Java Programming & Coding Interview Questions

This Q&A bank contains 50 questions on core Java syntax, collections, OOP, exception handling, and 20 works-out coding examples commonly asked during SDET technical loops.

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Java Programming Basics

Q1: What is Java, and why is it used in automation testing?

Java is a high-level, object-oriented programming language. It is popular in automation because of its platform independence (runs anywhere with a JVM), rich ecosystem of libraries (Selenium, RestAssured), and robust parallel threads support.

Q2: What is the difference between JDK, JRE, and JVM?

• JDK (Java Development Kit): Developer tools (including compiler javac) and JRE.
• JRE (Java Runtime Environment): Libraries and JVM to execute compiled Java.
• JVM (Java Virtual Machine): Converts bytecode to machine code during execution.

Q3: What are Java Data Types, and how are they used in test automation?

Java supports:

• Primitives: int (for counts), boolean (for assertion states), double (for prices), char.
• References: String (for element texts), classes, arrays.

Q4: What is the difference between primitive and reference data types?

• Primitive data types store the value directly in memory stack.
• Reference data types store the memory address of the object in heap storage.

Q5: What are Java Operators?

Operators are symbols used to perform operations (e.g. arithmetic +, relational ==, logical &&, and assignment =).

Q6: Explain Conditional Statements in Java.

Conditional statements check logic flows:

• if-else
• switch-case

Q7: What is the difference between if-else and switch-case?

• if-else handles range checks and complex boolean statements.
• switch-case checks equality against specific constant values (e.g., matching browser name inputs).

Q8: What are Loops in Java, and how are they used in QA?

Loops repeat actions:

• for (when runs are predetermined).
• while (repeats while conditions hold true).
• do-while (runs once before checking conditions).

Q9: Difference between break and continue in loops?

• break exits the loop immediately.
• continue skips the current iteration and jumps to the next loop evaluation.

Q10: What is an Array in Java?

An array is a fixed-size container that holds elements of a single data type. Example: String[] browsers = {"Chrome", "Firefox"};.

Q11: Difference between Array and ArrayList in Java?

• Array: Fixed-size structure.
• ArrayList: Dynamic-size collection that can grow and shrink dynamically as elements are added.

Q12: What is a String in Java, and why is it immutable?

A String represents characters. It is immutable, meaning its state cannot be updated after creation; modifications create new String instances in the String Pool to optimize memory.

Q13: Difference between String, StringBuilder, and StringBuffer?

• String is immutable.
• StringBuilder is mutable, fast, and not thread-safe.
• StringBuffer is mutable, thread-safe, but slower.

Q14: What is Object-Oriented Programming (OOP) in Java?

OOP is a design paradigm using classes and objects. Its four pillars are: Encapsulation, Inheritance, Polymorphism, and Abstraction.

Q15: Explain Encapsulation with an example.

Hiding variables using private and exposing access via public getter and setter methods to keep fields secure.

Q16: Explain Inheritance with an example.

Reusing code by letting one class inherit attributes and methods of another: class LoginTest extends BaseTest { ... }

Q17: Explain Polymorphism with examples.

Polymorphism allows methods to take different forms: Overloading (same method name, different args) and Overriding (child updates parent's implementation).

Q18: Explain Abstraction with an example.

Hiding execution details and exposing interfaces. For example, using the WebDriver interface to define standard browser actions.

Q19: Difference between Abstract Class and Interface?

• Abstract Class can have concrete methods, variables, and constructors. Supports single inheritance.
• Interface only has abstract or default methods, static constants, and supports multiple inheritance.

Q20: What is Method Overloading and Overriding?

• Overloading: Same name, different arguments (compile-time).
• Overriding: Subclass modifies parent class method behavior (runtime).

Q21: What is the difference between final, finally, and finalize()?

• final: Declares constants, blocks inheritance or overrides.
• finally: Block in try-catch executing cleanup.
• finalize(): Runs before garbage collection deletes objects.

Q22: What is Exception Handling in Java?

An execution mechanism to catch runtime errors (e.g. NullPointerException) using try-catch-finally blocks.

Q23: Difference between Checked and Unchecked Exceptions?

• Checked: Compile-time check (e.g. FileNotFoundException).
• Unchecked: Runtime check (e.g. ArrayIndexOutOfBoundsException).

Q24: What is a Wrapper Class in Java?

An object representation of primitive types (e.g. Integer wrapping int), enabling them to be stored in Java collections.

Q25: What is Autoboxing and Unboxing?

• Autoboxing: Automatic conversion of primitive types to wrapper objects.
• Unboxing: Automatic conversion of wrapper objects back to primitive types.

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Collections & Exception Handling

Q26: What are Collections in Java?

A set of classes and interfaces used to store and manipulate groups of objects (e.g., List, Set, Map).

Q27: Difference between List and Set?

• List allows duplicate values and maintains insertion order.
• Set requires unique values and does not guarantee insertion order.

Q28: Difference between HashMap and HashSet?

• HashMap stores key-value pairs (maps keys to values).
• HashSet stores only unique values.

Q29: How to iterate over a Collection?

Use an enhanced for loop, or retrieve an Iterator and execute: while(it.hasNext()) { ... }

Q30: Difference between ArrayList and LinkedList?

• ArrayList uses a dynamic array (faster for element lookups).
• LinkedList uses a doubly linked list (faster for insertions and deletions).

Q31: What is a HashMap in Java?

A collection storing key-value pairs using hashing mechanisms, allowing fast lookups by key.

Q32: What is Synchronization in Java?

Controlling thread access to shared resources to prevent data races and ensure thread safety.

Q33: How is multi-threading used in automation?

Running test suites concurrently on multiple threads to speed up regression testing runs.

Q34: What is the difference between throw and throws?

• throw triggers an exception instance in code.
• throws declares potential exceptions on the method signature.

Q35: What is the difference between == and .equals()?

• == compares references.
• .equals() compares content values.

Q36: What are Static Methods and Variables?

Fields/methods belonging to the class directly, not instances. Often used for framework utilities.

Q37: What are Constructors in Java?

Special methods having the same name as the class, invoked when objects are initialized.

Q38: Difference between default and parameterized constructors?

• Default: Takes no arguments.
• Parameterized: Accepts arguments to initialize instance fields.

Q39: What is the ‘this’ keyword in Java?

Refers to the current class instance. Used to resolve conflicts between instance and parameter names.

Q40: What is the ‘super’ keyword in Java?

Refers to parent class objects or constructors.

Q41: What is File Handling in Java?

The process of reading and writing file data using FileReader, FileWriter, and BufferedReader.

Q42: How to read a file in Java?

BufferedReader br = new BufferedReader(new FileReader("file.txt"));
String line;
while ((line = br.readLine()) != null) { ... }

Q43: How to write into a file in Java?

FileWriter writer = new FileWriter("file.txt");
writer.write("Text");
writer.close();

Q44: What is Serialization in Java?

Converting an object state into a byte stream for storage or network transfer.

Q45: What is Deserialization in Java?

Recreating a Java object in memory from a byte stream.

Q46: What are Java Streams?

Introduced in Java 8, streams process sequences of elements in a functional style (e.g. map, filter).

Q47: Difference between Lambda Expressions and Anonymous Class?

• Lambda: Short syntax representing functional interfaces.
• Anonymous Class: Complete inner class definition without names.

Q48: How to handle dates in Java?

Use the java.time package (e.g., LocalDate, LocalDateTime).

Q49: How to generate random data in Java?

Use the java.util.Random class, or Math.random().

Q50: How is Java integrated in Selenium Automation?

Java is the language used to write the scripts, and Selenium is the library driving the web browser.

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Essential Java Coding Scenarios

1. Reverse a String

public class ReverseString {
    public static void main(String[] args) {
        String str = "Automation";
        String rev = "";
        for (int i = str.length() - 1; i >= 0; i--) {
            rev += str.charAt(i);
        }
        System.out.println("Reversed: " + rev);
    }
}

2. Check if a String is a Palindrome

public class PalindromeCheck {
    public static void main(String[] args) {
        String str = "madam";
        String rev = new StringBuilder(str).reverse().toString();
        System.out.println(str.equals(rev) ? "Palindrome" : "Not Palindrome");
    }
}

3. Count Vowels in a String

public class CountVowels {
    public static void main(String[] args) {
        String str = "Quality Assurance";
        int count = 0;
        for (char c : str.toLowerCase().toCharArray()) {
            if ("aeiou".indexOf(c) != -1) count++;
        }
        System.out.println("Vowels: " + count);
    }
}

4. Swap Two Numbers Without a Temporary Variable

public class SwapNumbers {
    public static void main(String[] args) {
        int a = 5, b = 10;
        a = a + b;
        b = a - b;
        a = a - b;
        System.out.println("a=" + a + ", b=" + b);
    }
}

5. Find the Factorial of a Number

public class Factorial {
    public static void main(String[] args) {
        int num = 5, fact = 1;
        for (int i = 1; i <= num; i++) {
            fact *= i;
        }
        System.out.println("Factorial: " + fact);
    }
}

6. Fibonacci Series

public class Fibonacci {
    public static void main(String[] args) {
        int n = 7, a = 0, b = 1;
        System.out.print(a + " " + b);
        for (int i = 2; i < n; i++) {
            int c = a + b;
            System.out.print(" " + c);
            a = b; b = c;
        }
    }
}

7. Find the Largest Number in an Array

public class LargestInArray {
    public static void main(String[] args) {
        int[] arr = {10, 45, 23, 78, 56};
        int max = arr[0];
        for (int num : arr) {
            if (num > max) max = num;
        }
        System.out.println("Largest: " + max);
    }
}

8. Reverse an Array

public class ReverseArray {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        for (int i = arr.length - 1; i >= 0; i--) {
            System.out.print(arr[i] + " ");
        }
    }
}

9. Count Words in a String

public class WordCount {
    public static void main(String[] args) {
        String str = "Automation testing with Java";
        String[] words = str.trim().split("\\s+");
        System.out.println("Words: " + words.length);
    }
}

10. Remove Duplicates from an Array

import java.util.HashSet;

public class RemoveDuplicates {
    public static void main(String[] args) {
        int[] arr = {1, 2, 2, 3, 4, 4};
        HashSet<Integer> set = new HashSet<>();
        for (int n : arr) set.add(n);
        System.out.println(set);
    }
}

11. Find Duplicate Elements in an Array

public class FindDuplicates {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 2, 4, 5, 1};
        for (int i = 0; i < arr.length; i++) {
            for (int j = i + 1; j < arr.length; j++) {
                if (arr[i] == arr[j]) {
                    System.out.println("Duplicate: " + arr[i]);
                }
            }
        }
    }
}

12. Check Prime Number

public class PrimeCheck {
    public static void main(String[] args) {
        int num = 13;
        boolean prime = true;
        for (int i = 2; i <= num / 2; i++) {
            if (num % i == 0) { prime = false; break; }
        }
        System.out.println(prime ? "Prime" : "Not Prime");
    }
}

13. Reverse Words in a Sentence

public class ReverseWords {
    public static void main(String[] args) {
        String str = "I love Java";
        String[] words = str.split(" ");
        for (int i = words.length - 1; i >= 0; i--) {
            System.out.print(words[i] + " ");
        }
    }
}

14. Check Anagram Strings

import java.util.Arrays;

public class AnagramCheck {
    public static void main(String[] args) {
        String s1 = "listen", s2 = "silent";
        char[] a1 = s1.toCharArray();
        char[] a2 = s2.toCharArray();
        Arrays.sort(a1);
        Arrays.sort(a2);
        System.out.println(Arrays.equals(a1, a2) ? "Anagram" : "Not Anagram");
    }
}

15. Find the Second Largest Number in an Array

public class SecondLargest {
    public static void main(String[] args) {
        int[] arr = {10, 45, 78, 56};
        int largest = Integer.MIN_VALUE, second = Integer.MIN_VALUE;
        for (int num : arr) {
            if (num > largest) {
                second = largest;
                largest = num;
            } else if (num > second && num != largest) {
                second = num;
            }
        }
        System.out.println("Second Largest: " + second);
    }
}

16. Sum of Digits

public class SumDigits {
    public static void main(String[] args) {
        int num = 12345, sum = 0;
        while (num > 0) {
            sum += num % 10;
            num /= 10;
        }
        System.out.println("Sum: " + sum);
    }
}

17. Remove White Spaces from a String

public class RemoveSpaces {
    public static void main(String[] args) {
        String str = "  Hello   World  ";
        System.out.println(str.replaceAll("\\s+", ""));
    }
}

18. Convert String to Integer

public class StringToInt {
    public static void main(String[] args) {
        String s = "123";
        int num = Integer.parseInt(s);
        System.out.println(num + 10);
    }
}

19. Find the Missing Number in an Array

public class MissingNumber {
    public static void main(String[] args) {
        int[] arr = {1, 2, 4, 5};
        int n = 5, total = n * (n + 1) / 2, sum = 0;
        for (int num : arr) sum += num;
        System.out.println("Missing: " + (total - sum));
    }
}

20. Read File and Print Content

import java.io.*;

public class ReadFile {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new FileReader("data.txt"));
        String line;
        while ((line = br.readLine()) != null) {
            System.out.println(line);
        }
        br.close();
    }
}