Question

what is or what type is private Object[] obj;?

Answer 1

Object is the ultimate ancestor of every class in the Java inheritance hierarchy.

From the API:

Class Object is the root of the class hierarchy. Every class has Object as a superclass. All objects, including arrays, implement the methods of this class.

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The declaration:

private Object[] obj; 

declares a reference variable called obj for an array of Objects that private to the class that declared it. (Note that this doesn’t create this array, only declares a variable capable of referencing it).