Java is there a map function

Question

I need a map function  Is there something like this in Java already    For those who wonder  I of course know how to implement this trivial function myself

User · Accepted Answer

There is no notion of a function in the JDK as of java 6.

Guava has a Function interface though and the
Collections2.transform(Collection<E>, Function<E,E2>)
method provides the functionality you require.

Example:

// example, converts a collection of integers to their
// hexadecimal string representations
final Collection<Integer> input = Arrays.asList(10, 20, 30, 40, 50);
final Collection<String> output =
    Collections2.transform(input, new Function<Integer, String>(){

        @Override
        public String apply(final Integer input){
            return Integer.toHexString(input.intValue());
        }
    });
System.out.println(output);

Output:

[a, 14, 1e, 28, 32]

These days, with Java 8, there is actually a map function, so I'd probably write the code in a more concise way:

Collection<String> hex = input.stream()
                              .map(Integer::toHexString)
                              .collect(Collectors::toList);

User · Answer

Since Java 8  there are some standard options to do this in JDK   Collection lt E gt  in       Object   mapped   in stream   map e - gt  doMap e   toArray       or List lt E gt  mapped   in stream   map e - gt  doMap e   collect Collectors toList       See java util Collection stream   and java util stream Collectors toList

User · Answer

There is a wonderful library called Functional Java which handles many of the things you d want Java to have but it doesn t   Then again  there s also this wonderful language Scala which does everything Java should have done but doesn t while still being compatible with anything written for the JVM

User · Answer

This is another functional lib with which you may use map  http   code google com p totallylazy   sequence 1  2  map toString      lazily returns  1    2

User · Answer

Even though it s an old question I d like to show another solution   Just define your own operation using java generics and java 8 streams   public static  lt S  T gt  List lt T gt  map Collection lt S gt  collection  Function lt S  T gt  mapFunction       return collection stream   map mapFunction  collect Collectors toList         Than you can write code like this   List lt String gt  hex   map Arrays asList 10  20  30  40  50   Integer  toHexString

User · Answer

Be very careful with Collections2 transform   from guava  That method s greatest advantage is also its greatest danger  its laziness   Look at the documentation of Lists transform    which I believe applies also to Collections2 transform        The function is applied lazily  invoked when needed  This is necessary   for the returned list to be a view  but it means that the function   will be applied many times for bulk operations like   List contains java lang Object  and List hashCode    For this to   perform well  function should be fast  To avoid lazy evaluation when   the returned list doesn t need to be a view  copy the returned list   into a new list of your choosing    Also in the documentation of Collections2 transform   they mention you get a live view  that change in the source list affect the transformed list  This sort of behaviour can lead to difficult-to-track problems if the developer doesn t realize the way it works   If you want a more classical  map   that will run once and once only  then you re better off with FluentIterable  also from Guava  which has an operation which is much more simple  Here is the google example for it   FluentIterable         from database getClientList            filter activeInLastMonth            transform Functions toStringFunction            limit 10          toList      transform   here is the map method  It uses the same Function lt    callbacks  as Collections transform    The list you get back is read-only though  use copyInto   to get a read-write list   Otherwise of course when java8 comes out with lambdas  this will be obsolete

[java] Java: is there a map function?

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