Some other Java related links

Regarding HashMaps:

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html

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import java.util.HashMap;

HashMap<String, String> userMap = new HashMap<String, String>();

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Also, need to call the "Set" library too:

import java.util.Set;

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The Map Interface:

https://docs.oracle.com/javase/tutorial/collections/interfaces/map.html

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Some of the most common used methods are: clear, containsKey, containsValue,isEmpty, keySet, remove, replace, size, values. Try these out to see what you can do with HashMaps.