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copied!<p>They are just totally different. Consider this example of a <code>volatile</code> integer:</p> <pre><code>volatile int i = 0; void incIBy5() { i += 5; } </code></pre> <p>If two threads call the function concurrently, <code>i</code> might be 5 afterwards, since the compiled code will be somewhat similar to this (except you cannot synchronize on <code>int</code>):</p> <pre><code>void incIBy5() { int temp; synchronized(i) { temp = i } synchronized(i) { i = temp + 5 } } </code></pre> <p>If a variable is volatile, every atomic access to it is synchronized, but it is not always obvious what actually qualifies as an atomic access. With an <code>Atomic*</code> object, it is guaranteed that every method is "atomic". </p> <p>Thus, if you use an <code>AtomicInteger</code> and <code>getAndAdd(int delta)</code>, you can be sure that the result will be <code>10</code>. In the same way, if two threads both negate a <code>boolean</code> variable concurrently, with an <code>AtomicBoolean</code> you can be sure it has the original value afterwards, with a <code>volatile boolean</code>, you can't. </p> <p>So whenever you have <em>more than one thread</em> modifying a field, you need to make it atomic or use explicit synchronization.</p> <p>The purpose of <code>volatile</code> is a different one. Consider this example</p> <pre><code>volatile boolean stop = false; void loop() { while (!stop) { ... } } void stop() { stop = true; } </code></pre> <p>If you have a thread running <code>loop()</code> and another thread calling <code>stop()</code>, you might run into an infinite loop if you omit <code>volatile</code>, since the first thread might cache the value of stop. Here, the <code>volatile</code> serves as a hint to the compiler to be a bit more careful with optimizations.</p>

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