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Figure 30: Object diagram of the clock display in .NET Integrating barcode data matrix in .NET Figure 30: Object diagram of the clock display

Figure 30: Object diagram of the clock display use visual .net datamatrix 2d barcode creation toattach gs1 datamatrix barcode for .net gs1 128 Objects creating objects The first q uestion we have to ask ourselves is: where do these three objects come from When we want to use a clock display, we might create a ClockDisplay object. We then assume that our clock display has hours and minutes. So by simply creating a clock display, we expect that we have implicitly created two number displays for the hours and minutes.

[concept box: object creation] As writers of the ClockDisplay class, we have to make this happen. We simply write code in the constructor of the ClockDisplay that creates and stores two NumberDisplay objects. Since the constructor is automatically executed when a ClockDisplay object is created, the NumberDisplay objects will automatically be created at the same time.

Here is the code of the ClockDisplay constructor that makes this work.. public clas .NET gs1 datamatrix barcode s ClockDisplay { private NumberDisplay hours; private NumberDisplay minutes; Remaining fields omitted. public ClockDisplay() { hours = new NumberDisplay(24); minutes = new NumberDisplay(60); updateDisplay(); } Methods omitted.

}. Each of the first two lines in the constructor creates a new NumberDisplay object and assigns it to a variable. The syntax of an operation to create a new object is new ClassName ( parameter-list ). The new ope .net framework datamatrix 2d barcode ration does two things: 1. It creates a new object of the named class (here: NumberDisplay).

2. It executes the constructor of that class. If the constructor of the class is defined to have parameters, then the actual parameters must be supplied in the n e w statement.

For instance, the constructor of class NumberDisplay was defined to expect one integer parameter: formal parameter. public Numb erDisplay(int rollOverLimit). Thus, the n .NET Data Matrix ew operation for the NumberDisplay class, which calls this constructor, must provide one actual parameter of type int to match the defined constructor header: actual parameter. new NumberDisplay(24);. This is the ECC200 for .NET same as for methods, discussed in Section [2.4].

With this constructor, we have achieved what we wanted: If someone now creates a ClockDisplay object, the ClockDisplay constructor will automatically execute and create two NumberDisplay objects. Then the clock display is ready to go. Exercise: 3-15 Create a ClockDisplay object by selecting the following constructor:.

new ClockDisplay(). Call its ge DataMatrix for .NET tTime method to find out the initial time the clock has been set to. Can you work out why it starts at that particular time 3-16 How many times would you need to call the tick method on a newly created ClockDisplay object to make its time reach 01:00 How else could you make it display that time .

Multiple constructors You might h .net vs 2010 Data Matrix ECC200 ave noticed when you created a ClockDisplay object that the popup menu offered you two ways to do that:. new ClockDi splay() new ClockDisplay(hour, minute). This is bec Data Matrix 2d barcode for .NET ause the ClockDisplay class contains two constructors. What they provide are alternative ways of initializing a ClockDisplay object.

If the constructor with no arguments is used, then the starting time displayed on the clock will be 00:00. If, on the other hand, you want to have a different starting time, you can set that up by using the second constructor. It is common for class definitions to contain alternative versions of constructors or methods that provide various ways of achieving a particular task via their distinctive sets of parameters.

This is known as overloading a constructor or method. [concept box: overloading]. Exercise: 3 -17 3-18 Look at the second constructor in ClockDisplay s source code. Explain what it does and how it does it. Identify the similarities and differences between the two constructors.

Why is there no call to updateDisplay in the second constructor, for instance . Method calls Internal method calls The last line of the first ClockDisplay constructor consists of the statement updateDisplay();. This statem barcode data matrix for .NET ent is a method call. As we have seen above, the ClockDisplay class has a method with the following signature:.

private void updateDisplay(). [concept bo Data Matrix for .NET x: internal method call] The method call above invokes this method. Since this method is in the same class as the call of the method, we also call it an internal method call.

Internal method calls have the syntax methodName ( parameter-list ); In our example, the method does not have any parameters, so the parameter list is empty. This is signified by the set of parentheses with nothing between them. When a method call is encountered, the matching method is executed, and then execution returns to the method call and continues at the next statement after the call.

For a method signature to match the method call, both the name and the parameter list of the method must match. Here, both parameter lists are empty, so they match. This need to match against both method name and parameter lists is important because there may be more than one method of the same name in a class if that method is overloaded.

In our example, the purpose of this method call is to update the display string. After the two number displays have been created, the display string is set to show the time indicated by the number display objects. The implementation of the updateDisplay method will be discussed below.

3.11.2 External method calls.

Now let us examine the next method: timeTick. The definition is:. public void .net framework barcode data matrix timeTick() { minutes.increment(); if(minutes.

getValue() == 0) { hours.increment();. // it just rolled over!. Were this d .NET data matrix barcodes isplay connected to a real clock, this method would be called once every sixty seconds by the electronic timer of the clock. For now, we just call it ourselves to test the display.

When the timeTick method is called, it first executes the statement. minutes.increment();. [concept bo Data Matrix 2d barcode for .NET x: external method call] This statement calls the increment method of the minutes object. Thus, when one of the methods of the ClockDisplay object is called, it in turn calls a method of another object to do part of the task.

A method call to a method of another object is referred to as an external method call. The syntax of an external method call is object . methodName ( parameter-list ) This syntax is known as dot notation.

It consists of an object name, a dot, the method name, and parameters for the call. It is particularly important to appreciate that we use the name of an object here and not the name of a class. We use the field minutes rather than NumberDisplay.

The timeTick method then has an if statement to check whether the hours should also be incremented. As part of the condition in the if statement, it calls another method of the minutes object: getValue. This method returns the current value of the minutes.

If that value is zero, then we know that the display just rolled over, and we should increment the hours. That is exactly what the code does. If the value of the minutes is not zero, then we re done.

We don t have to change the hours in that case. Thus, the if statement does not need an else part. We should now also be able to understand the remaining three methods of the ClockDisplay class (see Figure 29).

The method setTime takes two parameters, the hour and the minute, and sets the clock to the specified time. Looking at the method body, we can see that it does so by calling the setValue methods of both number displays, the one for the hours and the one for the minutes. Then it calls updateDisplay to update the display string accordingly, just as the constructor does.

The getTime method is trivial it just returns the current display string. Since we always keep the display string up to date, this is all there is to do. Finally, the updateDisplay method is responsible for updating the display string so that the string correctly reflects the time as represented by the two number display objects.

It is called every time the time of the clock changes. It works by calling the getDisplayValue methods of each of the NumberDisplay objects. These methods return the value of each separate number display.

It then uses string concatenation to concatenate these two values with a colon in the middle to a single string..
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