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Yes, the IDier class needs to have those methods defined.

Put a method in the base class that will keep track.

The base class IDier does not have the method so you can not use an IDier object to get to methods in an sub class.
You could cast the object from the array to be the class that has the method, but...

You still have not answered my question:
What data type is the variable: ecosysteem?

Does that data type/class have the methods that are not being found?

If you do not know the datatype of...

Then you can only use references to those two classes to call those methods.

What data type is the variable: ecosysteem?
Is it an int or a String or one of your classes or what?


Then you can not use the ecosysteem variable as a reference to call that method. The...

Where are the methods in the error messages defined?

What data type is the variable: ecosysteem?
Does it have a getVerplaatst() method?

If the array IDier contains objects with the get/setBoolean methods, you use indexes to get the one element in the array you want to work with:
IDier[i][j].get/setBoolean()

Can you post a code structure that shows the problem?
Where is the boolean variable?
From where do you need to see the value of the variable?

Play computer with your code, do the statements one by one, keeping track of the values of the variables as you proceed and see what the code does.