How LongAdder performs better than AtomicLong

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does that mean LongAdder aggregates the values internally and update it later?

Yes, if I understand your statement correctly.

Each Cell in a LongAdder is a variant of an AtomicLong. Having multiple such cells is a way of spreading out the contention and thus increasing throughput.

When the final result (sum) is to be retrieved, it just adds together the values of each cell.

Much of the logic around how the cells are organized, how they are allocated etc can be seen in the source: http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/f398670f3da7/src/java.base/share/classes/java/util/concurrent/atomic/Striped64.java

In particular the number of cells is bound by the number of CPUs:

/** Number of CPUS, to place bound on table size */
static final int NCPU = Runtime.getRuntime().availableProcessors();

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