Java & Spring Boot interview questions — Level I · Beginner

18 questions with a short answer, a full explanation and the mistakes interviewers listen for. Every question has a difficulty level from I (Beginner) to V (Expert).

How Java Runs — JDK, JRE, JVM & Your First Program · Java Foundations — Zero to Basics

• What is the difference between the JDK, the JRE and the JVM?I
• What does 'write once, run anywhere' mean and how does Java achieve it?I
• Explain the signature `public static void main(String[] args)` piece by piece.I
• Why must a public class's file name match the class name?I
• What is bytecode, and how is it different from machine code?I
• At a beginner level, what is the difference between the stack and the heap?I
• Which Java versions matter, and what is an LTS release?I

Control Flow — Loops, Conditionals & Switch · Java Foundations — Zero to Basics

• What is the difference between an array's length and a String's length in Java?I
• How do you correctly compare the contents of two arrays, and why doesn't == work?I

Efficiency & Big-O — Measuring Speed From Zero · Java Foundations — Zero to Basics

• What does Big-O notation actually describe?I
• Why do we drop constants and lower-order terms, so O(2n + 5) becomes O(n)?I
• Which is faster for large n: O(n) or O(n^2), and why?I
• What is the difference between time complexity and space complexity?I

SQL Mastery & Joins · Databases & Persistence

• Explain the difference between INNER, LEFT, RIGHT, and FULL OUTER JOIN, and when an unmatched-row requirement forces an outer join.I
• What's the difference between UNION and UNION ALL, and why does it matter for performance?I

Strings — Practice Programs · Practice Programs — Master Every Pattern

• How do you reverse a string in Java, and which approach is best?I
• What does String.compareTo return, and how does it order strings?I

JVM Internals — Top Interview Q&A · Interview Rapid-Fire — Top Q&A

• What is the difference between Heap and Stack memory?I