Java collection/map apply method equivalent?

Question

I would like to apply a function to a Java collection, in this particular case a map. Is there a nice way to do this? I have a map and would like to just run trim() on all the values in the map and have the map reflect the updates.

Answer 1

With Java 8's lambdas, this is a one liner:

map.replaceAll((k, v) -> v.trim()); 

For the sake of history, here's a version without lambdas:

public void trimValues(Map<?, String> map) {   for (Map.Entry<?, String> e : map.entrySet()) {     String val = e.getValue();     if (val != null)       e.setValue(val.trim());   } } 

Or, more generally:

interface Function<T> {   T operate(T val); }  public static <T> void replaceValues(Map<?, T> map, Function<T> f) {   for (Map.Entry<?, T> e : map.entrySet())     e.setValue(f.operate(e.getValue())); }  Util.replaceValues(myMap, new Function<String>() {   public String operate(String val)   {     return (val == null) ? null : val.trim();   } }); 

Answer 2

I don't know a way to do that with the JDK libraries other than your accepted response, however Google Collections lets you do the following thing, with the classes com.google.collect.Maps and com.google.common.base.Function:

Map<?,String> trimmedMap = Maps.transformValues(untrimmedMap, new Function<String, String>() {   public String apply(String from) {     if (from != null)       return from.trim();     return null;   } } 

The biggest difference of that method with the proposed one is that it provides a view to your original map, which means that, while it is always in sync with your original map, the apply method could be invoked many times if you are manipulating said map heavily.

A similar Collections2.transform(Collection<F>,Function<F,T>) method exists for collections.