Using Statics

1. Using Statics

2. Review: class and instance variables • int is a data type;3 is a value (or instance) of that type • A class is a data type;an 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 can’t refer to an instance variable if you don’t have an instance • You “always” have the class • The keyword static marks a variable as a class variable

3. Example of a static variable class Person { String name; // Instance variable boolean alive; // Instance variable static int population; // Class variable Person(String name) { // Constructor this.name = name; alive = true; population++; } public void die() // Method alive = false; population--; }

4. Review: class and instance methods • An instance method “belongs to” an individual object—you can use it only by “sending a message” to that object • Example: String s = myTextField.getText(); • Example: saddamHussein.die(); • A class (static) method belongs to a class • Examples: • y = Math.abs(x); • if (Character.isLetter(ch)) { ... } • population = (int)(0.9 * population); // war

5. Static context • An application requires a public static void main(String args[]) method • It must be static because, before your program starts, there aren’t 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 variables—this is a static context, not an object • non-static variable xxx cannot be referenced from a static context

6. About the term • Static poisoning refers the fact that, in an application, you can’t 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 use—I made it up • There is a simple solution to this problem

7. error error error error 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

8. all are wrong Another example public class JustAdd { int x; int y; int z; public static void main(String args[]) { x = 5; y = 10; z = x + y; } }

9. A solution • public class JustAdd { int x; int y; int z; • public static void main(String args[]) {JustAdd myself = new JustAdd(); • myself.x = 5; • myself.y = 10; • myself.z = myself.x + myself.y; • }}

10. A better solution • public class JustAdd { int x; int y; int z; • public static void main(String args[]) {new JustAdd().doItAll(); } • void doItAll() { • x = 5; • y = 10; • z = x + y; • }}

11. The best solution • Know when a variable or method should be static! • A variable should be static if: • It logically describes the class as a whole • There should be only one copy of it • A method should be static if: • It does not use or affect the object that receives the message (it uses only its parameters) • When you are writing a “main” class with a main method: • Would it possibly make sense to have more than one of this “main” object? • If so, create one in your main method and use it

12. 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 } }

13. The End