Java Generics extends Loop

Question

how can i loop through object which extends from a class? i give an example: I have an Abstract class (AbstractDTO) and 2 classes which extends AbstractDTO:

public abstract class AbstractDTO
{
    private int id;

    /**
     * @return the id
     */
    public int getId()
    {
        return id;
    }
    /**
     * @param id the id to set
     */
    public void setId(int id)
    {
        this.id = id;
    }
}


public class FirstDTO extends AbstractDTO
{

}

public class SecondDTO extends AbstractDTO
{

}

Now i got a method which excpects 2 Collections of objects which extends AbstractDTO. I want to add all objects of the second list to the first list, if they are not in the first list already. how can i do this?

i tried the following:

public static void summarizeCollection(List<? extends AbstractDTO> firstList, Set<? extends AbstractDTO> secondList)
    {
        for(AbstractDTO second : secondList)
        {
            boolean exists = false;
            for (AbstractDTO first : firstList)
            {
                if (second.getId() == first.getId())
                {
                    exists = true;
                }
            }
            if(!exists)
            {
                firstList.add(second);
            }
        }

    }

I got an error in the line " firstList.add(second);" cause my second object is of the type AbstractDTO and not of a class which extends AbstractDTO.

Can someone help me with that? Thx alot ;)

score 3 · Accepted Answer · edited May 23 '17 at 10:28

The compiler cannot be sure that the elements in secondList are of the same type of the elements declared in firstList, thus you get a compiler error here:

firstList.add(second);

This is caused for the definition of the arguments:

List<? extends AbstractDTO> firstList, Set<? extends AbstractDTO> secondList

If you're sure both List and Set are of the same type and this type extends AbstractDTO, then declare your method like this:

public static <T extends AbstractDTO> void summarizeCollection(List<T> firstList, Set<T> secondList)

Then, inside the method, change this for loop:

for(AbstractDTO second : secondList)

into

for(T second : secondList)

And on.

More info:

What is PECS (Producer Extends Consumer Super)?

score 3 · Answer 2 · answered Apr 24 '15 at 18:09

The compiler error is there because the compiler cannot guarantee that the wildcard from the List is equal to the wildcard from the Set.

You can solve this by making the method generic. You need a type variable to represent the type of the List, which must be an AbstractDTO, say T. You can make the Set a wildcard bounded by T, so that a Set of any type that is T or extends T can have its elements added to the List.

public static <T extends AbstractDTO> void summarizeCollection(
    List<T> firstList, Set<? extends T> secondList)

Then both for loops' variables should be of type T.

for(T second : secondList)

and

    for (T first : firstList)

This will allow the call to add to compile.

Java Generics extends Loop

2 Answers2