Collections are just data structures. The classes that are part of the collections API are not Java-specific structures, and exist in many languages. Here's a basic rundown of the ones that you will use 99% of the time: Legend: parent class/interface (child classes) List (ArrayList): Essentially a resizable array that you can add elements to without having to specify an index. Set (HashSet): A collection of unique elements. Does not remember insertion order (unlike an ArrayList), but very quick to search for elements. Map (HashMap): A collection of key-value pairs with unique keys. LinkedList: A series of nodes that are linked to each other to allow for very fast insertion and deletion of elements. Commonly used when a stack or queue is needed. PriorityQueue: A queue that automatically orders newly added elements. Here's some additional reading if you want. More on reddit.com

What is the difference between Collection and List in Java? When should I use which? More on stackoverflow.com

What is the main difference between Collection and Collections in Java? More on stackoverflow.com

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