What are the basics of concurrency in Java?

A thread is an independent path of execution with its own stack but shared heap. Threads improve throughput for blocking IO and use multiple cores for CPU work. They also introduce races when they read and write shared mutable state without coordination.

public class Counter {
    private int value;

    public void increment() {
        value++;
    }

    public int value() {
        return value;
    }
}

Runnable task = () -> {
    for (int i = 0; i < 10_000; i++) {
        counter.increment();
    }
};

The three failure modes

• Atomicity: a compound action can be interrupted halfway
• Visibility: one thread may not see another thread's write
• Ordering: the compiler or CPU may reorder independent operations
• Deadlock: threads wait on each other's locks forever

Prefer high-level tools over raw threads: executor services, concurrent collections, atomics and synchronisers. Confine mutable state to one thread where possible. Use start to launch a thread and run only to execute its body on the current thread.