1. Static Class Methods CSIS 3701: Advanced Object Oriented Programming

2. Need for Static Components Calling standard OOP method: objectname.methodname(parameters) Does not make sense for functions: –Example: sqrt function in Math class –Theoretically, must construct a “ Math ” object first! Math m = new Math(); y = m.sqrt(x); Assumes that object exists

3. Static Methods Static methods not associated with object Syntax: classname.methodname(params) Example: All methods in Math class: y = Math.sqrt(x);

4. Standard Output Printing to “standard output” –Status window in NetBeans –DOS screen when app run independently System.out.println(String); Class representing link to OS standard output object Often used for debugging –Displaying breakpoints, variable values –Not used in final applications –Standard input very rarely used

5. Parsing Parsing strings to numeric values getText() String parsing numeric value for math Key: Each simple type has corresponding class int  Integer double  Double boolean  Boolean char  Char Built-in tools to manipulate simple type values

6. Parsing Parsing syntax: type = Classname.parseType(String); Examples: int h = Integer.parseInt(hourField.getText()); double area = Double.parseDouble(areaString);

7. String Format Printing numbers with fixed number of digits String.format(“format", value); same % syntax as C Example: String radiusString = String.format(“%.3f", radius); 3 digits after decimal

8. Popups Static methods in JOptionPane class Simple message : JOptionPane.showMessageDialog( location, String); displayed Example: JOptionPane.showMessageDialog(null, "The radius is " + radiusString); Usual values: null  center of screen this  same location as app

9. Popups Prompting for values: String reply = JOptionPane. showInputDialog(location,String); Returns string entered by user Example: String areaString = JOptionPane.showInputDialog(null, "Enter the area of a circle"); Note: returns null if Cancel button pressed instead

10. Creating Static Methods Syntax: public static returntype methodname (params) {…} Are there useful functions that do not require an object? –Example: translating hour, minute to am/pm form in Clock class public static String ampm(int h, int m) { String post = "am"; if (h > 12) { h -= 12; post = "pm"; } return ""+h+":"+m+post; } Call as Clock.ampm(12, 34);

11. Static Member Variables Can declare member variables static: private static type variablename; One variable “shared” by all instances of class public class Clock { private int hour; private static int MAXHOUR; Clock object hour: 14 Clock object hour: 2 Clock object hour: 12 23 MAXHOUR

12. Static Constants Generally used to implement class constants –If value never changes, more efficient to keep single copy instead of one for each object Example: maximum values for hour/minute –Easier to change in future public static final int MAXHOUR = 23; public static final int MAXMINUTE = 59; … public boolean isHour(int h) { return 0 <= h && h <= MAXHOUR; } Java syntax for “constant”

13. Static Constants Often make public to help user –Safe since user cannot change value public static final int MAXHOUR = 23; public static final int MAXMINUTE = 59; –Access using syntax classaname.constantname int h = Integer.parseInt(hourField.getText()); int m = Integer.parseInt(minuteField.getText()); if (h <= Clock.MAXHOUR && m <= Clock.MAXMINUTE) { clock.setHour(h); clock.setMinute(m); }

14. Static Constants Built into many classes –Example: Math.PI Often used as flags in place of integers –Example: Alignment of labels JLabel l = new JLabel(label, alignment); 1  left align 2  center align 3  right align Don’t want to have to memorize this!

15. Static Constants Internally (sort of): public class JLabel …{ public static final LEFT = 1; public static final CENTER = 2; public static final RIGHT = 3; Example of use: JLabel timeLabel = new JLabel(“Time: ”, JLabel.RIGHT); Just have to remember constant names

16. Static and main public static void main(String[] args) {...} When application started: – main method called – main method calls constructor to create app No object exists when main called! – main method must be static so can be called before any object exists yet