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 ;)

Answer 1

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)?

Answer 2

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.