F.11. final Instance Variables

The principle of least privilege is fundamental to good software engineering. In the context of an application, it states that code should be granted only the amount of privilege and access that it needs to accomplish its designated task, but no more. This makes your programs more robust by preventing code from accidentally (or maliciously) modifying variable values and calling methods that should not be accessible.

Let’s see how this principle applies to instance variables. Some of them need to be modifiable and some do not. You can use the keyword final to specify that a variable is not modifiable (i.e., it’s a constant) and that any attempt to modify it is an error. For example,

Click here to view code image

private final int INCREMENT;

declares a final (constant) instance variable INCREMENT of type int. Such variables can be initialized when they’re declared. If they are not, they must be initialized in every constructor of the class. Initializing constants in constructors enables each object of the class to have a different value for the constant. If a final variable is not initialized in its declaration or in every constructor, a compilation error occurs.

Image Software Engineering Observation F.6

Declaring an instance variable as final helps enforce the principle of least privilege. If an instance variable should not be modified, declare it to be final to prevent modification.

Image Common Programming Error F.7

Attempting to modify a final instance variable after it’s initialized is a compilation error.

Image Error-Prevention Tip F.2

Attempts to modify a final instance variable are caught at compilation time rather than causing execution-time errors. It’s always preferable to get bugs out at compilation time, if possible, rather than allow them to slip through to execution time (where experience has found that repair is often many times more expensive).

Image Software Engineering Observation F.7

A final field should also be declared static if it’s initialized in its declaration to a value that’s the same for all objects of the class. After this initialization, its value can never change. Therefore, we don’t need a separate copy of the field for every object of the class. Making the field static enables all objects of the class to share the final field.

..................Content has been hidden....................

You can't read the all page of ebook, please click here login for view all page.
Reset