Java 8 Streams

• A Stream is a pipeline of functions that can be evaluated
• Streams can transform data
• A Stream is not a data structure
• Streams cannot mutate data

Most importantly, a stream isn’t a data structure. You can often create a stream from collections to apply a number of functions on a data structure, but a stream itself is not a data structure. That’s so important, I mentioned it twice! A stream can be composed of multiple functions that create a pipeline that data that flows through. This data cannot be mutated. That is to say the original data structure doesn’t change. However the data can be transformed and later stored in another data structure or perhaps consumed by another operation.
We stated that a stream is a pipeline of functions, or operations. These operations can either be classed as an intermediate operation or a terminal operation. The difference between the two is in the output which the operation creates. If an operation outputs another stream, to which you could apply a further operation, we call it an intermediate operation. However, if the operation outputs a concrete type or produces a side effect, it is a terminal type. A subsequent stream operation cannot follow a terminal operation, obviously, as a stream is not returned by the terminal operation!

Intermediate operations

An intermediate operation is always lazily executed. That is to say they are not run until the point a terminal operation is reached. We’ll look in more depth at a few of the most popular intermediate operations used in a stream.

• filter – the filter operation returns a stream of elements that satisfy the predicate passed in as a parameter to the operation. The elements themselves before and after the filter will have the same type, however the number of elements will likely change.
• map – the map operation returns a stream of elements after they have been processed by the function passed in as a parameter. The elements before and after the mapping may have a different type, but there will be the same total number of elements.
• distinct – the distinct operation is a special case of the filter operation. Distinct returns a stream of elements such that each element is unique in the stream, based on the equals method of the elements.

Here’s a table that summarises this, including a couple of other common intermediate operations.

Function	Preserves count	Preserves type	Preserves order
map	✅	❌	✅
filter	❌	✅	✅
distinct	❌	✅	✅
sorted	✅	✅	❌
peek	✅	✅	✅

Terminal operations

A terminal operation is always eagerly executed. This operation will kick off the execution of all previous lazy operations present in the stream. Terminal operations either return concrete types or produce a side effect. For instance, a reduce operation which calls the Integer::sum operation would produce an Optional, which is a concrete type. Alternatively, the forEach operation does not return a concrete type, but you are able to add a side effect such as print out each element. The collect terminal operation is a special type of reduce which takes all the elements from the stream and can produce a Set, Map or List. Here’s a tabulated summary.

Function	Output	When to use
reduce	concrete type	to cumulate elements
collect	list, map or set	to group elements
forEach	side effect	to perform a side effect on elements

All Terminal Operations.