How is the volatile statement implemented and functioning in Java to ensure visibility and order of variable changes between threads?

In Java, the keyword volatile is used for variables to ensure proper visibility of changes between threads and to prevent reordering of operations by the compiler and processor.

When a variable is declared as volatile:

• Reading and writing such a variable occurs directly from main memory, not from the thread's cache. This guarantees that one thread will see the changes made by another.
• The volatile instruction creates so-called "memory barriers" that prohibit reordering of operations around the read/write of this variable.

Thus, volatile guarantees:

1. Visibility: changes to the variable are immediately visible to all threads.
2. Order: read/write operations with volatile will not be reordered relative to other operations related to this variable.

However, volatile does not ensure atomicity of complex operations (e.g., increment), for which synchronization or atomic classes are needed.

Example:

public class Counter {
    private volatile int count = 0;

    public void setCount(int value) {
        count = value; // write is immediately visible to other threads
    }

    public int getCount() {
        return count; // always reads the current value
    }
}