Streams - Java 8 Lambdas - page 26

I'm sure you've written some code that looks like this: it's called the filter pattern. The
central idea of filter is to retain some elements of the Stream , while throwing others out.
Example 3-11 shows how you would write the same code in a functional style.
Example 3-11. Functional style
List < String > beginningWithNumbers
= Stream . of ( "a" , "1abc" , "abc1" )
. filter ( value -> isDigit ( value . charAt ( 0 )))
. collect ( toList ());
assertEquals ( asList ( "1abc" ), beginningWithNumbers );
Much like map , filter is a method that takes just a single function as an argument—here
we're using a lambda expression. This function does the same job that the expression in the
if statement did earlier. Here, it returns true if the String starts with a digit. If you're re-
factoring legacy code, the presence of an if statement in the middle of a for loop is a pretty
strong indicator that you really want to use filter .
Because this function is doing the same job as the if statement, it must return either true or
false for a given value. The Stream after the filter has the elements of the Stream before-
hand, which evaluated to true . The functional interface for this type of function is our old
friend from the previous chapter, the Predicate (shown in Figure 3-6 ).
Figure 3-6. The Predicate interface
flatMap
NOTE
flatMap (see Figure 3-7 ) lets you replace a value with a Stream and concatenates all the
streams together.