What is the difference between atomic / volatile / synchronized?

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How do atomic / volatile / synchronized work internally?

What is the difference between the following code blocks?

Code 1

private int counter;

public int getNextUniqueIndex() {
    return counter++; 
}

Code 2

private AtomicInteger counter;

public int getNextUniqueIndex() {
    return counter.getAndIncrement();
}

Code 3

private volatile int counter;

public int getNextUniqueIndex() {
    return counter++; 
}

Does volatile work in the following way? Is

volatile int i = 0;
void incIBy5() {
    i += 5;
}

equivalent to 

Integer i = 5;
void incIBy5() {
    int temp;
    synchronized(i) { temp = i }
    synchronized(i) { i = temp + 5 }
}

I think that two threads cannot enter a synchronized block at the same time… am I right? If this is true then how does atomic.incrementAndGet() work without synchronized? And is it thread-safe?

And what is the difference between internal reading and writing to volatile variables / atomic variables? I read in some article that the thread has a local copy of the variables – what is that?

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