null object

what is the difference?
1) return world.equals(“hello”);
2) return “hello”.equals(world);
equals should obey three rules:
reflexitivity, symmetricity, transitivity
the above two are the same if world is not null.
if world is null, 1) throw an nullPointerException
2) return false
petabyte
= 1024 terabytes
stack overflow一般是要处理的数据过大
大于stack允许的大小
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