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Putting an Object in a Safe State

  • January 26, 2005
  • By Matt Weisfeld
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Class Attributes

As mentioned earlier, it is possible for two or more objects to share attributes. In Java and C++ you do this by making the attribute static:

public class Count {   static int count;   public method1() {   }}

By declaring count as static, this attribute is allocated a single piece of memory for the class. Thus, all objects of the class use the same memory location for a. Essentially, each class has a single copy, which is shared by all objects of that class (see Figure 7).

Figure 7: Class attributes.

There are many valid uses for class attributes; however, you must be aware of potential synchronization problems. Let's instantiate two Count objects:

Count Count1 = new Count();Count Count2 = new Count();

For the sake of argument, let's say that the object Count1 is going merrily about its way and is using count as a means to keep track of the pixels on a computer screen. This is not a problem until the object Count2 decides to use attribute count to count sheep. The instant that Count2 records its first sheep, the data that Count1 was saving is lost.

Operator Overloading

Some O-O languages allow you to overload an operator. C++ is an example of one such language. Operator overloading allows you to change the meaning of an operator. For example, most people, when they see a plus sign, assume that it represents addition. If you see this equation

X = 5 + 6;

you expect that X would contain the value 11. And in this case, you would be correct.

However, there are times when a plus sign could represent something else. For example, in the following code:

String firstName = "Joe", lastName = "Smith";String Name = firstName + " " + lastName;

You would expect that Name would contain Joe Smith. The plus sign here has been overloaded to perform string concatenation.

String Concatenation

String concatenation is when two strings are combined to create a single string.

In the context of strings, the plus sign does not mean addition of integers or floats, but concatenation of strings.

What about matrix addition? You could have code like this:

Matrix A, B, C;C = A + B;

Thus, the plus sign now performs matrix addition, not addition of integers or floats.

Overloading is a powerful mechanism. However, it can be downright confusing for people who read and maintain code. In fact, developers can confuse themselves.

Java does not allow the option of overloading operators. The language itself does overload the plus sign for string concatenation, but that is it. The designers of Java must have decided that operator overloading was more of a problem than it was worth. If you must use operator overloading, take care not to confuse the people who will use the class.

Multiple Inheritance

As the name implies, multiple inheritance allows a class to inherit from more than one class. In practice this seems like a great idea. Objects are supposed to model the real world, are they not? And there are many real-world examples of multiple inheritance. Parents are a good example of multiple inheritance. Each child has two parents—that's just the way it is. So it makes sense that you can design classes by using multiple inheritance. And in some O-O languages, such as C++, you can.

However, this situation falls into a category similar to operator overloading. Multiple inheritance is a very powerful technique, and in fact, some problems are quite difficult to do without it. Multiple inheritance can even solve some problems quite elegantly. However, multiple inheritance can significantly increase the complexity of a system.

As with operator overloading, the designers of Java decided that the increased complexity of allowing multiple inheritance far outweighed its advantages, so they eliminated it from the language. In some ways, the Java language construct of interfaces compensates for this; however, the bottom line is that Java does not allow conventional multiple inheritance.

Behavioral and Implementation Inheritance

Java interfaces are a mechanism for behavioral inheritance, whereas abstract classes are used for implementation inheritance. The bottom line is that Java interfaces provide interfaces, but no implementation, whereas abstract classes may provide both interfaces and implementation.

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