CSE115: Introduction to Computer Science I

1. CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall 645-4739 alphonce@buffalo.edu

2. Phones off Signs out

3. Agenda • non-void method • accessor method • method with parameters • mutator method • method calls in detail • association relationship

4. Accessor method • A method which returns information, the value of an object’s property. • These methods typically have names that start with “get”.

5. Accessor example • simple example of a method that returns a value public class EcoSystem { private example1.Terrarium _t; public EcoSystem() { _t = new example1.Terrarium(); } public example1.Terrarium getTerrarium() { return _t; } }

6. Mutator methods • A method which accepts information and sets the value of an object’s property. • So-called because method changes (mutates) the value of a property. • Mutator methods typically have names that start with “set”.

7. Mutator example public class EcoSystem { private example1.Terrarium _t; public EcoSystem() { _t = new example1.Terrarium(); } public void setTerrarium(example1.Terrarium t){ _t = t; } }

8. EcoSystemes = new EcoSystem(); Blackboard example EcoSystem es Terrarium _t

9. EcoSystemes = new EcoSystem(); example1.Terrarium terra = new example1.Terrarium(); Blackboard example EcoSystem es Terrarium _t Terrarium terra

10. EcoSystemes = new EcoSystem(); example1.Terrarium terra = new example1.Terrarium(); es.setTerrarium(terra); Blackboard example EcoSystem es Terrarium _t Terrarium terra

11. EcoSystemes = new EcoSystem(); es 1025 Blackboard example runtimestack invocation record 875 950 Terrarium heap 1025 950 _t EcoSystem EcoSystem

12. EcoSystemes = new EcoSystem(); example1.Terrarium terra = new example1.Terrarium(); es 1025 Blackboard example terra 875 runtimestack 875 Terrarium 950 Terrarium heap 1025 950 _t EcoSystem EcoSystem

13. EcoSystemes = new EcoSystem(); example1.Terrarium terra = new example1.Terrarium(); es.setTerrarium(terra); es 1025 Blackboard example terra 875 t 875 runtimestack invocation record 875 Terrarium 950 Terrarium heap 1025 950 _t EcoSystem EcoSystem

14. EcoSystemes = new EcoSystem(); example1.Terrarium terra = new example1.Terrarium(); es.setTerrarium(terra); es 1025 Blackboard example terra 875 t 875 runtimestack Control transfers to called method. Argument’s value is assigned to parameter. After method body is executed, control returns to statement after method call. _t = t; 875 Terrarium _t = t assignment copies contents of parameter to the instance variable 950 Terrarium heap 1025 875 _t EcoSystem EcoSystem

15. EcoSystemes = new EcoSystem(); example1.Terrarium terra = new example1.Terrarium(); es.setTerrarium(terra); es 1025 Blackboard example terra 875 runtimestack After method call, invocation record is removed from runtime stack. 875 Terrarium 950 Terrarium heap 1025 875 _t EcoSystem EcoSystem

16. java.awt.Color class • Instances of the java.awt.Color class represent colors. • A color is described by how much of each of the three colors red, green and blue are mixed together. • There are some predefined color objects in the java.awt.Color class, such as: java.awt.Color.RED java.awt.Color.BLUE

17. getColor • Both the Ant and the Caterpillar have getColor methods. • By calling their getColor methods we can find out the color of our Ant and Caterpillar objects. • The getColor method requires no argument in the method call.

18. Example Suppose that the variable a refers to an Ant. Then the following method call gets a reference to the color of the Ant object: a.getColor() We can assign that reference to a variable: java.awt.Colorcol = a.getColor();

19. setColor • Both the Ant and the Caterpillar have setColor methods. • By calling their setColor methods we can change the color of our Ant and Caterpillar objects. • The setColor method requires that a reference to a java.awt.Color object be provided as an argument in the method call.

20. Example Suppose again that the variable a refers to an Ant, and suppose further that thevariable c refers to a Caterpillar. Then the following method call sets the color of the Ant object to red: a.setColor(java.awt.Color.RED) Likewise, the following method call sets the color of the Caterpillar object to blue: c.setColor(java.awt.Color.BLUE)

21. Making c and a have the same color c.setColor(a.getColor()) • Swapping colors with two additional variables: java.awt.ColorcolorOne = a.getColor(); java.awt.ColorcolorTwo = c.getColor(); c.setColor(colorOne); a.setColor(colorTwo); • Exercise: can you swap the colors with just one additional variable.

22. Revisiting Clifford

23. Another relationship • One collar for its life…? • A dog has-a tail; the tail is a part of the dog. • We need something different to model a relationship like dog and collar.

24. Association • Also called “knows a”. • A relationship of knowing (e.g. Dog-Collar as opposed to Dog-Tail) – (back to Clifford!) • No necessary lifetime link • We’ll look at two different implementations of “knows a”: • The first we will see today, and is very similar to our implementation of “has a”. • The second, which we will see next time, is a bit more complex but is also more flexible.

25. First implementation In Java code, the first involves 3 changes to the “knowing” class: • Declaration of instance variable of the “known” class/type (because the “knowing” object will want to communicate with the “known” object). • Assignment of existing “known” instance to the instance variable (because the instance variable must refer to an object). • Parameter of “known” class in “knowing” class constructor (because the creator of an instance of the “knowing” class needs to supply an instance of the “known” class).

26. Dog – Collar example in Java public class Dog { private Collar _collar; public Dog(Collar collar) { _collar = collar; } }