Thread Basics & Overview

A process is an independent program with its own isolated memory, while a thread is a lightweight execution unit within a process that shares the heap but has its own stack.

Extend Thread (bad: wastes inheritance), implement Runnable (good: separates task), or use Callable with ExecutorService (best: returns result, managed pool).

A Java thread moves through six states: NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, and TERMINATED, and a terminated thread cannot be restarted.

BLOCKED means the thread is forced to wait for a lock held by another thread, while WAITING means the thread voluntarily paused and needs an explicit signal to resume.

start() creates a new OS thread and calls run() on it for true parallelism, while calling run() directly executes the method on the current thread with no concurrency.

Java concurrency tools form a layered toolkit: low-level Thread and synchronized, mid-level ExecutorService and Lock, high-level CountDownLatch, CyclicBarrier, and Semaphore, plus concurrent collections, atomics, and CompletableFuture.

Concurrency is multiple tasks making progress by interleaving (structure, one core possible), while parallelism is multiple tasks executing at the exact same instant on different cores (execution, multi-core required).