How to Get attributes list from ArrayList of objects

Question

Suppose I have an object that looks like:

public class Obj {
String foo;
String bar;
}

If I create an arraylist of type Obj and populate it, is there a way I can return a list of all the objects in the array's foo attribute from the ArrayList?

EDIT: I should have been more clear, I did not want to do this via iteration

Answer 1

Try this:

Collection<String> names = CollectionUtils.collect(personList, TransformerUtils.invokerTransformer("getName"));  

Use apache commons collection api.

Answer 2

You'll have to iterate through your List<Obj> and collate the foo entries into a new List

eg

List<String> foos = new ArrayList<String>();
for (Obj obj : objs) {
  foos.add(obj.foo)
}

or for Java 8 and beyond, use streams thus:

objs.stream().map(o -> o.foo).collect(toList());

Answer 3

Using Guava you could create a view of the foo property of the objects in the List using Lists.transform like this:

public class Obj {
  String foo;
  String bar;

  public static final Function<Obj, String> FOO = new Function<Obj, String>() {
    public String apply(Obj input) {
      return input.foo;
    }
  };
}

public void someMethod(List<Obj> objs) {
  List<String> foos = Lists.transform(objs, Obj.FOO);
  ...
}

Unlike other solutions, this is purely a view of the List<Obj> and as such it doesn't allocate a whole separate ArrayList or some such in memory and can be created in almost no time regardless of the size of your List<Obj> . Additionally, if you change the original List<Obj> , the foos list will reflect that change.

In Java 8 (due in 2012 sometime), this will become a lot easier with lambda expressions and method references. You'll be able to do something like this:

List<Obj> objs = ...
List<String> foos = objs.map(#Obj.getFoo);

Answer 4

The answer of @Surodip uses a compact solution based on Apache Commons Collections . But that solution is not typesafe, since the Transfomer references the property via string expression: TransformerUtils.invokerTransformer("getName")

Here is a more verbose, but typesafe solution using Apache Commons Collections:

Collection<String> values = CollectionUtils.collect(messages, new Transformer<Obj, String>(){

            @Override
            public String transform(Obj input) {
                return input.getFoo();
            }

        });

The above solutions uses Apache Commons Collection Version >= 4, which supports generics for type safety.

Below is the less typesafe version for Apache Collections Version < 4, which does not use generics:

Collection values = CollectionUtils.collect(messages, new Transformer(){

            @Override
            public Object transform(Object input) {
                Obj obj = (Obj) input;
                return obj.getFoo();
            }

        });

Answer 5

遍历列表并创建一组所有foo属性。

Answer 6

something like this?

List<String> foos = new ArrayList<String>();
for (Obj obj : objList )
{
    foos.addElement(obj.foo);
}