11.5. Maps
A Map defines mappings from keys to values. The <key, value> pair is called an entry. A map does not allow duplicate keys, in other words, the keys are unique. Each key maps to one value at the most, implementing what is called a single-valued map. Thus, there is a many-to-one relation between keys and values. For example, in a student-grade map, a grade (value) can be awarded to many students (keys), but each student has only one grade.
Both the keys and the values must be objects. This means that primitive values must be wrapped in their respective wrapper objects, if they are to be put in a map.
A map is not a collection and the Map interface does not extend the Collection interface. However, the mappings can be viewed as a collection ...
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