Learning Java requires you to understand the principles of object-oriented programming. From your first year CPSC courses, you should have learned what an object is, what a class is, how objects and classes are related, and how objects communicate by using messages. If you do not know about these concepts, or if you simply wish to refresh your memory, then read the following sections:

• What is an Object?
• What is a Message?
• What is a Class?
• What is Inheritance?
• What is an Interface?

What is an Object?

If you look around you, you will find many examples of real-world objects: your book, your computer, your pet, etc. In object-oriented programming, programs are viewed as collections of objects. Objects have three characteristics: state, behavior and identity. We can use a bank account to illustrate these concepts:

• State: id, name, balance
• Behaviour: deposit, withdraw, etc.
• Identity: Joe's account is similar to Jane's, but its state has different values.

You can see that an object is essentially a group of values and behaviours (also called methods, or member functions) that gives itself an identity. The following illustration is a common visual representation of a software object:

The object diagram shows that the object's variables make up the center, or nucleus, of the object. Methods surround and hide the object's nucleus from other objects in the program. Packaging an object's variables within the protective custody of its methods is called encapsulation. However, very often an object may wish to expose some of its variables or hide some of its methods. In Java, an object can specify one of four access levels for each of its variables and methods. The access level determines which other objects and classes can access that variable or method. We will talk more about accessing control later.

What is a Message?

A single object alone is generally not very useful. Instead, an object usually appears as a component of a larger program or application that contains many other objects. Through the interaction of these objects, programs can perform more complex functions. Software objects interact and communicate with each other by sending messages to each other. When object A wants object B to perform some action (call one of its methods), object A sends a message to object B:

A message is comprised of three components:

• The object to which the message is addressed.
• The name of the method to perform.
• Any parameters needed by the method.

• The behaviour of an object is expressed through its methods, and thus (aside from direct variable access) message passing supports all possible interactions between objects.
• Objects do not need to be in the same process or even on the same machine to send and receive messages back and forth to each other.

What is a Class?

In the real world, you often have many objects of the same kind. For example, your bank account is just one of many bank accounts at your bank. In terms of object-oriented programming, objects that share the same kinds of state and behaviour belong to the same class. In other words, a class defines the state and behaviour common to all objects of a certain kind, and any object belonging to that class is an instance of that class. We can use a bicycle example to illustrate these concepts. The bicycle class defines the variables and methods for a bicycle:

Two instances of the bicycle class could be:

A class can define two types of variables and methods: class and instance. The bicycle class above defines instance variables and classes. Each object of the bicycle class can assign its own value to the variables. However, if we define the bicycle class and make an object from the class this way:

Assuming that the bicycle class contains the same instance variables as before, the variable numberOfGears is a class variable. That is, this property is shared by all objects of the class (bicycles). If the class assigns a value (e.g. 18) to this variable, then all instances of the class will use that value.

The difference between instance methods and class methods may be less easy to understand, but it can be summarized in two points:

• Instance methods operate on the instance variables of the current object, and also have access to the class variables.
• Class methods can only access class variables directly. They can only access instance variables through a specific instance of the class (object). Also, since class methods are not applied to a specific object of the class, you call them without specifying an instance.

What is Inheritance?

In the real world, cats and lions are felines, and all felines are mammals. This type of relationship defines an inheritance hierarchy depicted as:

The same pattern applies to object oriented design. Child (derived) classes, also called subclasses, can inherit properties and methods from a parent (base) class, also called superclass:

A subclass inherits instance variables and methods from a superclass. A subclass can also have its own methods, which can override or extend methods of the superclass. This means we do not have to rewrite methods over and over for each class that needs to use them. When a subclass inherits from a superclass, the subclass obtains a copy of all the methods and instance variables contained in the superclass' class definition. Consider an example class called employee that represents people working at a company. All employee objects contain methods such as:

• add_skill - add a skill the employee knows
• has_skill - check to see if an employee has a given skill
• salary - determine how much to pay an employee

Then we can use a class hourly to represent employees who are paid by the hour. Class hourly is a subclass of class employee, and inherits all of its instance variables and methods. Thus, an hourly object can respond to methods such as add_skill and has_skill, even though they are not written explicitly in the definitions of the hourly class.

Stated simply, inheritance allows you to start with a previously defined base class (e.g., employee) and then modify or extend this base class in any way you like. Recall that it is possible to add new methods or instance variables to a subclass. For example, the hourly class could implement the method hours, which calculates the number of hours an employee has worked over a period of time. Moreover, a subclass can replace or override methods that it inherits. For instance, the hourly class can redefine the method salary that it inherited from the employee class.

We usually use inheritance in two types of situations:

• when a subclass is a minor modification of its superclass.
• when many subclasses share common high-level data structures and/or methods. For example, platypus, llama, and rhinoceros would be subclasses of a mammal class.

We are not limited to just one layer of inheritance. The inheritance tree, or class hierarchy, can be as deep as needed. Methods and variables are inherited down through the levels. In general, the farther down in the hierarchy a class appears, the more specialized its behavior.

What is an Interface?

Later you will learn that in Java, interfaces are as important as classes. An interface can be used to interact with objects without knowing exactly which class they belong to. For example, the header files in C++ programs are called interface files because they contain prototypes for all functions implemented in the corresponding source files. Using these interface files, classes implemented in the program can interact with each other by calling each other's methods or referencing each other's variables.

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