[java/de] Corrects the O-Notation of hashmaps

[Weyaaron]

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About the changes: The old version was wrong. O(n) is not "constant time". I decided to include wikipedia references for clarity.

[Weyaaron]

Given that a similar PR was open for quite some time, Is there something I can contribute to ensure merging in a timely manner?

[vendethiel]

Thanks!

[vendethiel]

I initially wanted to wait for a german reviewer but it’s been a while, so I’ll merge as-is :-).