Scope

Navigate Classes and Objects topic: Defining classes Inheritance Interfaces Overloading methods and constructors Object Lifecycle Scope Nested classes Generics

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.

Code listing 3.14: Scope.java public class Scope { public void method1(int i) { i = i++; method2(); int j = i * 2; } public void method2() { int k = 20; } public static void main(String[] args) { method1(10); } }

In code listing 3.14, i is visible within the entire method1 method but not in the method2 and the main 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.

Code section 3.50: Local variables. { ... // myNumber is NOT visible { // myNumber is NOT visible int myNumber; // myNumber is visible { ... // myNumber is visible } // myNumber is visible } // myNumber is NOT visible ... }

Access modifiers

You surely would have noticed by now, the words public, protected and private 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 public. You do not need to specify the access modifier. It will default to public. For clarity it is considered a good practice to put the public keyword.

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

Summary

	Class	Nested class	Method, or Member variable	Interface	Interface method signature
public	visible from anywhere	same as its class	same as its class	visible from anywhere	visible from anywhere
protected	N/A	its class and its subclass	its class and its subclass, and from its package	N/A	N/A
package	only from its package	only from its package	only from its package	N/A	N/A
private	N/A	only from its class	only from its class	N/A	N/A

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.

Code section 3.51: Encapsulation. private String name; /** * This is a getter method because it accesses data from the object. */ public String getName() { return name; } /** * This is a setter method because it changes data in the object. */ public boolean setName(String newName) { if (newName == null) { return false; } else { name = newName; return true; } }

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

Test your knowledge

Question 3.15: Consider the following class.

Question 3.15: Question15.java public class Question15 { public static final int QKQKQKQK_MULTIPLIER = 2; public int ijijijijijijijijijAwfulName = 20; private int unununununununununCrummyName = 10; private void mememememememeUglyName(int i) { i = i++; tltltltltltltltltlBadName(); int j = i * QKQKQKQK_MULTIPLIER; } public void tltltltltltltltltlBadName() { int k = ijijijijijijijijijAwfulName; } public static void main(String[] args) { mememememememeUglyName(unununununununununCrummyName); } }

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

Answer

1. unununununununununCrummyName
2. mememememememeUglyName()

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.