How to avoid applied lazily Lists.transform in guava?

Question

Map<String,Object> map = Maps.newHashMap();
map.put("test","123");
map.put("fuyou001","456");
map.put("id",145);
List<Map<String,Object>> list = Lists.newArrayList();
list.add(map);
Lists.transform(list, new Function<Map<String, Object>, Object>() {
  @Override
  public Object apply(@Nullable Map<String, Object> input) {
    System.out.println("test:" + input);
    return input;
  }
});
System.out.println(list);`

Console is not display "test...."

how to avoid applied lazily
I also try

List<Map<String,Object>> newList = new ArrayList<Map<String, Object>>(list.size());
Collections.copy(newList,list);

but not effect

Answer 1

Functions in general should not have side effects; that's your real problem.

That said, if you insist on applying the transformation immediately, do a copy: Lists.newArrayList(Lists.transform(list, function)).

Answer 2

Using an ImmutableList will force the values to be computed eagerly, and thus, no need for an extra copy afterwards. This is perhaps a more elegant solution:

list = ImmutableList.copyOf(Lists.transform(list, 
  new Function<Map<String, Object>, Object>() {
  @Override
  public Object apply(@Nullable Map<String, Object> input) {
    System.out.println("test:" + input);
    return input;
  }
}));
System.out.println(list);`

Answer 3

To complement Louis' answer, you're using Lists.transform() as if it modified the original list, like Collections.sort(). It doesn't.

You have to use the return value of Lists.transform() to see something happen, keeping in mind that it's a view which gets evaluated each and every time you call it. So if you need to use the result several times, as Louis said, do a copy in a new List (ArrayList, ImmutableList, etc.).