Static Poisoning

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Static Poisoning
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Review class and instance variables

• int is a data type3 is a value (or instance) of
  that type
• A class is a data typean object is a value
  (instance) of that type
• A class variable belongs to the class as a whole
  there is only one of it
• An instance variable belongs to individual
  objects there is one of it for each object, but
  none for the class as a whole
• You cant refer to an instance variable if you
  dont have an instance
• You always have the class
• The keyword static marks a variable as a class
  variable

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Review class and instance methods

• An instance method belongs to an individual
  objectyou can use it only by sending a message
  to that object
• Example String s myTextField.getText()
• A class (static) method belongs to a class
• Examples
• y Math.abs(x)
• if (Character.isLetter(ch)) ...

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Static context

• An application requires a public static void
  main(String args) method
• It must be static because, before your program
  starts, there arent any objects to send messages
  to
• This is a static context (a class method)
• You can send messages to objects, if you have
  some objects myTextField.setText("Hello")
• You cannot send a message to yourself, or use any
  instance variablesthis is a static context, not
  an object
• non-static variable xxx cannot be referenced from
  a static context

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About the term

• Static poisoning refers the fact that, in an
  application, you cant access non-static
  variables or methods from a static context, so
  you end up making more and more things static
• Static poisoning is not a term that is in
  widespread useI made it up
• There is a simple solution to this problem

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An example of static poisoning
public class StaticPoison int x
int y public static void main(String
args) doOneThing()
void doOneThing() x 5
doAnotherThing() void
doAnotherThing() y 10
static
static
static
static
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Another example
public class Narcissus int x int y
int z public static void main(String
args) x 5 y 10
z x y
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An attempted solution

• public class Narcissus int x int y
  int z
• public static void main(String args)
  Narcissus myself new Narcissus()
• myself.x 5
• myself.y 10
• myself.z myself.x myself.y

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The best solution

• public class Narcissus int x int y
  int z
• public static void main(String args)
  new Narcissus().doItAll()
• void doItAll()
• x 5
• y 10
• z x y

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Summary

• In an application, frequently the best way to
  write the main method is as follows
• class MyClass public static void
  main(String args) MyClass myself
  new MyClass() myself.doAllTheWork()
  
• void doAllTheWork() // note not static
  

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The End