Atomic operations

An atomic operation is an operation which is performed as a single unit of work without the possibility of interference from other operations.
In Java the language specification guarantees that that reading or writing a variable is atomic (unless the variable is of type long or double). Long and double are only atomic if they declared as volatile.
The operation i++ it not atomic in Java for the standard variables (int, long, etc).
Here i++ is not a single instructions its basically 3 instructions :
i.) Read i
ii.) Increment i
iii.) Set i
These 3 instructions may not happen atomically.

It first reads the value which is currently stored in i (atomic operations) and then it adds one to it (atomic operation). But between the read and the write the value of i might have changed. Java

Since Java 1.5 the java language provides atomic variables, e.g. AtomicInteger or AtomicLong which provide methods like getAndDecrement(), getAndIncrement() and getAndSet() which are atomic.