Frontpage web hosting - Methods 127 Have you noticed throughout the book
Methods 127 Have you noticed throughout the book so far that parentheses appear when using the new keyword to instantiate an object? For example, a new Radio object is instantiated as follows: Radio r = new Radio(); I have not explained those empty parentheses, but I am ready to now. Any time you see parentheses, it looks as if a method is being invoked. This is exactly what is happening when you use the new keyword, except that a method is not invoked. One of the constructors in the class is invoked. In fact, the only time you can invoke a constructor is when the object is being instantiated. Constructors are similar to methods, but keep in mind that they are not methods. They behave quite differently, as you will see in Chapter 6, Understanding Inheritance. Classroom Q & A Q: Wait a minute. We haven t added a constructor to any of the classes we have seen up until now. Do you have to add a constructor to every class? A: You are right. We must not have to add a constructor because our classes seem to be fine without one, so the answer to your question is no. If you do not add a constructor to your class, the compiler writes one for you. This free constructor is known as the default constructor. Q: How does the compiler know what I want my default constructor to do? A: The compiler doesn t know. The default constructor that you get for free has an empty parameter list and doesn t do anything. Q: Then why bother? It seems like a waste of code to have a constructor that doesn t do anything. A: Because every class must have a constructor. When you instantiate a class, the new operator must invoke a constructor. The fact that the compiler adds a default constructor to your class is purely for convenience. In almost all development situations you find yourself in, you will add at least one constructor yourself to all of your classes. Before we write our own constructors, though, let s take a quick look at this default constructor.
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