Access Modifiers public and private

Most instance-variable declarations are preceded with the keyword private (as in line 7). Like public, keyword private is an access modifier. Variables or methods declared with access modifier private are accessible only to methods of the class in which they’re declared. Thus, variable courseName can be used only in methods setCourseName, getCourseName and displayMessage of (every object of) class GradeBook.

Declaring instance variables with access modifier private is known as data hiding or information hiding. When a program creates (instantiates) an object of class GradeBook, variable courseName is encapsulated (hidden) in the object and can be accessed only by methods of the object’s class. This prevents courseName from being modified accidentally by a class in another part of the program. In class GradeBook, methods setCourseName and getCourseName manipulate the instance variable courseName.