How to determine the upper boundary of a class hierarchy in object-oriented programming?

In object-oriented programming, the top boundary of the class hierarchy is usually the base (parent) class from which all other classes in the hierarchy inherit.

In Java, this class is often the Object class, as all classes implicitly inherit from it unless specified otherwise.

Example:

public class MyClass {
    // ...
}

// Equivalent to
public class MyClass extends Object {
    // ...
}

Thus, the Object class is the upper boundary of the class hierarchy in Java. It defines basic methods such as toString(), equals(), hashCode(), and others, which are available to all objects.