java interview Questions

Wrapper classes: Why they matter ? Wrapper classes convert primitives (int, boolean, etc.) into objects. You need them because: • Collections like ArrayList only store objects, not primitives.  • They provide utility methods for type conversions. • They let you use primitives where objects are required. Each primitive has its wrapper: int → Integer, double → Double, char → Character, and so on.

'==' vs '.equals()': A common interview trap This distinction catches many freshers off guard. For primitive types, == performs value comparison. But for objects, it only checks if two references point to the same memory address. To compare object values properly, use .equals(), many classes override this method to provide meaningful comparisons.

When to use map and flatMap? map(): It is used where we have to map the elements of a particular collection to a specific function, and then we need to return the stream that contains the updated results. Example: Multiply all the elements of a list by 3 and return the updated list. flatMap(): It is used where we have to transform or flatten the string, as we can't flatten our string using map(). Example: Get the first Character of all the String present in a List of Strings and return the result in form of a stream.

What is Functional Interface in Java 8? An interface with only one abstract method is known as a functional interface but there is no restriction, in a functional interface you can have n number of default methods and static methods.

-------Date/Time API-------- This section gives you to handle the ever-changing world of dates and times within your Java programs. Explore working with calendars, timestamps and time manipulation – essential skills for building applications that deal with deadlines, scheduling or even historical data analysis.

Comparable and Comparator ----------------------------------------- Comparable and Comparator are interfaces used to order objects. They are particularly useful in sorting operations and collections that require natural ordering. Here we will learn about Comparable and Comparator in depth.

--- ---Streams------- Stream API is introduced in Java 8 and is used to process collections of objects with the functional style of coding using the lambda expression. So to understand what stream API is, you must have knowledge of both lambda and functional interfaces.

import java.util.Arrays; import java.util.LinkedHashSet; public class ArrayExample {     public static void main(String[] args) throws CloneNotSupportedException     {         //Array with duplicate elements         Integer[] numbers = new Integer[] {1,2,3,4,5,1,3,5};                  //This array has duplicate elements         System.out.println( Arrays.toString(numbers) );                  //Create set from array elements         LinkedHashSet<Integer> linkedHashSet = new LinkedHashSet<>( Arrays.asList(numbers) );                  //Get back the array without duplicates         Integer[] numbersWithoutDuplicates = linkedHashSet.toArray(new Integer[] {});                  //Verify the array content         System.out.println( Arrays.toString(numbersWithoutDuplicates) );     } }

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What is the role of a Java Backend Developer? A Java Backend Developer designs, develops, and maintains server-side logic, database interactions, and application architecture using Java technologies.

What are the primary data storage options in Java Backend? Primary data storage options include relational databases (MySQL, PostgreSQL), NoSQL databases (MongoDB, Cassandra), and in-memory databases (Redis, Hazelcast).

What is the difference between Java EE and Java SE? Java SE (Standard Edition) is the core Java platform, while Java EE (Enterprise Edition) provides additional features for enterprise-level application development, such as distributed computing and web services.

How does Java support concurrent programming? Java supports concurrent programming through the java.util.concurrent package, thread synchronization, executor frameworks, atomic variables, and advanced concurrency utilities.

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