Android™ How to Program, Second Edition

J.9. Maps

Maps associate keys to values. The keys in a Map must be unique, but the associated values need not be. If a Map contains both unique keys and unique values, it’s said to implement a one-to-one mapping. If only the keys are unique, the Map is said to implement a many-to-one mapping—many keys can map to one value.

Maps differ from Sets in that Maps contain keys and values, whereas Sets contain only values. Three of the several classes that implement interface Map are Hashtable, HashMap and TreeMap, and maps are used extensively in Android. Hashtables and HashMaps store elements in hash tables, and TreeMaps store elements in trees—the details of the underlying data structures are beyond the scope of this book. Interface SortedMap extends ...