what is Collections?
The Java Collections API's provide a set of classes and interfaces that makes it easier to handle collections of objects. In a sense Collection's works a bit like arrays, except their size can change dynamically, and they have more advanced behavior than arrays.
Rather than having to write your own collection classes, Java provides these ready-to-use collection classes. Most of the Java collections are located in the java.util package. Java also has a set of concurrent collections in the java.util.concurrent package.
Following thins comes under Collections API
Overview of Interfaces Iterable Collection Generic Collections List Set SortedSet NavigableSet Map SortedMap NavigableMap Queue Deque Stack hashCode() and equals() Sorting
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