What does the expression   account1.accountNumber   mean?

A good answer might be:

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

The dot operator is how you access a part 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 happed when the the object was constructed:

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

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

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

However it is common for a class have private variables and methods. Only methods of the class can access these. This will be discussed further in the next chapter. To begin with, our programs will look like this, all in one source file:

class SomeClass
{
  instance variables, constructors, and methods

}

class  TesterClass
{
  public static void main ( String[] args )
  {
    SomeClass someObject = new SomeClass( . . .  );

    someObject . instanceVariable = . . .;
    someObject . method( ... ) ;

    . . .

  }
}

QUESTION 9:

Can the main() method of TesterClass see the statements in the methods of SomeClass?