Three months ago, I began a mini-series of articles about designing objects with a discussion of design principles that focused on proper initialization at the beginning of an object's life. In this Design Techniques article, I'll be focusing on the design principles that help you ensure proper cleanup at the end of an object's life.
Why clean up?
Every object in a Java program uses computing resources that are finite. Most obviously, all objects use some memory to store their images on the heap. (This is true even for objects that declare no instance variables. Each object image must include some kind of pointer to class data, and can include other implementation-dependent information as well.) But objects may also use other finite resources besides memory. For example, some objects may use resources such as file handles, graphics contexts, sockets, and so on. When you design an object, you must make sure it eventually releases any finite resources it uses so the system won't run out of those resources.
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