Basic concepts
The Java container library takes the idea of "holding your objects" and divides it into two distinct concepts, expressed as the basic interfaces of the library:
1.
Collection: a sequence of individual elements with one or more rules applied to them. A List must hold the elements in the way that they were inserted, a Set cannot have duplicate elements, and a Queue produces the elements in the order determined by a queuing discipline (usually the same order in which they are inserted).
2.
Map: a group of key-value object pairs, allowing you to look up a value using a key. An ArrayList allows you to look up an object using a number, so in a sense it associates numbers to objects. A map allows you to look up an object using another object. It’s also called an associative array, because it associates objects with other objects, or a dictionary, because you look up a value object using a key object just like you look up a definition using a word. Maps are powerful programming tools.
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