Advanced Collections and Collectors - Java 8 Lambdas - page 75

collector—but it is quite an instructive example both of how custom collectors work and of
how to refactor legacy code into Java 8.

Example 5-17 is a reminder of our Java 7 String -joining example.

Example 5-17. Using a for loop and a StringBuilder to pretty-print the names of artists
StringBuilder builder = new new StringBuilder ( "[" );

for for ( Artist artist : artists ) {

iif ( builder . length () > 1 )
builder . append ( ", " );

String name = artist . getName ();
builder . append ( name );

}
builder . append ( "]" );
String result = builder . toString ();

It's pretty obvious that we can use the map operation to transform the Stream of artists into a
Stream of String names. Example 5-18 is a refactoring of this code to use streams and map .

Example 5-18. Using a forEach and a StringBuilder to pretty-print the names of artists
StringBuilder builder = new

new StringBuilder ( "[" );

artists . stream ()
. map ( Artist: : getName )
. forEach ( name -> {
iif ( builder . length () > 1 )
builder . append ( ", " );

builder . append ( name );

});
builder . append ( "]" );
String result = builder . toString ();

This has made things a bit clearer in the sense that the mapping to names shows us what has
been built up a bit more quickly. Unfortunately, there's still this very large forEach block
that doesn't fit into our goal of writing code that is easy to understand by composing high-
level operations.

Let's put aside our goal of building a custom collector for a moment and just think in terms
of the existing operations that we have on streams. The operation that most closely matches
what we're doing in terms of building up a String is the reduce operation. Refactoring
Example 5-18 to use that results in Example 5-19 .