1.
Modifiers are Java keywords that provide information to compiler
about the nature of the code, data and classes.
2.
Access modifiers - public, protected, private
·
Only applied to class level variables. Method variables are
visible only inside the method.
·
Can be applied to class itself (only to inner classes declared at class level, no such thing as protected or private top level
class)
·
Can be applied to methods and constructors.
·
If a class is accessible, it doesn't mean, the members are also accessible. Members' accessibility determines what is accessible
and what is not. But if the class is not accessible, the members are not accessible, even though they are declared public.
·
If no access modifier is specified, then the
accessibility is default package visibility. All classes in the same
package can access
the feature. It's called as friendly access. But friendly is not a
Java keyword. Same directory is same package in Java's consideration.
·
'private' means only the class can access it, not even sub-classes.
So, it'll cause access denial to a sub-class's own variable/method.
·
These modifiers dictate, which classes can access the features. An instance of a class can access the private
features of another instance of the same class.
·
'protected' means all classes in the same
package (like default) and sub-classes in any package can access the
features. But
a subclass in another package can access the protected members in
the super-class via only the references of subclass or its subclasses. A
subclass in the same package
doesn't have this restriction. This ensures that classes from
other packages are accessing only the members that are part of their
inheritance hierarchy.
·
Methods cannot be overridden to be more private. Only the direction shown in following figure is permitted
from parent classes to sub-classes.
private
Ã
friendly (default) Ã
protected Ã
public
Parent classes
Sub-classes
3.
final
·
final features cannot be
changed.
·
The final modifier applies to
classes, methods, and variables.
·
final classes cannot be
sub-classed.
·
You can declare a variable in
any scope to be final.
·
You may, if necessary, defer initialization of a final local variable. Simply declare the local variable and initialize it later (for
final instance variables. You must initialize them at the time of declaration or in constructor).
·
final variables cannot be
changed (result in a compile-time error if
you do so )
·
final methods cannot be
overridden.
·
Method arguments marked final
are read-only. Compiler error, if trying to assign values to final
arguments inside the method.
·
Member variables marked final are not initialized by
default. They have to be explicitly assigned a value at declaration or
in an initializer block.
Static finals must be assigned to a value in a static initializer
block, instance finals must be assigned a value in an instance
initializer or in every constructor.
Otherwise the compiler will complain.
·
A blank final is a final variable whose declaration lacks an initializer.
·
Final variables that are not assigned a value at the declaration and method arguments that are marked final are called blank
final variables. They can be assigned a value at most once.
·
Local variables can be declared final as well.
·
If a final
variable holds a reference to an object, then the state of the
object may be changed by operations on the object, but the variable will
always refer
to the same object.
·
This applies also to arrays,
because arrays are objects; if a final
variable holds a reference to an array, then the components of the
array may be changed by operations on the array, but the variable will
always
refer to the same array
·
A
blank final instance variable must be definitely assigned
at the end of every constructor of the class in which it is
declared; otherwise a compile-time error occurs.
·
A class can be declared final if its definition is complete
and no subclasses are desired or required.
·
A compile-time error occurs if
the name of a final class
appears in the extends
clause of another class
declaration; this implies that a final
class cannot have any subclasses.
·
A compile-time error occurs if
a class is declared both final
and abstract,
because the implementation of such a class could never be completed.
·
Because a final class never has any subclasses,
the methods of a final
class are never overridden
4.
abstract
·
Can be applied to classes and
methods.
·
For deferring implementation to
sub-classes.
·
Opposite of final, final
can't be sub-classed, abstract must be sub-classed.
·
A class should be declared
abstract,
1.
if it has any abstract methods.
2.
if it doesn't provide implementation to any of the abstract
methods it inherited
3.
if it doesn't provide implementation to any of the methods in an
interface that it says implementing.
·
Just terminate the abstract
method signature with a ';', curly braces will give a compiler error.
·
A class can be abstract even if
it doesn't have any abstract methods.
5.
static
·
Can be applied to nested
classes, methods, variables, free floating code-block (static initializer)
·
Static variables are
initialized at class load time. A class has only one copy of these
variables.
·
Static methods can access only
static variables. (They have no this)
·
Access by class name is a
recommended way to access static methods/variables.
·
Static initializer code is run
at class load time.
·
Static methods may not be overridden to be non-static.
·
Non-static methods may not be overridden to be static.
·
Abstract methods may not be static.
·
Local variables cannot be declared as static.
·
Actually, static methods are
not participating in the usual overriding mechanism of invoking the
methods based on the class of the object at runtime. Static method binding
is done at compile time, so the method to be invoked is determined by the
type of reference variable rather than the actual type of the object it
holds at runtime.
Let's
say a sub-class has a static method which 'overrides' a static method
in a parent class. If you
have a reference variable of parent class type and you assign a child
class object to that variable and invoke the static method, the method
invoked will be the parent class method, not the child class method.
The following code explains this.
public class
StaticOverridingTest {
public static void main(String s[]) {
Child c = new Child();
c.doStuff(); // This will invoke
Child.doStuff()
Parent p = new Parent();
p.doStuff(); // This will invoke
Parent.doStuff()
p = c;
p.doStuff(); // This will invoke
Parent.doStuff(), rather than Child.doStuff()
}
}
class Parent {
static int x = 100;
public static void doStuff() {
System.out.println("In
Parent..doStuff");
System.out.println(x);
}
}
class Child extends Parent {
static int x = 200;
public static void doStuff() {
System.out.println("In
Child..doStuff");
System.out.println(x);
}
}
6.
native
·
Can be applied to methods only.
(static methods also)
·
Written in a non-Java language, compiled for a single
machine target type.
·
Java classes use lot of native
methods for performance and for accessing hardware Java is not aware of.
·
Native method signature should
be terminated by a ';', curly braces will provide a compiler error.
·
native doesn't affect access
qualifiers. Native methods can be private.
·
Can pass/return Java objects from native methods.
·
System.loadLibrary is
used in static initializer code to load native libraries. If the library
is not loaded when the static method is called, an UnsatisfiedLinkError
is thrown.
7.
transient
·
Can be applied to class level
variables only.(Local variables cannot be declared transient)
·
Transient variables may not be
final or static.(But compiler allows the declaration, since it doesn't
do any harm. Variables marked transient are never serialized. Static
variables are not serialized anyway.)
·
Not stored as part of
object's persistent state, i.e. not written out during serialization.
·
Can be used for security.
8.
synchronized
·
Can be applied to methods or parts of methods only.
·
Used to control access to
critical code in multi-threaded programs.
9.
volatile
·
Can be applied to variables
only.
·
Can be applied to static variables.
·
Cannot be applied to final variables.
·
Declaring a variable volatile
indicates that it might be modified asynchronously, so that all threads
will get the correct value of the variable.
·
Used in multi-processor
environments.
Modifier
|
Class
|
Inner
classes (Except local and anonymous classes)
|
Variable
|
Method
|
Constructor
|
Free
floating Code block
|
public
|
Y
|
Y
|
Y
|
Y
|
Y
|
N
|
protected
|
N
|
Y
|
Y
|
Y
|
Y
|
N
|
(friendly)
No access modifier
|
Y
|
Y (OK for all)
|
Y
|
Y
|
Y
|
N
|
private
|
N
|
Y
|
Y
|
Y
|
Y
|
N
|
final
|
Y
|
Y (Except anonymous
classes)
|
Y
|
Y
|
N
|
N
|
abstract
|
Y
|
Y (Except anonymous
classes)
|
N
|
Y
|
N
|
N
|
static
|
N
|
Y
|
Y
|
Y
|
N
|
Y
(static initializer)
|
native
|
N
|
N
|
N
|
Y
|
N
|
N
|
transient
|
N
|
N
|
Y
|
N
|
N
|
N
|
synchronized
|
N
|
N
|
N
|
Y
|
N
|
Y
(part of method, also need to specify an object on which a lock
should be obtained)
|
volatile
|
N
|
N
|
Y
|
N
|
N
|
N
|
