Read Chapter 5: Objects and Classes.
OO concepts of modularity, abstraction, composition, and hierarchy are very powerful and require intense attention to detail. The previous subunit on how Python implements name bindings gave a glimpse of the detail that is involved. This chapter gives a detailed presentation of OO in Java. The terminology in this reading is a little different; name-binding is called name scope. The chapter begins with an explanation of the data and procedures of a class (called class variables and class methods). The class data can be fixed for all objects, in which case, it is called static. Static variables are common to all objects. There is only one copy and, thus only one value, stored in the class.
Recall from the Python OO overview that an environment associates a name with a 'value'. A static variable is in the class environment table, which points to the one copy. In contrast, a class can also have non-static variables and each object of the class contains its own copy of them. In the terminology for Java, each object has a name, which is a pointer to the location of the object's instance variables.
The next detail to note is how objects are created and initialized (i.e. values assigned to its instance variables and its methods names) by assigning values to them in the class and by constructors. A consequence of the concepts of modularity and abstraction is reuse – in writing a OO program we can use classes that have been written by others and are part of the language or contained in class libraries associated with the language. Recall the generic computing paradigm consisted of several states: requirements, design, implementation, and validation. In software engineering, it is referred to as the program process.
Section 5.3 gives insight into writing programs using classes. What classes, what objects, and how are they related? Are questions of program design. Section 5.4 illustrates the design and implementation stages of the process. The latter sections continue with the VERY important OO details of inheritance and polymorphism. They create a class hierarchy that enables code to be shared among classes and among similar, but different, objects. Whereas, Java has simple inheritance (a subclass can extend one superclass), C++ has multiple inheritance ( a subclass can extend 2 or more superclasses). Java does, however, have a restricted kind of multiple inheritance, called interfaces, where an interface can be implemented by 2 or more classes. As you finish the reading, you should appreciate how the concepts are connected. If you understand the variable names, in particular, the object names, 'this' and 'super', the class name 'Object', and the dot naming convention (e.g. ClassName.objectName.methodname), you should have a good understanding of the concepts and details presented in this reading.