9: Constructing New Objects in Java

Make ANSI/AIM Code 39 in Java 9: Constructing New Objects

I describe the oddball case (in which a constructor looks like it s being inherited) later in this chapter, in the section The default constructor So the code in Listing 9-5 has four constructors Each constructor has the name TemperatureNice, and each constructor has its own, uniquely identifiable parameter list That s the boring part The interesting part is that each constructor makes a call to something named super, which is a Java keyword In Listing 9-5, super stands for a constructor in the parent class The statement super() in Listing 9-5 calls the parameterless Temperature() constructor that s in Listing 9-1 That parameterless constructor assigns 00 to the number variable and ScaleNamefahrenheit to the scale variable The statement super(number, scale) in Listing 9-5 calls the constructor Temperature(double number, ScaleName scale) that s in Listing 9-1 In turn, the constructor assigns values to the number and scale variables In a similar way, the statements super(number) and super(scale) in Listing 9-5 call constructors from Listing 9-1 The computer decides which of the Temperature class s constructors is being called by looking at the parameter list after the word super For instance, when the computer executes super(number, scale); from Listing 9-5, the computer says to itself, The number and scale variables in parentheses have types double and ScaleName But only one of the Temperature constructors in Listing 9-1 has two parameters with types double and ScaleName The constructor s header looks like this: public Temperature(double number, ScaleName scale) So, I guess I ll execute the statements inside that particular constructor

In Listing 9-5, I define what it means to be in the TemperatureNice class Now it s time to put this TemperatureNice class to good use Listing 9-6 has code that uses TemperatureNice

class UseTemperatureNice { public static void main(String args[]) { TemperatureNice temp = new TemperatureNice(); tempsetNumber(700); tempsetScale(ScaleNamefahrenheit); tempdisplay(); temp = new TemperatureNice(320); tempdisplay(); temp = new TemperatureNice(ScaleNamecelsius); tempdisplay(); temp = new TemperatureNice(273, ScaleNamekelvin); tempdisplay(); } } The code in Listing 9-6 is very much like its cousin code in Listing 9-3 The big differences are as follows: Listing 9-6 creates instances of the TemperatureNice class That is, Listing 9-6 calls constructors from the TemperatureNice class, not the Temperature class Listing 9-6 takes advantage of the display method in the Temperature Nice class So the code in Listing 9-6 is much tidier than its counterpart in Listing 9-3 A run of Listing 9-6 looks exactly like a run of the code in Listing 9-3 The run is shown previously in Figure 9-1

The main message in the previous section is that subclasses don t inherit constructors So what gives with all the listings back in 8 In Listing 8-6, a statement says FullTimeEmployee ftEmployee = new FullTimeEmployee();

But, here s the problem: The code defining FullTimeEmployee (Listing 8-3) doesn t seem to have any constructors declared inside it So, in Listing 8-6, how can you possibly call the FullTimeEmployee constructor Here s what s going on When you create a subclass and don t put any explicit constructor declarations in your code, then Java creates one constructor for you It s called a default constructor If you re creating the public FullTimeEmployee subclass, the default constructor looks like the one in Listing 9-7

Listing 9-7:

public FullTimeEmployee() { super(); } The constructor in Listing 9-7 takes no parameters, and its one statement calls the constructor of whatever class you re extending (Woe be to you if the class that you re extending doesn t have a parameterless constructor) You ve just read about default constructors, but watch out! Notice one thing that this talk about default constructors doesn t say: It doesn t say that you always get a default constructor In particular, if you create a subclass and define any constructors yourself, Java doesn t add a default constructor for the subclass (and the subclass doesn t inherit any constructors, either) So how can this trip you up Listing 9-8 has a copy of the code from Listing 8-3, but with one constructor added to it Take a look at this modified version of the FullTimeEmployee code

Listing 9-8:

Look, I Have a Constructor!

class FullTimeEmployee extends Employee { private double weeklySalary; private double benefitDeduction; public FullTimeEmployee(double weeklySalary) { thisweeklySalary=weeklySalary; } public void setWeeklySalary(double weeklySalaryIn) { weeklySalary = weeklySalaryIn; } public double getWeeklySalary() {

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