Convert List<Object> to List<CustomClass> [duplicate]

Question

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?

Answer 1

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.

Answer 2

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;

Answer 3

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;
}

Answer 4

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.