Introduction

• The key concept in object-oriented programming is the object:
• An object encapsulates:
  • data (an objects's attributes)
  • methods (an object's behaviors)
• Objects hide information about their implementation from their users:
  • To use an object, we must interact with it according to its methods.
  • We cannot (if it is designed properly) fiddle directly with its attributes.
  • Users should not need to know how an object is implemented in order to use it.
    • This is true of all the library methods that we have used.
• In C, the unit of programming is the function (method, in Java); the data which the function processes is secondary.
• In Java, the unit of programming is the object, and programming is object-centered, giving appropriate attention to both an object's attributes and its behaviors (i.e., its data and its methods).
• Java programmers focus on creating their own user-defined classes.
  • A class's data elements are called its instance variables.
• Just as an instance of a primitive data type, such as int, is called a variable, an instance of a class is called an object.