Detect type of object in ArrayList

Question

I'm attempting to make a class that will convert ArrayLists of objects into ArrayLists of other objects. i.e.

ArrayList<Foo> convert(ArrayList<Bar> input){
     //conversion logic
}

ArrayList<Bar> convert(ArrayList<Foo> input){
     //conversion logic
}

Unfortunately Java doesn't want to have two functions with the same name and what it believes to be the same inputs and outputs.

I'm attempting to go a different route. Instead of multiple functions with the same name, I want to make one function that accepts an ArrayList, determines which type of object is inside, does the proper conversion, and returns an ArrayList:

ArrayList convert(ArrayList input){
     //conversion logic for Foo

     //conversion logic for Bar
}

Is something like this possible?

Is there any subtype relationship between Bar and Foo for your purposes? — notnoop
– notnoop, Commented Jun 24, 2009 at 18:56
Side issue: For flexibility the input should be an interface, such as List or even better Collection. Actually the return type should also be an interface. — Steve Kuo
– Steve Kuo, Commented Jun 24, 2009 at 19:07
Could you please clarify your goal? Do you want to create a generic converter or conversion between few types? In the latter case, you could just include the source or target type name in the method name. — akarnokd
– akarnokd, Commented Jun 24, 2009 at 19:23
I'm using GWT and google app engine, since GWT doesn't play nicely with GAE I need to have two copies of my entities: One for GWT (just pojo), one for GAE (with annotations). My end goal is to make converting the objects as easy as possible, since there will be A LOT of converting back and forth. — KevMo
– KevMo, Commented Jun 24, 2009 at 20:07

Michael Myers · Accepted Answer · 2009-06-24 18:43:10Z

4

How about an interface:

public class Converter<From, To> {
    List<To> convert(List<From> input);
}

And then have as many implementations as you want. For example:

private static final Converter<Foo, Bar> fooToBarConverter = new Converter<Foo, Bar>() {
    public List<Bar> convert(List<Foo> input) {
        ...
    }
}

answered Jun 24, 2009 at 18:43

Michael Myers♦

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Comments

akarnokd · Accepted Answer · 2009-06-24 19:06:00Z

4

Try:

public <T, U> ArrayList<U> convert(Class<T> typeIn, ArrayList<T> input){     
    // dispatch on typeIn
}

Or better yet

public <T, U, V extends ArrayList<U>> V convert(Class<T> typeIn, 
Class<V> typeOut, ArrayList<T> input){     
    // dispatch on typeIn
    return typeOut.cast(yourConversionResult);
}

Because you might return ArrayList<Foo> or ArrayList<Bar> within the same method and having the proper cast will help you return them without compiler warnings.

Edit: The return type cast for the second sample wasn't going to work. Tried to fix it

edited Jun 24, 2009 at 19:06

answered Jun 24, 2009 at 18:43

akarnokd

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4 Comments

Michael Myers Over a year ago

I'm thinking this might be better than my answer. (By the way, you can add a "public" in there to get the syntax highlighting off of XML mode).

KevMo Over a year ago

I'm not familiar with the <T, U> syntax. Could you provide a more verbose example, or perhaps a link?

Michael Myers Over a year ago

@KevMo: It means the method is generic, even though the containing class might not be.

akarnokd Over a year ago

Thanks. Not sure about the return type cast though.

james · Accepted Answer · 2009-06-24 19:55:04Z

0

You are basically describing a "map" in terms of functional programming. Take a list of objects, apply some operation to each object and accumulate the results in another list. There are libraries out there that implement this stuff already, although i haven't looked recently. I know commons collections has this for pre-generics collections.

the gist of the solution is (similar to mmeyers solution):

public interface Function<From,To> {
  public To apply(From);
}

public <From,To> List<To> map(List<From> fromList, Function<From,To> fun)  {
  // call fun.apply() on every element in fromList and return a new result list ...
}

answered Jun 24, 2009 at 19:55

james

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1 Comment

Michael Myers Over a year ago

You are #18 to misspell my name on SO. Congratulations. ;)

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