How to add element in ArrayList<? extends SomeType> via a method

Question

i want to add element extending Type to ArrayList.

ArrayList<? extends Type>

i defined a method as below

public <T extends Type> void addItem(T itemToAdd, Class<?> classType) {
    ArrayList<? extends Type> allItems = getAllItems(classType);
    allItems.add(itemToAdd);
}

T itemToAdd is error. because i can't add someother type. i thought but i don't know to mention it and its error!

How to add the item via a method call?

Is it a typo that you have both SomeType and Type in your question? — Duncan Jones
– Duncan Jones, Commented Apr 30, 2014 at 8:27

AlexR · Accepted Answer · 2014-04-30 08:32:55Z

2

Think about your definition: ArrayList<? extends Type>. It means list of elements each of them extends Type. It does not mean that each of them is of type T as itemToAdd. To make this code to compile you have to ensure this fact, i.e. use T in all relevant places:

public <T extends Type> void addItem(T itemToAdd, Class<T> classType) {
    List<T> allItems = getAllItems(classType);
    allItems.add(itemToAdd);
}

It probably means that you should change definition of getAllItem() to

public <T extends Type> List<T> getAllItems(Class<T> classType)

BTW please pay attention that I changed your ArrayList to List. Although it is out of the topic but avoid using concrete classes at the left part of assignment operator, as a return value of method and as a method parameter.

answered Apr 30, 2014 at 8:32

AlexR

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4 Comments

Dineshkumar Over a year ago

so wherever we mention T, eg: List<T> it means that it returns a list of <T extends Type> ?

AlexR Over a year ago

Exactly. But T is always the same over whole the way.

Dineshkumar Over a year ago

but when i'm calling function everytime i need to pass class<T> also anyway to eliminate that? just like calling <Type>method()?

AlexR Over a year ago

You do not need it for method addItem. You probably need it for getAllItems but since you have not shared code of this method I cannot say whether you can eliminate this parameter. Try to ask another question and share more code.

FrancoisT · Accepted Answer · 2014-04-30 08:33:46Z

0

If you define for your List all object must have the same type

If you want to insert multiple class defined simple List and you can insert objects extends Type

Example :

List<Type> list = gellAllItems(class);
list.add(T);

answered Apr 30, 2014 at 8:33

FrancoisT

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Comments

songyuanyao · Accepted Answer · 2014-04-30 08:42:55Z

0

I'm not sure about your declaration of the return value of getAllItems, but you can't put itemToAdd with type T extends Type into ArrayList<? extends Type>. The type of element may be unmatch (different type although both are the derived class of Type), so the ArrayList should be declared as

ArrayList<? super Type> allItems;

which can guarantee itmeToAdd can be put into the ArrayList, or just:

ArrayList<Type> allItems;

edited Apr 30, 2014 at 8:42

answered Apr 30, 2014 at 8:37

songyuanyao

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