Hiring guide for Java Engineers

Java Developer Hiring Guide

Java, a high-level computer programming language, was developed by Sun Microsystems in 1995, now owned by Oracle Corporation. It is an object-oriented language, designed to be simple and secure, and is used for developing platform-independent web and mobile applications. Java's "Write Once, Run Anywhere" principle ensures compatibility across different operating systems. Its syntax is derived from C and C++, making it familiar to many programmers. Today, Java is widely used in enterprise-scale applications, Android app development, and Internet of Things (IoT) technology.

Ask the right questions secure the right Java talent among an increasingly shrinking pool of talent.

First 20 minutes

General Java app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in Java application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

How would you describe the main principles of Object Oriented Programming in Java?
The main principles of OOP in Java are encapsulation, inheritance, polymorphism, and abstraction. Encapsulation is the mechanism of hiding data implementation by wrapping data and functions into a single unit. Inheritance is a mechanism where an object acquires all the properties and behaviors of a parent object. Polymorphism allows an object to take on many forms. Abstraction is a process of hiding the implementation details and showing only functionality.
What are Java Class Loaders?
Java Class Loaders are part of the Java Runtime Environment that dynamically loads Java classes into the JVM. They are used to load classes and interfaces. There are three built-in class loaders in Java: Bootstrap ClassLoader, Extensions ClassLoader and System ClassLoader.
Describe the difference between an Interface and an Abstract class in Java.
An abstract class can have method implementations, but an interface can't. All methods of an Interface are implicitly abstract and public. An abstract class can have both abstract and non-abstract methods. An interface can have variables, but they're implicitly final and static.
How would you handle exceptions in Java?
In Java, exceptions are handled using a try-catch block. We can also use a finally block to ensure that a certain section of code runs regardless of whether an exception is thrown or not.
What are Java Annotations?
Annotations are a form of metadata that provide data about a program that is not part of the program itself. They have no direct effect on the operation of the code they annotate. Annotations can be read from source files, class files, or reflectively at runtime.
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What you’re looking for early on

Does the candidate demonstrate a strong understanding of Java fundamentals?
How well does the candidate understand object-oriented programming?
Is the candidate familiar with the Java ecosystem and related tools?
Can the candidate solve problems effectively and efficiently?

Next 20 minutes

Specific Java development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

Describe the difference between Checked and Unchecked Exceptions in Java.
Checked exceptions are checked at compile-time, while unchecked exceptions are checked at runtime. Checked exceptions need to be declared in a method or constructor's throws clause if they can be thrown by the execution of the method or constructor and propagate outside the method or constructor boundary.
What are Java Generics?
Java Generics add a layer of abstraction over types, allowing code to be written in a generic manner, reducing bugs and adding an extra layer of security. They were added to provide compile-time type checking and removing risk of ClassCastException that was common while working with collection classes.
How would you implement multi-threading in Java?
There are two ways to create a thread in Java - by extending the Thread class and by implementing the Runnable interface. After creating a thread, the start() method is used to make the thread eligible for running.
What are the differences between the methods sleep() and wait() in Java?
The sleep() method is a static method that pauses the execution of the current thread for a specified time. The wait() method is used in synchronization for inter-thread communication and it releases the lock while waiting.
Describe the difference between HashMap and TreeMap in Java.
HashMap is a general purpose Map implementation. It provides a performance of O(1) for get() and put() operations. TreeMap is a sorted map implementation. It provides a performance of O(log(n)) for get() and put() operations. HashMap does not maintain any order, whereas TreeMap maintains elements in sorted order.
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The ideal back-end app developer

What you’re looking to see on the Java engineer at this point.

At this point, a skilled Java engineer should demonstrate strong problem-solving abilities, proficiency in Java programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with Java.

What does the following Java code do?
public class HelloWorld {
 public static void main(String[] args) {
 System.out.println("Hello, World!");
This Java code prints the string 'Hello, World!' to the standard output.
What does the 'final' keyword signify in the following Java code?
public final class Test {
 public void show() {
 System.out.println("This is a final class.");
In this Java code, the 'final' keyword is used to prevent a class from being subclassed. The class 'Test' cannot be extended by any other class.
What will be the output of the following Java code?
import java.util.Arrays;
public class Test {
 public static void main(String[] args) {
 int[] array = {1, 2, 3, 4, 5};
This Java code sorts an integer array in ascending order and then prints it. The output will be '[1, 2, 3, 4, 5]'.
What does the following Java code do?
public class Test extends Thread {
 public void run() {
 System.out.println("Thread is running.");
 public static void main(String[] args) {
 Test t = new Test();
This Java code creates a new thread and starts it. The 'run' method of the thread prints the string 'Thread is running.' to the standard output.

Wrap-up questions

Final candidate for Java Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle Java application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

How would you prevent SQL Injection in Java?
In Java, SQL Injection can be prevented by using Prepared Statements and Callable Statements. They work by precompiling the SQL statement which means special characters do not have any meaning and cannot change the intent of the SQL statement.
What are the differences between equals() and == in Java?
The == operator checks if the two references point to the exact same object, whereas the equals() method checks if the two objects are logically equivalent, according to the implementation of equals() in their class.
How would you manage memory in Java?
In Java, memory management is mostly handled by the Garbage Collector. However, we can assist the Garbage Collector by making sure we dereference objects that are no longer needed, and by using the 'finalize' method to free up other non-Java resources.

Java application related

Product Perfect's Java development capabilities

Beyond hiring for your Java engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in Java projects, and can engage in multiple capacities.