[Chapter 13] 13.4 Modifiers

13.4 Modifiers

There are quite a few Java keywords that serve as modifiers for Java classes, interfaces, methods, and fields. They are described in Table 13.4.

Table 13.4: Java Modifiers
Modifier	Used On	Meaning
abstract	class	The class contains unimplemented methods and cannot be instantiated.
	interface	All interfaces are `abstract`. The modifier is optional in interface declarations.
	method	No body is provided for the method (it is provided by a subclass). The signature is followed by a semicolon. The enclosing class must also be `abstract`.
final	class	The class may not be subclassed.
	field	The field may not have its value changed (compiler may precompute expressions).
	method	The method may not be overridden (compiler may optimize).
	variable	Java 1.1 and later: the local variable or method or exception parameter may not have its value changed.
native	method	The method is implemented in C, or in some other platform-dependent way. No body is provided; the signature is followed by a semicolon.
none (package)	class	A non-`public` class is accessible only in its package
	interface	A non-`public` interface is accessible only in its package
	member	A member that is not `private`, `protected`, or `public` has package visiblity and is accessible only within its package.
private	member	The member is only accessible within the class that defines it.
protected	member	The member is only accessible within the package in which it is defined, and within subclasses.
public	class	The class is accessible anywhere its package is.
	interface	The interface is accessible anywhere its package is.
	member	The member is accessible anywhere its class is.
static	class	In Java 1.1, a class delared `static` is a toplevel class, not an inner class.
	field	A `static` field is a "class field." There is only one instance of the field, regardless of the number of class instances created. It may be accessed through the class name.
	initializer	The intializer is run when the class is loaded, rather than when an instance is created.
	method	A `static` method is a "class method." It is not passed as an implicit `this` object reference. It may be invoked through the class name.
synchronized	method	The method makes non-atomic modifications to the class or instance, and care must be taken to ensure that two threads cannot modify the class or instance at the same time. For a `static` method, a lock for the class is acquired before executing the method. For a non-`static` method, a lock for the specific object instance is acquired.
transient	field	The field is not part of the persistent state of the object, and should not be serialized with the object.
volatile	field	The field may be accessed by unsynchronized threads, so certain code optimizations must not be performed on it.

Table 13.5 summarizes the visibility modifiers; it shows the circumstances under which class members of the various visibility types are accessible.

Table 13.5: Class Member Accessibility
Accessible to:	Member Visibility
Accessible to:	public	protected	package	private
Same class	yes	yes	yes	yes
Class in same package	yes	yes	yes	no
Subclass in different package	yes	yes	no	no
Non-subclass, different package	yes	no	no	no