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Putting an Object in a Safe State

  • January 26, 2005
  • By Matt Weisfeld
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Introduction

This is the ninth installment in a series of articles about fundamental object-oriented (OO) concepts. The Object-Oriented Thought Process is intended for anyone who needs to understand the basic object-oriented concepts before jumping into the code. Click here to start at the beginning of the series.

In keeping with the code examples used in the previous articles, Java will be the language used to implement the concepts in code. One of the reasons that I like to use Java is because you can download the Java compiler for personal use at the Sun Microsystems Web site http://java.sun.com/. You can download the J2SE 1.4.2 SDK (software development kit) to compile and execute these applications and will provide the code listings for all examples in this article. I have the SDK 1.4.0 loaded on my machine. I will provide figures and the output (when appropriate) for these examples. See the previous articles in this series for detailed descriptions for compiling and running all the code examples in this series.

In the last column we continued our discussion on data hiding, as in data protection. When designed properly, encapsulation provides a built-in way to enforce certain levels of data protection. Yet, protecting the integrity of the data does not ensure the integrity of the object itself.

A goal of every object-oriented design is to put the object into what is called a safe-state. By this we mean that the object is in a state that will not cause the system to be unstable. There are several mechanisms that are provided to assist in the construction of safe objects. In this article we explore some of the topics.

Constructors

Constructors are a new concept for people doing structured programming. Constructors do not normally exist in non-O-O languages such as C and Basic. Earlier we spoke about special methods that are used to construct objects. In Java and C++, as well as other O-O languages, constructors are methods that share the same name as the class and have no return type. For example, a constructor for the Cabbie class would look like this:

public Cabbie(){   /* code to construct the object */}

The compiler will recognize that the method name is identical to the class name and consider the method a constructor.

Return Value

Note again that a constructor does not have a return value. If you provide a return value, the compiler will not treat the method as a constructor.

When Is a Constructor Called?

When a new object is created, one of the first things that happens is that the constructor is called. Check out the following code:

Cabbie myCabbie = new Cabbie();

The new keyword creates a new instance of the Cabbie class, thus allocating the required memory. Then the constructor is called, passing the arguments in the parameter list. The developer must do the appropriate initialization within the constructor.

Thus, the code new Cabbie() will instantiate a Cabbie object and call the Cabbie method, which is the constructor.

What's Inside a Constructor?

The most important function of a constructor is to initialize the memory allocated when the new keyword is encountered. In short, code included inside a constructor should set the newly created object to its initial, stable state.

For example, if you have a counter object with an attribute called count, you need to set count to zero in the constructor:

count = 0;

Initializing Attributes

Initializing attributes is a common function performed within a constructor.

The Default Constructor

If you write a class and do not include a constructor, the class will still compile and you can still use it. If the class provides no explicit constructor, such as in C++ and Java, a default constructor will be provided.

The default constructor calls only the constructor of the superclass. For example, if a constructor is not provided for the Cabbie class, the following default constructor is inserted:

public Cabbie(){   super();}

Perhaps the default constructor may be sufficient in some cases; however, in most cases some sort of memory initialization should be performed. Regardless of the situation, it is good programming practice to always include at least one constructor in a class. In any case, if there are attributes in the class, they should be initialized in a constructor.

Providing a Constructor

The rule of thumb is that you should always provide a constructor, even if you do not plan on doing anything inside it. You can provide a constructor with nothing in it and then add to it later. While there is technically nothing wrong with using the default constructor provided by the compiler, it is always nice to know exactly what your code looks like.

Using Multiple Constructors

In many cases, an object can be constructed in more than one way. To accommodate this situation you need to provide more than one constructor. For example, let's consider the Count class presented below.

public class Count {   int count;   public Count(){      count = 0;   }}

On the one hand we simply want to initialize the attribute count to count to zero: We can easily accomplish this by having a constructor initialize count to zero as follows:

public Count(){   count = 0;}

On the other hand, we might want to pass an initialization parameter that allows count to be set to various numbers:

public Count (int number){   count = number;}

This is called overloading a method (overloading pertains to all methods, not just constructors). Most O-O languages provide functionality for overloading a method.

Overloading Methods

Overloading allows a programmer to use the same method name over and over, as long as the signature of the method is different each time. The signature consists of the method name and a parameter list (see Figure 1).

Thus, the following methods all have different signatures:

public void getCab();// different parameter listpublic void getCab (String cabbieName);// different parameter listpublic void getCab (int numberOfPassengers);

Figure 1: The components of a signature.

By using different signatures, you can construct objects differently depending on the constructor used.





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