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You can imagine creating a method called initialize( ) for every class you write. The name is a hint that it should be called before using the object. Unfortunately, this means the user must remember to call the method. In Java, the class designer can guarantee initialization of every object by providing a special method called a constructor. If a class has a constructor, Java automatically calls that constructor when an object is created, before users can even get their hands on it. So initialization is guaranteed.
The next challenge is what to name this method. There are two issues. The first is that any name you use could clash with a name you might like to use as a member in the class. The second is that because the compiler is responsible for calling the constructor, it must always know which method to call. The C++ solution seems the easiest and most logical, so it’s also used in Java: The name of the constructor is the same as the name of the class. It makes sense that such a method will be called automatically on initialization.
Here’s a simple class with a constructor: (See page 97 if you have trouble executing this program.)
Now, when an object is created:
storage is allocated and the constructor is called. It is guaranteed that the object will be properly initialized before you can get your hands on it.
Note that the coding style of making the first letter of all methods lower case does not apply to constructors, since the name of the constructor must match the name of the class exactly.
Like any method, the constructor can have arguments to allow you to specify how an object is created. The above example can easily be changed so the constructor takes an argument:
Constructor arguments provide you with a way to provide parameters for the initialization of an object. For example, if the class Tree has a constructor that takes a single integer argument denoting the height of the tree, you would create a Tree object like this:
If Tree(int) is your only constructor, then the compiler won’t let you create a Tree object any other way.
Constructors eliminate a large class of problems and make the code easier to read. In the preceding code fragment, for example, you don’t see an explicit call to some initialize( ) method that is conceptually separate from definition. In Java, definition and initialization are unified concepts – you can’t have one without the other.
The constructor is an unusual type of method because it has no return value. This is distinctly different from a void return value, in which the method returns nothing but you still have the option to make it return something else. Constructors return nothing and you don’t have an option. If there were a return value, and if you could select your own, the compiler would somehow need to know what to do with that return value.
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