easyoopfundamentalsThe four OOP principlesWhat are the four core principles of Object-Oriented Programming and how does Java support each one?▾
easyoopinterfacesfundamentalsInterface vs abstract classWhat is the difference between an interface and an abstract class in Java? When should you use each?▾
easyoopcollectionsfundamentalsequals() and hashCode() contractExplain the equals/hashCode contract in Java. What breaks if you violate it?▾
easystringsmemoryfundamentalsString pool and immutabilityWhy is String immutable in Java? What is the String pool and how does it work?▾
easyprimitivesfundamentalspitfallsAutoboxing and unboxingWhat is autoboxing in Java? What are the performance and correctness pitfalls?▾
easyexceptionsfundamentalsChecked vs unchecked exceptionsWhat is the difference between checked and unchecked exceptions in Java? When should you use each?▾
easylanguage-featuresjava10+var (local variable type inference)What does the 'var' keyword do in Java? What are its limitations?▾
mediumgenericstype-systemGenerics and type erasureHow do Java generics work? Explain type erasure, bounded wildcards (? extends T, ? super T), and the PECS rule.▾
mediumcollectionsdata-structuresJava Collections Framework — List, Map, SetDescribe the main List, Map, and Set implementations in Java. How do ArrayList, LinkedList, HashMap, LinkedHashMap, TreeMap, HashSet, and TreeSet differ?▾
mediumstreamsjava8+functionalJava 8 Streams APIHow does the Java Streams API work? Explain intermediate vs terminal operations, lazy evaluation, and common operators.▾
mediumlambdasfunctionaljava8+Lambdas and functional interfacesWhat is a functional interface? Name the key ones from java.util.function and explain how lambdas and method references relate to them.▾
mediumimmutabilityoopthread-safetyImmutability in JavaHow do you design an immutable class in Java? What are the rules and why does immutability matter?▾
mediummemorygcjvmJava Memory Model and garbage collectionDescribe the JVM memory areas (heap, stack, metaspace). How does garbage collection work and what are the main GC algorithms?▾
mediumconcurrencyjvmthread-safetyConcurrency basics — synchronized, volatile, and happens-beforeWhat do synchronized and volatile mean in Java? Explain the happens-before relationship in the Java Memory Model.▾
mediumrecordsjava16+oopRecords (Java 16)What are Java records? What do they provide automatically and what are their limitations?▾
hardgenericstype-systemadvancedGenerics — wildcards, capture, and heap pollutionExplain wildcard capture, unchecked warnings, heap pollution, and the @SafeVarargs annotation.▾
hardsealed-classespattern-matchingjava17+Sealed classes and pattern matchingWhat are sealed classes in Java? How do they interact with pattern matching in switch expressions?▾
hardjvmclassloadingadvancedClassLoader architecture and class loadingHow does the Java ClassLoader hierarchy work? Explain the delegation model, class loading phases, and when you might write a custom ClassLoader.▾
hardreflectionproxyadvancedframeworksReflection and dynamic proxiesHow does Java reflection work? What are dynamic proxies (java.lang.reflect.Proxy) and what are they used for?▾
hardconcurrencyasyncjava8+advancedjava.util.concurrent — executors, futures, CompletableFutureCompare ExecutorService, Future, and CompletableFuture. How do you compose asynchronous tasks with CompletableFuture?▾
hardvirtual-threadsconcurrencyjava21+loomVirtual threads (Project Loom, Java 21)What are virtual threads? How do they differ from platform threads and when should you use them?▾
hardcollectionsinternalsadvancedHashMap internals and collision handlingDescribe the internal structure of HashMap in detail. How are collisions handled, when does treeification happen, and what are the resize mechanics?▾
hardpattern-matchingjava21+language-featuresPattern matching — instanceof and switchExplain the evolution of pattern matching in Java from instanceof (Java 16) through switch patterns (Java 21). What are deconstruction patterns?▾
hardjvmperformancejitadvancedJIT compilation and performanceHow does the JVM's JIT compiler work? Explain tiered compilation, inlining, escape analysis, and how to interpret JVM warm-up.▾