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I am returning a List of objects with just the type Object. However I know that these objects are of class CustomClass in this case.

When I attempt to cast the original list to the CustomClass I get an error.

This is how I'm working around it currently and it works but I don't like the fact I have a for loop just to do this.

List<Object> objects = getObjects(); List<CustomClass> customObjects = new ArrayList<>(); for ( Object object : objects ) { if ( object instanceof CustomClass ) customObjects.add( (CustomClass) object ); }

Any ideas?

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  • 3
    Can you update your getObjects() method to return List<CustomClass>? Commented May 26, 2015 at 15:58
  • Code for getObjects please? What's the error? ClassCastException? Commented May 26, 2015 at 16:00
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    I'm pretty sure this is a duplicate (though did not search for "it"). However, this is the usual way to do it (although it could be generalized, e.g. to have a utility method like List<Custom> c = filter(objects, Custom.class);) Commented May 26, 2015 at 16:00
  • Not really it's a database method where I pass in an object factory that creates the objects I want... Commented May 26, 2015 at 16:00
  • @SMA: There is no error. The OP just wants to eliminate the ugly for-loop. Commented May 26, 2015 at 16:02

4 Answers 4

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If you know in advance that all of your objects are actually CustomClass objects, you can perform an unsafe cast:

@SuppressWarnings("unchecked") List<CustomClass> list = (List<CustomClass>)(List<?>)getObjects();

This is the fastest solution; practically in runtime nothing is performed except the variable assignment. However if you're wrong and your list actually contains other objects, you may have an unexpected ClassCastException later.

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If you find yourself doing this kind of filtering/casting often, you can write your own type-safe generic method:

List<Object> objects = getObjects(); List<CustomClass> customObjects = myFilter(objects, CustomClass.class); static <E> List<E> myFilter(List<?> lst, Class<E> cls) { List<E> result = new ArrayList<E>(); for (Object obj : lst) { if (cls.isInstance(obj)) result.add(cls.cast(obj)); } return result; }
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If you are absolutely, positively sure that your list contains only CustomClass objects, you can do this, and silence a lot of warnings in the process

// Note, absense of generic parameter here, this is a first warning you will see List genericList = getObjects( ); // Expect another 'unchecked' warning here List< CustomClass > typedList = (List<CustomClass>) genericList;
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A couple of tips: When preparing the List, ensure you add CustomClass objects to the list.

customObjects.add( (CustomClass) object );

When returning object from your method getObjects(), this will return list of CustomClass and not list of Object.

List<CustomClass> objects = getObjects();

Hope this helps.

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