Thread vs Process in Java

Direct Answer

A process is a running program with its own address space and operating-system resources. A thread is an execution path inside a process. Threads in one Java process share heap objects and class state, while each has its own call stack. Sharing makes communication convenient but requires safe coordination for mutable data.

Code Example

public class SharedStateDemo {
    private static int shared; // class state reachable by both threads

    public static void main(String[] args) throws InterruptedException {
        Runnable task = () -> {
            for (int i = 0; i < 1_000; i++) {
                shared++;       // unsafe compound update
            }
        };
        Thread first = new Thread(task);
        Thread second = new Thread(task);
        first.start();
        second.start();
        first.join();
        second.join();
        System.out.println(shared); // 2,000 is not guaranteed
    }
}

Both threads can reach the same class state because they run in one process. A separate process cannot access this Java variable directly; cross-process communication needs an operating-system or external mechanism, which can include explicitly shared memory.

Common Mistakes & Pitfalls

• Treating threads like isolated processes: threads in one process can reach the same heap objects and class state.
• Saying each process switch copies all memory: modern virtual-memory systems do not require that model.
• Saying processes can never share memory: they can use explicit shared-memory facilities, though ordinary Java objects are not directly shared across processes.
• Quoting one fixed cost: creation and switching vary by platform and thread type.
• Assuming more threads means more CPU: parallel computation is still limited by available processors.