12 Java Developer interview questions and answers

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How would you define Java classes? Would you please provide examples?

Since a class in Java is the blueprint that includes all of the data, your contractor should have a firm grasp of how Java classes operate. Therefore, when asking for information about the functionalities of java classes, look for answers that mention classes that are made up of fields, attributes, methods, or behaviors. Contractors should also be able to describe class methods that are specific to java objects. Your contractor should be familiar with common class names, including subclass, abstract class, superclass, public class, base class, child class, parent class, final class, string class, and object class.

What is the difference between a local variable and an instance variable?

Your freelancer should understand the difference between and applications of these two variables. The freelancer should be able to tell you that the local variable is used within a method, default constructor, or a block of code and has only a local scope. They should confirm that the instance variable is the one that is used outside or globally. The ability to distinguish between these two variables is an indicator that the coder knows the importance of isolating local and global variables and how to save space within the code. 

What are important features of the Java programming language?

A qualified contractor should be able to identify the core concepts used to code within Java. These concepts are all interlinked, so your developer should be able to link OOC concepts such as inheritance, abstraction, encapsulation, and polymorphism and how they work within the code blocks. Look for answers that include some of the following information:

  • Java is platform-independent and uses the Java Virtual Machine (JVM) to allow Java programs to operate on multiple platforms without any modifications.
  • Java has a high-performance output since it uses a Just-In-Time (JIT) compiler allowing excellent Java performance. 
  • Java uses multi-threading language to run together and implement the interface.

Describe what contexts that final keyword can be used

Your qualified freelancer should understand the functionality of the final keyword as a non-access modifier. They should also incorporate the following information within their answer: 

  • Final variable: its value can't be changed once assigned. If it was assigned a no value, only a class constructor could assign a value to it. 
  • Final method: cannot be overridden by the inheriting class.
  • Final class: this value can't be extended by any subclass class, but it can extend to other classes and potentially new strings. 

Can you explain the thread lifecycle in Java?

When you work with object-oriented programming in Java, it is important to grasp the life cycle of an object. A freelancer should know about the lifecycle phases from new to runnable to terminated. Understanding these main methods in the lifecycle process will confirm that your contractor can understand the code and how to identify each specific lifecycle phase. 

Why are arrays important in Java?

Arrays are an essential data structure within Java and are used by almost every Java program. They are also used to implement many other data structures, such as array lists and strings. Your java developer freelancer should know how to create an array and dynamically grow other linked lists and add in new objects. A qualified contractor should be able to provide an example of an array to generate its class. 

A good example is to ask how to store a specific number of people in a list: For example, String[] array = new String[200] (will store the names of 200 people). 

What is the difference between method overloading and method overriding?

While these names sound similar, their function is completely different. Your freelancer should be able to define both of these concepts with a firm understanding of how each affects the code. Common answers should include that method overloading is a compile-time polymorphism while method overriding is runtime polymorphism. Understanding the differences between these two methods confirms that your contractor knows the differences between compile-time and runtime and how they function within the code.

Why is a string class important to Java?

Strings represent a fundamental piece of code within Java programs. Your freelancer should be comfortable using StringBuilder and identifying which string buffers are mutable strings. A grasp of string classes shows that your contractor is familiar with communicating into the java program and comfortable creating and implementing mutable strings.

What is the runtime environment?

Your freelancer should be able to describe the basic elements of a runtime system. Look for answers that include how the runtime environment implements part of the core behavior of the computer language and provides the ability to be modified via an API. Make sure your contractor can distinguish between runtime and the lifecycle phase of the program to confirm they have a deeper knowledge of the systems. 

Describe the differences between instance variables and local variables.

Instance variables are variables that are within a class but outside any of the methods. These variables are started when the class is loaded. Variables defined inside constructors, methods, or blocks are called local variables. A freelancer who knows the difference between instance and local variables shows that the contractor can build locally or globally.

Can you override the static method?

Your freelancer should know that you cannot override the static method. This is because the static variables are a part of the class and not a part of the object. Understanding static variables shows that your programmer contractor knows the difference between class functionality and object functionality.

Why is exception handling needed?

Understanding Exception handling is an important part of the data structure of Java because it helps maintain the desired flow of the program. Exception handling keeps everything run smoothly, even when unexpected events occur. Your programmer should understand the concept of how to structure exception handling within the data structure and be able to find any compile-time or runtime errors. A knowledge of exception handling will show you that your java programmer will find any errors within the code. 



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  • $40 hourly
    David M.
    Java Developer
    • 5.0
    • (3 jobs)
    Belgrade, CENTRAL SERBIA
    vsuc_fltilesrefresh_TrophyIcon Java
    I am 26 year old programmer from Serbia. I mainly work with Unity and C#. but am also know many other languages like Python, VB, HTML,C,C++,Java, Android, Arduino etc.
  • $90 hourly
    Amar K.
    Java Developer
    • 5.0
    • (27 jobs)
    Bengaluru, KA
    vsuc_fltilesrefresh_TrophyIcon Java
    Google App Engine
    Software Development
    Web Development
    Machine Learning
    Big Data
    Google Cloud Platform
    Amazon Web Services
    Apache Airflow
    Apache Spark
    Data Engineering
    I pride myself on achieving a ๐—ฝ๐—ฒ๐—ฟ๐—ณ๐—ฒ๐—ฐ๐˜ ๐—ฟ๐—ฒ๐—ฐ๐—ผ๐—ฟ๐—ฑ ๐—ผ๐—ณ ๐Ÿฑ-๐˜€๐˜๐—ฎ๐—ฟ ๐—ฟ๐—ฎ๐˜๐—ถ๐—ป๐—ด๐˜€ ๐—ฎ๐—ฐ๐—ฟ๐—ผ๐˜€๐˜€ ๐—ฎ๐—น๐—น ๐—ฝ๐—ฟ๐—ผ๐—ท๐—ฒ๐—ฐ๐˜๐˜€. My expertise in ๐—ฐ๐—น๐—ผ๐˜‚๐—ฑ ๐—ฑ๐—ฎ๐˜๐—ฎ ๐—ฒ๐—ป๐—ด๐—ถ๐—ป๐—ฒ๐—ฒ๐—ฟ๐—ถ๐—ป๐—ด and ๐—ณ๐˜‚๐—น๐—น-๐˜€๐˜๐—ฎ๐—ฐ๐—ธ development has been honed through experience with premier institutions like ๐—š๐—ผ๐—น๐—ฑ๐—บ๐—ฎ๐—ป ๐—ฆ๐—ฎ๐—ฐ๐—ต๐˜€, ๐— ๐—ผ๐—ฟ๐—ด๐—ฎ๐—ป ๐—ฆ๐˜๐—ฎ๐—ป๐—น๐—ฒ๐˜†, a member of the ๐—•๐—ถ๐—ด ๐—™๐—ผ๐˜‚๐—ฟ and a ๐—™๐—ผ๐—ฟ๐˜๐˜‚๐—ป๐—ฒ ๐Ÿฑ๐Ÿฌ๐Ÿฌ company. With over 9 years of experience in Data Engineering and Programming, I bring a commitment to excellence and a passion for perfection in every project I undertake. My approach is centered around delivering not just functional, but ๐—ต๐—ถ๐—ด๐—ต๐—น๐˜† ๐—ฒ๐—ณ๐—ณ๐—ถ๐—ฐ๐—ถ๐—ฒ๐—ป๐˜ ๐—ฎ๐—ป๐—ฑ ๐—ผ๐—ฝ๐˜๐—ถ๐—บ๐—ถ๐˜‡๐—ฒ๐—ฑ code, ensuring top-quality outputs that consistently impress my clients. My expertise combined with extensive experience on both GCP and AWS Cloud platforms, allows me to provide solutions that are not only effective but also innovative and forward-thinking. I believe in going beyond the basics, striving for excellence in every aspect of my work, and delivering results that speak for themselves. ๐—–๐—ต๐—ผ๐—ผ๐˜€๐—ฒ ๐—บ๐—ฒ ๐—ถ๐—ณ ๐˜†๐—ผ๐˜‚ ๐—ฝ๐—ฟ๐—ถ๐—ผ๐—ฟ๐—ถ๐˜๐—ถ๐˜‡๐—ฒ ๐˜๐—ผ๐—ฝ-๐—ป๐—ผ๐˜๐—ฐ๐—ต ๐—พ๐˜‚๐—ฎ๐—น๐—ถ๐˜๐˜† ๐—ถ๐—ป ๐˜†๐—ผ๐˜‚๐—ฟ ๐—ฝ๐—ฟ๐—ผ๐—ท๐—ฒ๐—ฐ๐˜๐˜€ ๐—ฎ๐—ป๐—ฑ ๐—ฎ๐—ฝ๐—ฝ๐—ฟ๐—ฒ๐—ฐ๐—ถ๐—ฎ๐˜๐—ฒ ๐—ฎ ๐—ณ๐—ฟ๐—ฒ๐—ฒ๐—น๐—ฎ๐—ป๐—ฐ๐—ฒ๐—ฟ ๐˜„๐—ต๐—ผ ๐—ฎ๐˜‚๐˜๐—ผ๐—ป๐—ผ๐—บ๐—ผ๐˜‚๐˜€๐—น๐˜† ๐—บ๐—ฎ๐—ธ๐—ฒ๐˜€ ๐—ผ๐—ฝ๐˜๐—ถ๐—บ๐—ฎ๐—น ๐—ฑ๐—ฒ๐—ฐ๐—ถ๐˜€๐—ถ๐—ผ๐—ป๐˜€, ๐˜€๐—ฒ๐—ฒ๐—ธ๐—ถ๐—ป๐—ด ๐—ฐ๐—น๐—ฎ๐—ฟ๐—ถ๐—ณ๐—ถ๐—ฐ๐—ฎ๐˜๐—ถ๐—ผ๐—ป๐˜€ ๐—ผ๐—ป๐—น๐˜† ๐˜„๐—ต๐—ฒ๐—ป ๐—ฎ๐—ฏ๐˜€๐—ผ๐—น๐˜‚๐˜๐—ฒ๐—น๐˜† ๐—ป๐—ฒ๐—ฐ๐—ฒ๐˜€๐˜€๐—ฎ๐—ฟ๐˜†. ๐—”๐—ฟ๐—ฒ๐—ฎ๐˜€ ๐—ผ๐—ณ ๐—˜๐˜…๐—ฝ๐—ฒ๐—ฟ๐˜๐—ถ๐˜€๐—ฒ: - ๐—–๐—น๐—ผ๐˜‚๐—ฑ: GCP (Google Cloud Platform), AWS (Amazon Web Services) - ๐—ฃ๐—ฟ๐—ผ๐—ด๐—ฟ๐—ฎ๐—บ๐—บ๐—ถ๐—ป๐—ด ๐—Ÿ๐—ฎ๐—ป๐—ด๐˜‚๐—ฎ๐—ด๐—ฒ: Java, Scala, Python, Ruby, HTML, Javascript - ๐——๐—ฎ๐˜๐—ฎ ๐—˜๐—ป๐—ด๐—ถ๐—ป๐—ฒ๐—ฒ๐—ฟ๐—ถ๐—ป๐—ด: Spark, Kafka, Crunch, MapReduce, Hive, HBase, AWS Glue, PySpark, BiqQuery, Snowflake, ETL, Datawarehouse, Databricks, Data Lake, Airflow, Cloudwatch ๐—–๐—น๐—ผ๐˜‚๐—ฑ ๐—ง๐—ผ๐—ผ๐—น๐˜€: AWS Lambda, Cloud Functions, App Engine, Cloud Run, Datastore, EC2, S3, - ๐——๐—ฒ๐˜ƒ๐—ข๐—ฝ๐˜€: GitHub, GitLab. BitBucket, CHEF, Docker, Kubernetes, Jenkins, Cloud Deploy, Cloud Build, - ๐—ช๐—ฒ๐—ฏ & ๐—”๐—ฃ๐—œ: SpringBoot, Jersey, Flask, HTML & JSP, ReactJS, Django ๐—ฅ๐—ฒ๐˜ƒ๐—ถ๐—ฒ๐˜„๐˜€: "Amar is a highly intelligent and experienced individual who is exceeding expectations with his service. He has very deep knowledge across the entire field of data engineering and is a very passionate individual, so I am extremely happy to have finished my data engineering project with such a responsible fantastic guy. I was able to complete my project faster than anticipated. Many thanks...." "Amar is an exceptional programmer that is hard to find on Upwork. He combines top-notch technical skills in Python & Big Data, excellent work ethic, communication skills, and strong dedication to his projects. Amar systematically works to break down complex problems, plan an approach, and implement thought-out high-quality solutions. I would highly recommend Amar!" "Amar is a fabulous developer. He is fully committed. Is not a clock watcher. Technically very very strong. His Java and Python skills are top-notch. What I really like about him is his attitude of taking a technical challenge personally and putting in a lot of hours to solve that problem. Best yet, he does not charge the client for all those hours, He still sticks to the agreement. Very professional. It was a delight working with him. and Will reach out to him if I have a Java or Python task."
  • $100 hourly
    Alek G.
    Java Developer
    • 5.0
    • (25 jobs)
    Paramus, NJ
    vsuc_fltilesrefresh_TrophyIcon Java
    MySQL Programming
    Hi! I'm just your average senior software whiz with a ton of hands-on know-how. Throughout my career journey, I've been the captain of some pretty massive squads and cooked up seriously scalable stuff using all the cool tech toys like: + Python (with Flask and Django) + JS (with React and Angular) + C++ + Java (with struts & spring). + MySQL + AWS/GCP + Stripe/Paypal/Auth.net By the way, my work clock follows Eastern Standard Time (EST), and I'm chilling right here in the good ole US of A ;)
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