What does the expression account1.accountNumber mean?

Answer:

The expression means to use the variable accountNumber that is part of the object referred to by account1.

What Members of an Object can be Seen?

The dot operator is how you access a member of an object:

referenceToAnObject . partOfTheObject

Of course, an object has to exist, and there has to be a reference to it. In the program, this happened when the object was constructed:

CheckingAccount account1 
        = new CheckingAccount( "123", "Bob", 100 );

When a method has a reference to an object, the method can:

access the object's instance variables using dot notation, and
call the object's methods using dot notation.

However, it is common for a class to have private variables and methods. Only methods of the class can access these. This will be discussed further in the next chapter.

QUESTION 9:

(Thought Question: ) Do you think that the following would work?

class CheckingAccountTester
{
  public static void main( String[] args )
  {
    CheckingAccount account1 = new CheckingAccount( "123", "Bob", 100 );
 
    account1.balance = 10000000;
  }
}