1. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 1 CS100A, Lecture 7, 22 Sept. 1998 Static fields and methods. Suppose that we would like to main- tain, somehow, the number of instances of Employee that were ever created. The following does not work! public class Employee { public int noEmps= 0; //No. of Emps //ever created public String name; // Constructor -- An Employee named n public Employee(String n) { noEmps= noEmps+1; name= n; }

2. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 2 Using the prefix qualifier static means that there is only ONE field noEmps for the whole class, not one for each instance of the class: public class Employee { static public int noEmps= 0; //No. Empls //ever created public String name; //Employee’s name // Constructor -- An Employee named n public Employee(String n) { noEmps= noEmps+1; name= n; } // Return no. of Employees ever created public int getNoEmps() {return noEmps;} }

3. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 3 static public void main (String args[ ]) { Employee v1= new Employee(); System.out.println("No. of emps" + v1.noEmps); Employee v2= new Employee(); System.out.println("No. of emps" + v2.noEmps); } Class Employee ----------------------------------------------------- noEmps _______ Name GriesName Cardie Frame for main v1 ____ v2 ____

4. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 4 Model of Execution A static field (or method) is called a class variable (or method). At runtime, a box for a class named C con- tains all the class variables and methods. When- ever an instance of class C is created, it is placed in this box. A parameter or local variable declared as C v; is placed in the frame for the method in which it appears. This model assures that the rule we gave for finding a variable, when it is referenced, works: look first in the frame for method being execut- ed, then in the surrounding box, etc.

5. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 5 You can (should) use the name of the class, instead of a class variable, to refer- ence a static field or method Employee v1= new Employee(“Gries”); Employe v2= new Employee(“Cardie”); Instead of writing System.out.println(v1.noEmps); write System.out.println (Employee.noEmps);

6. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 6 Make the static field private, so it can’t be changed from outside the class. Have a “get” method to read its value. public class Employee { static private int noEmps= 0; //No. Empls //ever created public String name; //Employee’s name // Constructor -- An Employee named n public Employee(String n) { noEmps= noEmps+1; name= n; } // Return no. of Employees ever created public int getNoEmps() {return noEmps;} }

7. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 7 Method Color contains static fields (and methods) public class Color implements java.io.Serializable { public final static Color red = new Color(255, 0, 0); public final static Color white = new Color(255, 255, 255); public final static Color yellow = new Color(255, 255, 0); public final static Color pink = new Color(255, 175, 175); (The three arguments are RGB (red-green-blue) values) public String toString() { return getClass().getName() + "[r=" + getRed() + ",g=" + getGreen() + ",b=" + getBlue() + "]"; }

8. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 8 A variable (or field) declared with prefix qualifier final may be assigned only once. Do it in the declaration. OKAY: final int x; x= 5; BETTER: final int x= 5;

9. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 9 Methods, as well as fields, may be static A static method may refer only to parameters, local variables, and static fields. It may not refer to non- static fields of the class. In which the method appears. // Return the number of Employees ever created static public int getNoEmps() {return noEmps;} A static method may be called only using the class name. It may not be called using a variable name. Employee v1= new Employee(“Gries”); WRONG: System.out.println(v1.getNoEmps()); RIGHT: System.out.println(Employee.getNoEmps());

10. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 10 Class Math contains ONLY static fields and methods public final class Math { public static final double E = 2.7182818284590452354; public static final double PI = 3.14159265358979323846; public static native double sin(double a); public static native double cos(double a); public static int round(float a) { return (int)floor(a + 0.5f); } public static int max(int a, int b) { return (a >= b) ? a : b; }

11. CS100A, Lect. 7, 22 Sept. 98. Static Fields and Methods 11 On the PC, class Color (in file Color.java), and lots of other methods of the “abstract window toolkit”, should be in directory Program Files / Metrowerks / CodeWarrior / Java Support / Java Source / java / awt Class Math is in file Math.java in directory Program Files / Metrowerks / CodeWarrior / Java Support / Java Source / java / lang On the Mac, it will be in a similar place. DON’T CHANGE THESE FILES. But look at them. In Visual J++, highlight a class name and push button F1 to obtain a specification of the class.