SCOPE

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SCOPE I/O

• CSC 171 FALL 2004
• LECTURE 5

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CSC171

• Room Change
• Thursday, September 23.
• CSB 209
• THERE WILL BE A (group) QUIZ!
• - topic the CS department at UR

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Scope and Accessibility

• Scope is the lifetime of a variable
• when where does it live
• Scope rules govern where in the program a
  variable is accessible
• Most times java provides explicit modifiers to
  control scope
• In two cases, scope rules govern accessibility
• class scope
• block scope

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Class Scope

• Within a class definition reference variables of
  this classs type can be used to access all
  members regardless of their accessibility
  modifiers

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• Class Light
• private int noOfWatts
• private String location
• private boolean indicator // true or false
• public void switchOn() indicator true
• public void switchOff() indicator false
• public boolean isOn() return indicator
• public static Light duplicate (Light oldLight)

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• public static Light duplicateLight (Light
  oldLight)
• Light newLight new Light()
• newLight.noOfWatts oldLight.noOfWatts
• newLight.indicator oldLight.indicator
• newLight.location
• new String(oldLight.location)
• return newLight
• // end of class

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Class Scope

• Method duplicateLight has
• parameter oldLight
• local variable newLight
• Even though the instance variables of the class
  are private, they are accessible though the
  references

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Block Scope

• Declarations and statements can be grouped into a
  block using braces
• The body of a method is a block
• Blocks can be nested
• General rule
• a variable declared in a block is in scope
  inside the block in which it is declared but is
  not accessible outside of the block

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• public static void main(String args)
• //String args // cannot redeclare
• char digit
• int index 5
• int k 3
• int k 4 // ok
• // int index // not ok
• int index 6 // ok

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Member Accessibility Modifiers

• Accessibility modifiers help a class define its
  relationship to its client members
• Accessibility of members can be one of
• public
• protected
• default (a.k.a package)
• private

-
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public Members

• Least restrictive
• Accessible everywhere
• Both class (static) and instance members

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protected Members

• Accessible in the package containing this class
• Accessible by all subclases of the class in any
  package where this class is visible
• Non-subclasses in other packages cannot access
  protected members
• Less restrictive than default accessibility

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default Members

• When no access modifier is specified for a member
• It is only accessible by another class in the
  same package where its class is defined
• Even if a class is visible in another (possibly
  nested) package, the member is not accessible.

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private Members

• Most restrictive
• Private members are not accessible from any other
  class.
• This applies to subclasses
• Whether in the same package or not
• Not to be confused with inheritance

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Other Modifiers for members

• static
• final
• abstract
• synchonized
• native
• transient
• volitile

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static

• Members of the class in which they are declared
• Not part of any instance of the class
• The class need not be instantiated to reference
  static members
• Static members a.k.a class members

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final

• A final variable is a constant
• Its value cannot be changed once initialized
• Applies to instance, static, and local vars
• Final methods cannot be overridden (redefined in
  subclasses)

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abstract (incomplete)

• An abstract method has the following syntax
• abstract ltreturn typegt ltnamegt (ltparamsgt)
• An abstract method does not have an
  implementation (no method body)
• Only a prototype is provided
• No instantiation is allowed
• Instantiable subclasses are forced to implement
  the method body

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advanced modifiers

• synchronized used for multi-threaded code
• native used for foreign (C,C) methods
• transient if you store it, you dont save the
  values
• volatile some other thread could change its
  value

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IO in JAVA

• If there is System.out
• There must be System.in
• And other methods
• Like pop ups

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• import javax.swing.JOptionPane
• public class Popup
• public static void main(String args)
• String input1 JOptionPane.showInputDialog("Fi
  rst number")
• String input2 JOptionPane.showInputDialog("Se
  cond number")
• int i1 Integer.parseInt(input1)
• int i2 Integer.parseInt(input2)
• System.out.println(input1 " " input2 "
  "
• (input1 input2))
• JOptionPane.showMessageDialog(null,
• input1 " " input2 " " i1 i2)
• System.exit(0)

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• import java.io.BufferedReader
• import java.io.InputStreamReader
• import java.io.IOException
• public class Console
• public static void main(String args) throws
  IOException
• InputStreamReader isreader new
  InputStreamReader(System.in)
• BufferedReader console new BufferedReader(isre
  ader)
• String input1 console.readLine()
• System.out.println(input1)