scope : Java Glossary

2007-11-07 by Roedy Green ©1996-2008 Canadian Mind Products
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scope

Visibility	Class scope
Gotchas	Links
Limiting Scope

Visibility

Java class variables have six possible levels of visibility to other classes:

Java Scope
Scope	Visibility	Purpose
published	visible to external users of a Javabean	Javabean properties and methods. There is no way to declare a method published in the Java source. You use a special getXXX/setXXX naming convention and write a separate declaration file.
public	visible to all classes	methods and variables of interest to users of the class.
protected	visible to classes outside the package that inherit the class, also to all classes in the package.	methods and variables of interests to third parties who may extend your class.
default aka package aka friendly	visible to all classes of the package.	methods and variables involved in cross-class communication within the package.
private	visible only within the class, not by inheritors, not by other classes in the package.	Variables and methods that should not or would not be changed by someone extending the class. Proper functioning of the class depends on them working precisely as written.
local	visible only to the block of the method in which the variables were declared.	Temporary working variables.

Gotchas

Though there is a default keyword in Java, you may not use it to explicitly mark a method or class as having default scope. Further there is no other keyword you can use for that purpose. There is not even an official descriptive name for the default scope, though programmers often informally call it “package” or “package private” scope. It is a bit like ordering a hybrid car from a Japanese company who insists on calling the standard colour “default”, and nowhere in the literature telling what shade it actually is.
There are no global variables. In order of increasing visibility are: local, private, package, protected, public, published.
There is no scope that will allow derived classes to see a method or variable, but hide it from the rest of the package. There is no scope that will allow derived classes within a package to see a method or variable, but hide it from the rest of the package.
You may not redefine a local variable inside a block nested within its scope. Local variables may hide instance variables however. Any you may reuse local variables inside blocks, so long as they don’t nest.

Limiting Scope

There are five benefits to limiting the scope of indexing variables to the loop block:

The optimiser does not need to worry about preserving loop index values in case you jump out of the loop. This means faster code.
It may reduce the size of the stack frame. Local variables in disjoint scopes within a method can share the same slots in the stack frame.
You can reuse indexing variable names safely.
You are less likely to create bugs where you inadvertently reuse a variable for two different purposes.
It is less confusing for programmers. They need not check for uses of the variable outside the loop.

I use the term "indexing variable" in the most general sense. It may not necessarily appear formally in a for or while loop.

Class scope

Permitted class scopes
Type	public	protected	default	private
standalone class
top level class
inner class
static inner class
anonymous class
local class

A top-level class is usually a default scope outer class sharing a *.java file with a public class. The term also can be used for public classes. You can have at most one public class per *.java file. A *.java file might contain only default scope classes.

anonymous class
class
inner classes
instance
local scope
nested class
package scope
private scope
protected scope
public scope
static

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