concept flatMap in category java

This is an excerpt from Manning's book Modern Java in Action: Lambdas, streams, reactive and functional programming. Login to get full access to this book.

A common data processing idiom is to select information from certain objects. For example, in SQL you can select a particular column from a table. The Streams API provides similar facilities through the map and flatMap methods.

Luckily there’s a solution to this problem using the method flatMap! Let’s see step-by-step how to solve it.

Using flatMap

You can fix this problem by using flatMap as follows:

List<String> uniqueCharacters =
  words.stream()
       .map(word -> word.split(""))        #1
       .flatMap(Arrays::stream)            #2
       .distinct()
       .collect(toList());

copy

Using the flatMap method has the effect of mapping each array not with a stream but with the contents of that stream. All the separate streams that were generated when using map(Arrays::stream) get amalgamated—flattened into a single stream. Figure 5.6 illustrates the effect of using the flatMap method. Compare it with what map does in figure 5.5.

Figure 5.6. Using flatMap to find the unique characters from a list of words

In a nutshell, the flatMap method lets you replace each value of a stream with another stream and then concatenates all the generated streams into a single stream.

We’ll come back to flatMap in chapter 11 when we discuss more advanced Java 8 patterns such as using the new library class Optional for null checking. To solidify your understanding of map and flatMap, try out quiz 5.2.

This is an excerpt from Manning's book Functional Programming in Java: How functional techniques improve your Java programs. Login to get full access to this book.

Don’t worry if you don’t understand what this code does. You’ll learn about this kind of code in later chapters. Note, however, that the flatMap method takes a function as its argument (in the form of a lambda), and that the implementation of this function (the code after the ->) defines a new lambda, which corresponds to a locally embedded function.

Think about the List class. Does this ring a bell? Yes, it leads to the flatMap method.

This is an excerpt from Manning's book Java 8 in Action: Lambdas, streams, and functional-style programming. Login to get full access to this book.

Luckily there’s a solution to this problem using the method flatMap! Let’s see step by step how to solve it.

We have to look at another method supported by Optional called flatMap!

So how can we solve this problem? Again, we can look at a pattern you’ve used previously with streams: the flatMap method. With streams, the flatMap method takes a function as an argument, which returns another stream. This function is applied to each element of a stream, which would result in a stream of streams. But flatMap has the effect of replacing each generated stream by the contents of that stream. In other words, all the separate streams that are generated by the function get amalgamated or flattened into a single stream. What you want here is something similar, but you want to flatten a two-level optional into one.

Like figure 10.2 for the map method, figure 10.4 illustrates the similarities between the flatMap methods of the Stream and Optional classes.

Figure 10.4. Comparing the flatMap methods of Stream and Optional

Here the function passed to the stream’s flatMap method transforms each square into another stream containing two triangles. The result of a simple map would then be a stream containing three other streams, each of them having two triangles, but the flatMap method flattens this two-level stream into a single stream containing six triangles in total. In the same way, the function passed to the optional’s flatMap method transforms the square contained in the original optional into an optional containing a triangle. If this function was passed to the map method, the result would be an optional containing another optional that, in turn, contains a triangle, but the flatMap method flattens this two-level optional into a single optional containing a triangle.