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 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 error

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

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