Getting a list of elements from an ArrayList

Question

Let's say I have a bean like below.

class Customer{
  private String code;
  private String name;
  private Integer value;
  //getters setters omitted for brevity
}

Then from a method I get a List<Customer> back. Now let's say I want to get a list of all member "name" from the List. Obviously I can traverse and build a List<String> of element "name" myself.

However, I was wondering if there is a short cut or more effiecient way to this technique that anyone knows . For instance, if I want to get a list of all keys in a Map object I get do map.keySet(). Something along that line is what I am trying to find out.

Answer 1

Guava has Lists.transform that can transform a List<F> to a List<T>, using a provided Function<F,T> (or rather, Function<? super F,? extends T>).

From the documentation:

public static <F,T>
   List<T> transform(
               List<F> fromList,
               Function<? super F,? extends T> function
           )

Returns a list that applies function to each element of fromList. The returned list is a transformed view of fromList; changes to fromList will be reflected in the returned list and vice versa.

The function is applied lazily, invoked when needed.

Similar live-view transforms are also provided as follows:

• Iterables.transform (Iterable<F> to Iterable<T>)
• Iterators.transform (Iterator<F> to Iterator<T>)
• Collections2.transform (Collection<F> to Collection<T>)
• Maps.transformValues (Map<K,V1> to Map<K,V2>)

Answer 2

Looks like you're looking for the Java equivalent of Perl's map function. This kind of thing might be added to the collections library once (if) Java receives closures. Until then, I think this is the best you can do:

List<String> list = new ArrayList<String>(customers.size());
for ( Customer c : customers ) {
    list.add(c.getName());
}

You could also write a map function that uses a simple interface to provide the mapping function. Something like this:

public interface Transform<I, O> {
    O transform(I in);
}
public <I, O> List<O> map(Collection<I> coll, Transform<? super I, ? extends O> xfrm) {
    List<O> list = new ArrayList<O>(coll.size());
    for ( I in : coll ) {
        list.add(xfrm.transform(in));
    }
    return list;
}

Answer 3

could use something like this: http://code.google.com/p/lambdaj/

Answer 4

I think this is something that you would have to code yourself, in a loop.

Answer 5

You can use LambdaJ's Converter interface and have the following line:

List<String> customerNames = convert(customerList, new Converter<Customer,String>() {
  public String convert(Customer customer) {
    return customer.getName();
  }
});

Answer 6

You need to use a loop, but the function you're looking for is called map in functional languages. It's possible to implement map in Java, although it tends to be fairly inelegant; here's the version I implemented ages ago in my "stuff Java should have but for some reason doesn't" library:

public interface MapFunction<T, U> {
    public U map(T source);
}

public static <T, U> U[] map(T[] objects, MapFunction<T, U> f) {
    if(objects.length == 0) {throw new IllegalArgumentException("Can't map onto an empty array");}
    @SuppressWarnings("unchecked") U[] rtn = (U[])Array.newInstance(f.map(objects[0]).getClass(), objects.length);
    for(int i = 0; i < objects.length; i++)
        rtn[i] = f.map(objects[i]);
    return rtn;
}

Using that, you could do:

List<Customer> list = yourFunction();
List<String> names = Arrays.asList(map(list.toArray(new Customer[0]), new MapFunction<Customer, String>() {
    public String map(Customer c) {
        return c.getName();
    }
}));

You could naturally change map to take collections instead of arrays, which would eliminate the need for Arrays.asList and List.toArray

Answer 7

Using Guava, you can use a Function along with Iterables.transform, Collections2.transform or Lists.transform to create an Iterable, Collection or List respectively.

Iterable<String> names = Iterables.transform(customers, 
    new Function<Customer, String>() {
      public String apply(Customer from) {
        return from.getName();
      }
    });

The returned Iterable is lazy and applies the function to the underlying list as you iterate through it. For a List<String> containing the names, you could use:

List<String> names = Lists.transform(...);

or

ImmutableList<String> names = ImmutableList.copyOf(Iterables.transform(...));

Of course, writing out the anonymous inner class Function implementation each time you want to do this is ugly and verbose, so you may want to make the Function a constant available from the Customer class, called Customer.NAME for instance.

Then the transformation looks much nicer (with static imports especially):

for (String name : transform(customers, Customer.NAME)) { ... }

I also wrote about using interfaces for particular properties of objects (such as name here) to help with consolidating such functions on my blog here.

Answer 8

.... is there a short cut or more efficient way

So, are you looking for a more efficient way to do this:

 List<String> names = new ArrayList<String>();
 for( Customer customer : yourCustomerList ) {
     names.add( customer.getName() );
 }

?!!!!

Or just a different way?

All the previous answer are not really more efficient in terms of runtime nor coding. They are however more flexible without a doubt.

Another alternative would be to include Scala Groovy in your Java code and use this:

list.map( _.name )

list.collect { it.name }

If compiled, Groovy classes may be used from Java, or you can plug in them as an script.

Here's a sample for the given Customer class using Groovy as script.

    List<Customer> customers = Arrays.asList( new Customer[]{
       new Customer("A","123",1),
       new Customer("B","456",2),
       new Customer("C","789",3),
       new Customer("D","012",4)
    });

    setVariable(customers, "list");
    evaluate("names = list.collect { it.name } ");
    List<String> names = (List<String>) getVariable("names");
    System.out.println("names = " + names);

Output:

names = [A, B, C, D]

_{note: I extracted method for readability, but you can see them below}

But, again that's just different, not really more efficient than the regular for loop.

Here's the complete source code. To run it you just need Java1.6 and Groovy in the classpath.