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What are the two primary ways to create a thread in Java (extending Thread versus implementing Runnable), and which is generally preferred? What is the synchronized keyword, and how does it use Java's intrinsic (monitor) locks to prevent race conditions on shared mutable state? What is the ExecutorService framework, and why is it generally preferred over manually creating and managing individual Thread objects? What causes a deadlock in multithreaded Java code, and what is the classic "lock ordering" strategy for preventing it? What are the atomic classes in java.util.concurrent.atomic (AtomicInteger, AtomicLong, AtomicReference), and how do they achieve thread-safe operations without traditional locking? What is thread starvation, and how can thread priority misuse or unfair lock scheduling contribute to it? What is the difference between wait()/notify()/notifyAll() (Object's intrinsic monitor methods) and the newer java.util.concurrent tools, and why are the older methods considered error-prone? What is a race condition, and how does it differ conceptually from a deadlock as two distinct categories of concurrency bugs? How does java.util.concurrent's ReentrantLock differ from the synchronized keyword, and what additional capabilities does it provide (tryLock, fairness, interruptible locking)? How do CompletableFuture's composition methods (thenApply, thenCompose, thenCombine) enable building asynchronous pipelines, and what's the difference between thenApply and thenCompose?
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Java › Concurrency & Threads

What is the difference between calling Thread.start() and Thread.run() directly, and why is calling run() directly a common beginner mistake?

Beginner

Answer

start() causes the JVM to allocate a new OS-level thread and asynchronously invoke run() on that new thread of execution; calling run() directly instead simply invokes it as a completely ordinary synchronous method call on the CURRENT thread, with no new thread created at all -- this is a common beginner mistake because the code compiles and appears to run without any error, but the intended concurrent execution silently never actually happens, since run() executes synchronously and blocks the caller just like any normal method call would.
Thread t = new Thread(() -> System.out.println("Running on: " + Thread.currentThread().getName()));

t.start();  // correct: prints a NEW thread name, e.g. "Thread-0", runs concurrently
t.run();    // mistake: prints "main", runs synchronously on the CURRENT thread, no concurrency at all
Real-world example A beginner's attempt to run several tasks "concurrently" by calling run() directly on several Thread objects (instead of start()) is confused when execution is actually happening sequentially, one after another, since each run() call was really just a normal synchronous method invocation on the main thread the whole time.

Common follow-ups: Can you call start() more than once on the same Thread object?;What exception is thrown if you attempt to call start() twice on the same Thread instance?

Background Tasks & Hosted Services;Design Patterns in Java
All 15 Concurrency & Threads questions