Streams - Java 8 Lambdas - page 23

map

NOTE

If you've got a function that converts a value of one type into another, map lets you apply
this function to a stream of values, producing another stream of the new values.

You'll probably notice fairly soon that you've been doing some kind of map operations for
years already. Say you are writing Java code that takes a list of strings and converts them to
their uppercase equivalents. You would loop over all the values in the list and call toUpper-
case on each element. You would then add each of the resulting values into a new List .
Example 3-8 is code written in this style.

Example 3-8. Converting strings to uppercase equivalents using a for loop
List < String > collected = new new ArrayList <>();

for for ( String string : asList ( "a" , "b" , "hello" )) {

String uppercaseString = string . toUpperCase ();
collected . add ( uppercaseString );

}

assertEquals ( asList ( "A" , "B" , "HELLO" ), collected );

map is one of the most commonly used Stream operations (see Figure 3-3 ). You could prob-
ably have guessed this, given how frequently you have implemented something similar to the
aforementioned for loop. Example 3-9 is the same example of turning a list of strings into
their uppercase equivalents using the stream framework.