Summary: Removes unnecessary imports and adds missing Override annotations
Reviewed-by: xuelei
Contributed-by: NFlorian Weimer <fweimer@redhat.com>

Reviewed-by: chegar, ksrini

Reviewed-by: weijun, wetmore

Reviewed-by: dholmes

Reviewed-by: wetmore

Reviewed-by: xdono

Reviewed-by: mchung

Reviewed-by: darcy, weijun

Summary: add NullPointerException description to those methods.
Reviewed-by: weijun