Special Object Methods

I have talked about methods that are called by other objects or possibly used by an object itself. But what happens when objects are created? What happens when they go away? If objects are self-contained units, then it would be a good idea to have methods to handle these situations.

These special methods do, in fact, exist and are called constructors and destructors.

A constructor is a special method that is automatically called when the object is created. Its purpose is to handle starting up the object. This is part of an object's mandate to be responsible for itself. The constructor is the natural place to do initializations, set default information, set up relationships with other objects, or do anything else that is needed to make a well-defined object. All object-oriented languages look for a constructor method and execute it when the object is created.

By using constructors properly it is easier to eliminate (or at least minimize) uninitialized variables. This type of error usually occurs from carelessness on the part of the developer. By having a set, consistent place for all initializations throughout your code (that is, the constructors of your objects) it is easier to ensure that initializations take place. Errors caused by uninitialized variables are easy to fix but hard to find, so this convention (with the automatic calling of the constructor) can increase the efficiency of programmers.

A destructor is a special method that helps an object clean up after itself when the object goes out of existence; that is, when the object is destroyed. All object-oriented languages look for a destructor method and execute it when the object is being deleted. As with the constructor, the use of the destructor is part of the object's mandate to be responsible for itself.

Destructors are typically used for releasing resources when objects are no longer needed. Since Java has garbage collection (auto-cleanup of objects no longer in use), destructors are not as important in Java as they are in C++. In C++, it is common for an object's destructor also to destroy other objects that are used only by this object.