• Origin story: Java was initially developed by James and his colleagues(a.k.a, the ‘green team’) for digital devices like set-top boxes, television, etc. It was called ‘Greentalk’ and ‘Oak’ before Gosling came up with the name ‘Java.’ This project was created to implement a virtual machine that is easier than C/C++.
• Widely popular: According to Daxx’s LinkedIn research, the USA has the highest number of Java programmers- 63K, followed by the UK with almost 14K Java developers.
• Top companies using Java: Top companies like Google, Instagram, Airbnb, Pinterest, Slack, Spotify, etc. use Java.
• Other trivia: In 1995, Time magazine featured Java as one of the Ten Best Products of 1995.

• User-friendly: Java has an English-like syntax, which makes it easy to learn. It is easy to advance from Core Java to Advanced Java for someone who has a functional knowledge of C and C++.
• Robust: Java is robust as it utilizes strong memory management. No use of pointers bypasses security dilemmas. It also has an automatic garbage collector, which runs on the Java Virtual Machine(JVM) to eliminate objects which are not being accepted by a Java application anymore.

The following are some of the tools used by Java developers at various stages of development:

1. JDK

Oracle made Java Development Kit or JDK, an easy-to-use tool that includes the Runtime Environment, Compiler, and APIs for Java development. Both beginners, as well as experienced developers, can use this tool with ease.

2. Java IDEs

A few of the most popular Java IDEs or Integrated Development Environments are

• IntelliJIDEA - developed by JetBrains, designed to maximize productivity.
• NetBeans - a free and open-source IDE by Apache
• Eclipse - this Java IDE is preferred due to its extensive assistance.

3. Java Testing Tools

• JUnit - Simple unit testing tool that makes it easy to write repeatable tests
• JMeter - Load testing tool that offers a range of services for running tests on the cloud.

4. JavaDoc

This is a documentation generator for the Java programming language, making it easier for other Java developers to work on a given project.

5. Jenkins

Jenkins is great for continuous integration and is used by Java developers for deploying apps on the internet. It can also be used for continuous delivery with ease.

Below are some key points that we at Flexiple have learned through trial and error - a process of filtering through over 15,000 developers. You can use these in your process to hire Java developers.

• Years of software development experience: Be specific about the technology you’re looking for and the number of years of experience needed. A proxy to years of experience can be the number of production-level apps worked on.
• Expertise in a specific framework: All frameworks have their peculiarities and nuances. While good Java developers can pick up other frameworks, it’s a plus point to hire a developer who already has expertise in it.
• Type of products built in the past: Based on the type of product you are trying to build - social network, SAAS product, etc. - you can prioritize certain developers over others.
• Exposure to specific industries: Developers who have built products in certain industries will already know key aspects of laying the foundation for a great product and will be able to predict possible problems.
• Experience working remotely: Hiring someone who has worked remotely in the past is an added advantage as they would have first-hand experience of everything right from how contracts work to communication and from timelines to deliverables.
• Strong communication: An essential thing to look at when hiring a freelancer is excellent communication skills. Since all the interaction happens online, freelancer should be able to effectively communicate to ensure a streamlined execution of all work.

You can check out a more detailed job description for Java developers here.

Now that you have made a quality JD, it can still be tricky to evaluate the skills of your applicants when you hire Java developers. To help you with that, we have created a pool of questions that a good Java developer should be comfortable with. You can check out some related questions here: Java, Hibernate, JPA.

It is important to note that the ability to answer these questions doesn't imply that you have a top quality candidate. But it definitely is a big step in that direction.

To help you navigate through these questions, we’ve categorized the interview questions in 3 parts:

A. Basic concepts: Includes all basic concepts used across languages but we've focused on its significance in Java. This section will give you an understanding of how strong their programming foundation is.

B. Advanced concepts: Includes all concepts that someone with higher expertise should know.

C. DS/Algorithm questions: To test the logical capability of the candidate.

A. Basic concepts

How are whitespaces removed from a string in Java?

The Java strip() is a string method that removes all leading and trailing whitespaces. Strip() uses the Character.isWhitespace() method internally to check for whitespaces.

This method identifies whitespaces using Unicodes and hence it is the recommended method to remove whitespaces.

stripLeading() and stripTrailing() are alternatives in case you are only looking to remove leading or trailing whitespaces respectively.

The code below is an example of the Java strip() method

String s = "  flexiple ";
        
s = s.strip();
        
System.out.println(s);

Write a code snippet to implement a Fibonacci series using recursion?

The code snippet below implements the Fibonacci series using recursion which is a common Java interview question.

public class FibonacciNumbers {
    public static int fibonacci(int n) {
        if (n <= 1)
            return n;
        return fibonacci(n - 1) + fibonacci(n - 2);
    }

    public static void main(String args[]) {
        int n = 10;
        System.out.println(fibonacci(n));
    }
}

B. Advanced concepts

How do you reverse a string in Java without using the reverse() methods?

Java does not come out of the box with the reverse() method although the reverse() method does exist in a few libraries such as StringBuffer or StringBuilder. Hence, reversing an array has become a common interview question.

public class StringReverse {

    public static void main(String[] args) {

        String str = "Flexiple";
        System.out.println(reverse(str));
    }

    public static String reverse(String in) {
        if (in == null)
            throw new IllegalArgumentException("Null is not valid”);

        StringBuilder out = new StringBuilder();

        char[] chars = in.toCharArray();

        for (int i = chars.length - 1; i >= 0; i--)
            out.append(chars[i]);

        return out.toString();
    }
}

What causes a deadlock scenario? Write code to create a deadlock.

A deadlock scenario is caused when two threads require the same locks to execute.

These scenarios occur when both threads have obtained one lock and are waiting to obtain the other lock. However, since both threads wait for the other to execute they block each other causing a deadlock.

Multi Threaded programs suffer from deadlocks because the synchronized keyword is used to make the methods thread-safe. This means that only one thread can lock and use the synchronized method. Other threads have to wait for the current thread to complete.

The code below creates two threads that are in a deadlock.

class Util
{
    static void sleep(long millis)
    {
        try
        {
            Thread.sleep(millis);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}
class Shared
{
    synchronized void test1(Shared s2)
    {
        System.out.println("test1-begin");
        Util.sleep(1000);

        s2.test2();
        System.out.println("test1-end");
    }

    synchronized void test2()
    {
        System.out.println("test2-begin");
        Util.sleep(1000);

        System.out.println("test2-end");
    }
}

class Thread1 extends Thread
{
    private Shared s1;
    private Shared s2;

    public Thread1(Shared s1, Shared s2)
    {
        this.s1 = s1;
        this.s2 = s2;
    }

    @Override
    public void run()
    {
        s1.test1(s2);
    }
}

class Thread2 extends Thread
{
    private Shared s1;
    private Shared s2;

    public Thread2(Shared s1, Shared s2)
    {
        this.s1 = s1;
        this.s2 = s2;
    }

    @Override
    public void run()
    {
        s2.test2(s1);
    }
}

public class Deadlock
{
    public static void main(String[] args)
    {
        Shared s1 = new Shared();

        Shared s2 = new Shared();

        Thread1 t1 = new Thread1(s1, s2);
        t1.start();

        Thread2 t2 = new Thread2(s1, s2);
        t2.start();

        Util.sleep(2000);
    }
}

C. Data Structure/ Algorithm

1. Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if: Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Note that an empty string is also considered valid.

class ValidParenthesesFunc {
    func isValid(_ s: String) -> Bool {
        var stc = [Character]()

        for char in s {
            if char == "(" || char == "[" || char == "{" {
                stc.append(char)
            } else if char == ")" {
                guard stc.count != 0 && stc.removeLast() == "(" else {
                    return false
                }
            } else if char == "]" {
                guard stc.count != 0 && stc.removeLast() == "[" else {
                    return false
                }
            } else if char == "}" {
                guard stc.count != 0 && stc.removeLast() == "{" else {
                    return false
                }
            }
        }
        return stc.isEmpty
    }
}

The above code will input 0(false). 

2.  Write a program to find whether a string or number is palindrome or not.

import java.util.Scanner;
public class Palindrome {
    public static void main (String[] args) {
        String original, reverse = "";
 Scanner in = new Scanner(System.in);
       int length;
        System.out.println("Enter a number or a string");
        original = in.nextLine();
        length = original.length();
        for (int i =length -1; i>;=0; i--) {
            reverse = reverse + original.charAt(i);
        }
        System.out.println("The reverse is: " +reverse);

        if(original.equals(reverse))
            System.out.println("The string is a palindrome");
        else
            System.out.println("The stringis not a palindrome");

    }
}

Output:
For String- Enter a number or a string
sam
The reverse is: mas
The string is not a palindrome

For Number-Enter a number or a string
99
The reverse is: 99
The number is a palindrome 

3. What will the output of the following code be?

class Test {
public static void main(String args[])
    {
        String str1 = new String("Hello World");
        String str2 = new String("Hello World");

        String str3 = "Hello World";
        String str4 = "Hello World";

        int a = 0, b = 0, c = 0;

        if (str3 == str4)
            a = 1;
        else
            a = 2;

        if (str1.equals(str3))
            b = 1;
        else
            b = 2;

        if (str1 == str4)
            c = 1;
        else
            c = 2;
        System.out.println("a= " + a + " b= " + b + " c= " + c);
    }
}

D. a=1 b=1 c=2 because a new memory is created when we make an object with the help of the ‘new’ keyword, 
and the reference variable contains the memory location. 
Here the memory is created with the same string twice, but since we are comparing objects and not strings, 
the object will point to a different memory location and so they are not equal. 

That is everything you need to know about hiring a freelance Java developer. As discussed, it isn't easy to find a quality freelance Java developer but this guide makes the process easier for you. To offload the entire hiring process, reach out to us at Flexiple. We've designed a high-quality, 6-step screening process to find the top 1%, freelance developers. You can find the best freelance Java developers here. We've already served over a hundred clients, earning great reviews for the quality of service.

Lastly, to quickly summarize it for you:

• Note your project requirements and hire accordingly. Do not go for the lowest or the highest-paid developer.
• Don’t hire without vetting- consider asking questions right from the basics to advanced to logical questions.
• Look for companies like Flexiple that help you find the perfect fit.

That is everything you need to know if you want to hire Java developers. Happy hiring! :)