I’ve sat on both sides of the Java interview table enough times to notice a pattern: candidates who can recite definitions often freeze the moment you ask “why” or “what happens if.” This guide is organized the way interviews actually flow — core language fundamentals, then collections, then concurrency, then the Streams/Optional questions that trip up even experienced developers, and finally the Spring basics that show up in almost every Java backend role today. Each question includes a real, runnable example and a note on the specific way candidates get it wrong, because the wrong answer is usually more instructive than the right one.