Classes and Objects: A Deeper Look - Java: How to Program Early Objects - page 338

by other programs. The JVM performs automatic garbage collection to reclaim the mem-
ory occupied by objects that are no longer used. When there are no more references to an
object, the object is eligible to be collected. Collection typically occurs when the JVM ex-
ecutes its garbage collector , which may not happen for a while, or even at all before a pro-
gram terminates. So, memory leaks that are common in other languages like C and C++
(because memory is not automatically reclaimed in those languages) are less likely in Java,
but some can still happen in subtle ways. Resource leaks other than memory leaks can also
occur. For example, an app may open a file on disk to modify its contents—if the app does
not close the file, it must terminate before any other app can use the file.
A Note about Class Object 's finalize Method
Every class in Java has the methods of class Object (package java.lang ), one of which is
method finalize . (You'll learn more about class Object in Chapter 9.) You should never
use method finalize , because it can cause many problems and there's uncertainty as to
whether it will ever get called before a program terminates.
The original intent of finalize was to allow the garbage collector to perform termi-
nation housekeeping on an object just before reclaiming the object's memory. Now, it's
considered better practice for any class that uses system resources—such as files on disk—
to provide a method that programmers can call to release resources when they're no longer
needed in a program. AutoClosable objects reduce the likelihood of resource leaks when
you use them with the try -with-resources statement. As its name implies, an AutoClos-
able object is closed automatically, once a try -with-resources statement finishes using the
object. We discuss this in more detail in Section 11.12.
Software Engineering Observation 8.8
Many Java API classes (e.g., class Scanner and classes that read files from or write files to
disk) provide close or dispose methods that programmers can call to release resources
when they're no longer needed in a program.
8.11 static Class Members
Every object has its own copy of all the instance variables of the class. In certain cases, only
one copy of a particular variable should be shared by all objects of a class. A static field —
called a class variable —is used in such cases. A static variable represents classwide infor-
mation —all objects of the class share the same piece of data. The declaration of a static
variable begins with the keyword static .
Motivating static
Let's motivate static data with an example. Suppose that we have a video game with Mar-
tian s and other space creatures. Each Martian tends to be brave and willing to attack other
space creatures when the Martian is aware that at least four other Martian s are present. If
fewer than five Martian s are present, each of them becomes cowardly. Thus, each Martian
needs to know the martianCount . We could endow class Martian with martianCount as
an instance variable . If we do this, then every Martian will have a separate copy of the in-
stance variable, and every time we create a new Martian , we'll have to update the instance
variable martianCount in every Martian object. This wastes space with the redundant cop-
ies, wastes time in updating the separate copies and is error prone. Instead, we declare mar-