How does a Java class make immutable: In case I explained the use of final in classes, the interviewer expected it to extend some class and be immutable in its own way. 2.- Explanation of threads: Explain how threads work, how they work, how they interact. I didn't remember some methods so I was honest and told him that + add what was his way of interacting, it means how .run() / sleep() vs wait() and use of notify() works, also the reserved word synchronize . At the end of the question and a couple of questions about collections API, he only answered, you are too stupid to answer the questions correctly. I hope you prepare better, make leetcode faster and learn all the basics of java much better. And he closed the Zoom session.