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Access Modifiers. Control which classes use a feature Only class-level variables may be controlled by access modifiers Modifiers 1. public 2. protected 3. private Non-inner classes can only be public. Friendly. Features with no access modifier default to friendly

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PowerPoint Slideshow about 'Modifiers' - richard_edik


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access modifiers
Access Modifiers
  • Control which classes use a feature
  • Only class-level variables may be controlled by access modifiers
  • Modifiers

1. public

2. protected

3. private

  • Non-inner classes can only be public
friendly
Friendly
  • Features with no access modifier default to friendly
  • Friendly features are accessible to any class in the same package
  • Classes outside the package may not access these features
  • Friendly classes may be subclassed but their variables/methods are not accessible by the subclass
modifiers
Modifiers
  • public
    • Can be used in any Java program without restriction
  • private
    • may only be used by the instance of the class that declares the variable or method
  • protected
    • only variables, methods, and inner classes can be declared protected
    • available to all classes in the same package
    • available to all subclasses( even those in different packages )
overriding methods
Overriding Methods
  • Methods may not be overwritten to be more private

Private

Friendly

Protected

Public

final modifier
Final Modifier
  • Final features may not be overwritten
  • A final class may not be subclassed
  • A final variable cannot be changed once it has been assigned a value
final modifier example
Final Modifier Example

class Walrus {

int weight;

Walrus( int w ) { weight = w };

}

class Tester {

final Walrus w1 = new Walrus(1500);

void test() {

w1 = new Walrus(1400); // Illegal

w1.weight = 1800; // Legal

}

}

abstract modifier
Abstract Modifier
  • An abstract class cannot be instantiated
  • This is a way to defer implementation to subclasses
  • An class with one more methods declared abstract cannot be instantiated
  • A class that is declared to implement an interface but does not implement all the methods of that interface must be abstract
  • Similar to virtual in C++
abstract example
Abstract Example

abstract class Stack {

protected int count = 0;

public abstract void push( Object o );

public abstract void pop();

public abstract Object top();

public abstract boolean isFull();

public boolean isEmpty() {

return count==0;

}

}

static modifier
Static Modifier
  • Associated with a class, not a particular instance of a class

public class StaticTest {

static int x = 0;

StaticTest() {

x++;

}

}

No matter how many instances of StaticTest we have the ‘x’ variable is the same for all

accessing static variables
Accessing Static Variables

StaticTest st = new StaticTest();

st.x = 69;

OR

StaticTest.x = 69

more about static
More about Static
  • Static methods cannot use non-static features of their class
  • They can access the class’s static data
  • Since static methods are not associated with an instance of a class there is no this variable
static initializers
Static Initializers

public class Demo {

int x = 5;

static {

x = 69;

}

public static void main( String[] ) {

System.out.println( ‘X = ‘ + x );

}

}

What is the value printed?

native modifier
Native modifier
  • Can only refer to methods
  • Indicates that the body of the method is to be found elsewhere
  • Namely in a file in another language
  • Call to native method is the same as if it was implemented in Java
transient modifier
Transient modifier
  • Only applies to variables
  • Transient variables will not be serialized
  • Transient variables cannot be final or static
synchronized modifier
Synchronized modifier
  • Used to control critical code in multi-threaded programs
  • Covered in chapter 7
volatile modifier
Volatile modifier
  • Not on exam
  • Used in multiprocessor environments
  • Applies only to variables
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