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Editorial Team
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Asked: 2026-05-31T00:59:02+00:00

I’m working on a project using OpenCV, and there is a function which takes

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I’m working on a project using OpenCV, and there is a function which takes a List as a parameter. 

Core.split(Mat m, List<Mat> mv);

I tried looking at documentation for List in Java however I’m confused as to how I would create a new list to use. I saw someone online say that list was just an Interface, but I’m not sure exactly what that means? (Sorry – Java isn’t my first language)

What I was doing was – 

private List<Mat> mList; 
private Mat listItem1; (etcetera) 

mList.add(listItem1)

The first part does not cause an error however when I try to mList.add(listItem1) this causes a NULL pointer exception. I’m assuming this is because I haven’t initialised the List. I thought this would be a case of mList = new List<Mat>(); however this gives me the error “Cannot instantiate the type List < Mat >”

Any help would be much appreciated – thanks! Sorry if this is a silly question.

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1 Answer

  1. Editorial Team

    List is an interface. You need to use an implementation of List. Try ArrayList.

    mList = new ArrayList<Mat>();

    Check the JavaDoc for List for other implementations.

    The reason you cannot instantiate List is because it is an interface. It provides a contract with which you can be assured that any implementers will have defined all of its methods. This way, you can work on a List reference without needing to know its exact implementation. (that is, ArrayList, LinkedList, Vector, etc…)

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