Java Programming/Scope

Scope
The scope of a class, a variable or a method is its visibility and its accessibility. The visibility or accessibility means that you can use the item from a given place.

Scope of method parameters
A method parameter is visible inside of the entire method but not visible outside the method.

In code listing 3.14,  is visible within the entire   method but not in the   and the   methods.

Scope of local variables
A local variable is visible after its declaration until the end of the block in which the local variable has been created.

Access modifiers
You surely would have noticed by now, the words, and  at the beginning of class's method declarations used in this book. These keywords are called the access modifiers in the Java language syntax, and they define the scope of a given item.

For a class

 * If a class has public visibility, the class can be referenced by anywhere in the program.
 * If a class has protected visibility, the class can be referenced only in the package where the class is defined.
 * If a class has private visibility, (it can happen only if the class is defined nested in another class) the class can be accessed only in the outer class.

For a variable

 * If a variable is defined in a public class and it has public visibility, the variable can be referenced anywhere in the application through the class it is defined in.
 * If a variable has protected visibility, the variable can be referenced only in the sub-classes and in the same package through the class it is defined in.
 * If a variable has package visibility, the variable can be referenced only in the same package through the class it is defined in.
 * If a variable has private visibility, the variable can be accessed only in the class it is defined in.

For a method

 * If a method is defined in a public class and it has public visibility, the method can be called anywhere in the application through the class it is defined in.
 * If a method has protected visibility, the method can be called only in the sub-classes and in the same package through the class it is defined in.
 * If a method has package visibility, the method can be called only in the same package through the class it is defined in.
 * If a method has private visibility, the method can be called only in the class it is defined in.

For an interface
The interface methods and interfaces are always. You do not need to specify the access modifier. It will default to. For clarity it is considered a good practice to put the keyword.

The same way all member variables defined in the Interface by default will become  once inherited in a class.

Summary
The cases in bold are the default.

Utility
A general guideline for visibilities is to only make a member as visible as it needs to be. Don't make a member public if it only needs to be private.

Doing so, you can rewrite a class and change all the private members without making compilation errors, even you don't know all the classes that will use your class as long as you do not change the signature of the public members.

Field encapsulation
Generally, it is best to make data private or protected. Access to the data is controlled by setter and getter methods. This lets the programmer control access to data, allowing him/her to check for and handle invalid data.

In the code section 3.51, the  method will only change the value of   if the new name is not null. Because  is conditionally changing name, it is wise to return a boolean to let the program know if the change was successful.

Question 3.15: Consider the following class.

List the fields and methods of this class that can be renamed without changing or even knowing the client classes.



Every field or method that is public can be directly called by a client class so this class would return a compile error if the field or the method has a new name.