null key or value. HashMap is generally preferred over HashTable if thread synchronization is not needed

Can you discribe collections in Java?

Can you discribe LinkedList in Java?

fibonacci series

Giving you an ArrayList that stores integer, find out the max and min value.

• Singleton, Facade and Factory design pattern

• What's dynamic binding?

In Java, what is an static variable?