Get the class of java.util.Arrays$ArrayList

Question

How can I know the instance of java.util.Arrays$ArrayList. I've tried this code snippet but it does not work at all :

if (myList instanceof Arrays) {
    //Do something here.
}

I have compared the object classtype with ArrayList and I had the same issue. But while I was inspecting the object, the classType was a :

class java.util.Arrays$ArrayList

The conditional statement below was the only solution I found:

else if (myList.getClass().toString().equals("class java.util.Arrays$ArrayList")) {
    //do something here
}

but I think that controlling the object type by using instanceof would be a great solution.

My question is : what is the classType of java.util.Arrays$ArrayList, so I can use instanceof for my control.

Thanks in advance,

Omar.

Why do you need to know at all that a class is an instance of `java.util.Arrays$ArrayList`? Your code shouldn't depend on this. — Jesper, Oct 23 '15 at 08:24

score 3 · Answer 1 · answered Oct 23 '15 at 08:25

3

in case the list comes from Arrays#asList, the returned ArrayList is NOT java.util.ArrayList, therefore a comparison to this class will always fail.

The question is, why do you need to know that it is exactly this implementation ? Maybe it is enough to check for java.util.List ?

In general it is questionable why you need the instanceof operator at all, often there are other design solutions better.

answered Oct 23 '15 at 08:25

Emerson Cod

1,990
3
21
39

It's because of the nature of the project I'm working on. I receive a Arrays.asList() list and I couldn't change it since I'm not working on this part of the project. At last, the solution was to ask the person who works on creating the list to send me a java.util.ArrayList and it worked! Thanks for the help anyway! – EKO Oct 26 '15 at 21:44
you can also simply wrap the retrieved list in a new list and then you can work with that further – Emerson Cod Oct 27 '15 at 10:54

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

2

java.util.Arrays$ArrayList is a private inner class of Arrays, returned by the Arrays.asList method.

Since it's a private class there is no way to get access to the class in a way that it can be used with the instanceof operator. You can do this check using reflection however:

Class<?> clazz = Class.forName("java.util.Arrays$ArrayList");
// ...
clazz.isAssignableFrom(myList)

But I do not recommend doing that. It's a good practice to program against an interface. In this case the List interface defines the behavior of the object. The actual class of the object shouldn't matter.

edited Jun 20 '20 at 09:12

Community

1
1

answered Oct 23 '15 at 08:36

fabian

80,457
12
86
114

Thanks for your help I didn't know that in the case of **ClassA$ClassB** ClassB is a private inner class of ClassA. – EKO Oct 26 '15 at 21:49

score -1 · Answer 3 · answered Oct 23 '15 at 08:20

-1

Why are you using Arrays ? You must use ArrayList like in this code

if (myList instanceof ArrayList) {
    //Do something here.
}

answered Oct 23 '15 at 08:20

ThomasThiebaud

11,331
6
54
77

Get the class of java.util.Arrays$ArrayList

3 Answers3

Linked

Related