Create a new project: a Foundation Command Line Tool called Appliances. In this program, you are going to create two classes: Appliance and OwnedAppliance (a subclass of Appliance). An instance of Appliance will have a productName and a voltage. An instance of OwnedAppliance will also have a set containing the names of its owners.

Figure 29.1  Appliance and its subclass, OwnedAppliance

Create a new file: an NSObject subclass named Appliance. In Appliance.h, create instance variables and property declarations for productName and voltage:

#​i​m​p​o​r​t​ ​<​F​o​u​n​d​a​t​i​o​n​/​F​o​u​n​d​a​t​i​o​n​.​h​>​

@​i​n​t​e​r​f​a​c​e​ ​A​p​p​l​i​a​n​c​e​ ​:​ ​N​S​O​b​j​e​c​t​ ​{​
 ​ ​ ​ ​N​S​S​t​r​i​n​g​ ​*​p​r​o​d​u​c​t​N​a​m​e​;​
 ​ ​ ​ ​i​n​t​ ​v​o​l​t​a​g​e​;​
}​
@​p​r​o​p​e​r​t​y​ ​(​c​o​p​y​)​ ​N​S​S​t​r​i​n​g​ ​*​p​r​o​d​u​c​t​N​a​m​e​;​
@​p​r​o​p​e​r​t​y​ ​i​n​t​ ​v​o​l​t​a​g​e​;​

@​e​n​d​

(We’ll talk about the copy property attribute in Chapter 30.)

In Appliance.m, synthesize the properties to create the accessor methods for the instance variables:

#​i​m​p​o​r​t​ ​"​A​p​p​l​i​a​n​c​e​.​h​"​

@​i​m​p​l​e​m​e​n​t​a​t​i​o​n​ ​A​p​p​l​i​a​n​c​e​

@​s​y​n​t​h​e​s​i​z​e​ ​p​r​o​d​u​c​t​N​a​m​e​,​ ​v​o​l​t​a​g​e​;​

.​.​.​

You would create an instance of Appliance like this:

A​p​p​l​i​a​n​c​e​ ​*​a​ ​=​ ​[​[​A​p​p​l​i​a​n​c​e​ ​a​l​l​o​c​]​ ​i​n​i​t​]​;​

Note that because Appliance doesn’t implement an init method, it will execute the init method defined in NSObject. When this happens, all the instance variables specific to Appliance are zero-ed out. Thus, the productName of a new instance of Appliance will be nil, and voltage will be zero.