1. Object interaction Creating cooperating objects 3.0

2. 2 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling A digital clock

3. 3 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Abstraction and modularization Abstraction is the ability to ignore details of parts to focus attention on a higher level of a problem. Modularization is the process of dividing a whole into well-defined parts, which can be built and examined separately, and which interact in well-defined ways.

4. 4 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Modularizing the clock display One four-digit display? Or two two-digit displays?

5. 5 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Implementation - NumberDisplay public class NumberDisplay { private int limit; private int value; Constructor and methods omitted. }

6. 6 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Implementation - ClockDisplay public class ClockDisplay { private NumberDisplay hours; private NumberDisplay minutes; Constructor and methods omitted. }

7. 7 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Object diagram

8. 8 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Class diagram

9. 9 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Primitive types vs. object types 32 object type primitive type SomeObject obj; int i;

10. 10 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Primitive types vs. object types 32 ObjectType a; int a; ObjectType b; 32 int b; b = a;

11. 11 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Source code: NumberDisplay public NumberDisplay(int rollOverLimit) { limit = rollOverLimit; value = 0; } public void increment() { value = (value + 1) % limit; }

12. 12 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Source code: NumberDisplay public String getDisplayValue() { if(value < 10) { return "0" + value; } else { return "" + value; }

13. 13 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Objects creating objects public class ClockDisplay { private NumberDisplay hours; private NumberDisplay minutes; private String displayString; public ClockDisplay() { hours = new NumberDisplay(24); minutes = new NumberDisplay(60); updateDisplay(); }

14. 14 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Method calling public void timeTick() { minutes.increment(); if(minutes.getValue() == 0) { // it just rolled over! hours.increment(); } updateDisplay(); }

15. 15 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Internal method /** * Update the internal string that * represents the display. */ private void updateDisplay() { displayString = hours.getDisplayValue() + ":" + minutes.getDisplayValue(); }

16. 16 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling ClockDisplay object diagram

17. 17 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Objects creating objects hours = new NumberDisplay(24); public NumberDisplay(int rollOverLimit); in class ClockDisplay: in class NumberDisplay: formal parameter actual parameter

18. 18 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Method calls internal method calls updateDisplay();... private void updateDisplay() external method calls minutes.increment();

19. 19 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Method calls (2) object. methodName ( parameter-list )

20. 20 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Concepts abstraction modularization classes define types class diagram object diagram object references primitive types object types object creation overloading internal/external method call debugger