Java streams allow you to process a sequence of elements in a sequential or parallel way. You can create a stream from different data sources, as a Collection, a File or an Array and apply to its elements a sequence of operations normally defined with lambda expressions. Those operations can be divided into two different classes:

• Intermediate operations: These operations return other Stream as a result and allow you to filter, transform, or sort the elements of the stream
• Terminal operations: These operations return a result after processing the elements of the stream

A stream has a source, zero or more intermediate operations, and a terminal operation. The two most important terminal operations are:

• The reduce operation, which allows you to obtain a unique result after processing the elements of the stream. This result usually is a summary of the processed data. The Reducing the elements of a stream recipe explains you how to use reduce operations in Java.
• The collect operation that allows you to generate a data structure with the results of processing the elements of the stream. This is also called a mutable reduction operation as the result is a mutable data structure.

In this recipe, we will learn how to execute collect operations in Java streams with the different versions of the collect() method and the auxiliary Collectors class.