Converting ArrayList of objects to ArrayList of this objects String names in Java

Question

I have an ArrayList of User objects. Now I need the ArrayList of these user's names only. Is there a way to use toString() on entire ArrayList and convert it to ArrayList of String names rather than doing this in for loop? I have also overridden toString in User class so it returns user's name, and I have tried ArrayList <String> names = usersList.toString() but it didn't work.

Answer 1

public ArrayList< String > toStringList( Collection< MyObject > objectList )
{
    ArrayList< String > stringList = new ArrayList< String >();

    for( MyObject myobj : objectList ) {
        stringList.add( myobj.toString() );
    }

    return stringList;
}

...

ArrayList< String > myStringList = toStringList( objectList );

There. One line, and you can reuse it with whatever collection of objects you have in the future.

Answer 2

The Object#toString() returns a String, not an ArrayList. That's your problem. You really need to loop over it. There's absolutely no reason for an aversion against looping. Just hide it away in some utility method if the code is bothering you somehow ;)

---

You can also wait for Java 7 being released and then use the powers of closures.

The following

List<User> users = getItSomehow();
List<String> names = new ArrayList<String>();
for (User user : users) {
    names.add(user.getName());
}

can then be shortened to like

List<User> users = getItSomehow();
List<String> names = users.map({ User user => user.getName() });

---

By the way, you can also put the for in a single line:

for (User user : users) names.add(user.getName());

It may however not be directly understandable for starters.

Answer 3

You can do this using the Google Collections API:

List<User> userList = ...;
List<String> nameList = Lists.transform(userList, new Function<User, String>() {
   public String apply(User from) {
      return from.toString(); // or even from.getName();
   }
});

The library has been renamed to Guava.

Answer 4

In a word: No. What you need is a map function, which you don't have in Java (yet, as BalusC points out). You can write something similar yourself, but you'll end up iterating over each element in your List regardless.