1. Week 5: Defining Classes and Methods

2. Review Loop statements while do-while for

3. Objective • Learn how to define classes • Learn how to define and use methods • Learn difference between class and object • Learn private variable – get, set method • Learn call by reference

4. Class (Design) amount of fuel __10__ speed __55__ license plate _135XJK_ amount of fuel __14__ speed __2__ license plate _Sues car_ Data: amount of fuel ______ speed ______ license plate _______ Functions: increaseSpeed: stop: amount of fuel __2__ speed __75__ license plate _351WLF_ • Not a real product • Just a design Class definition vs. Object Object (Product) • Real product based on a design (class) • Each product (object) can have own data

5. Define class & Create object OK! Here you are Class name Instance Variables Class Definition Memory Address public class Point { public double x; public double y; ….. public void increaseX(int increment ) { x = x + increment; } public void decreaseX(int decrement ) { x = x – decrement; } } ... public class Point { public double x; public double y; ….. public void increaseX(int increment ) { x = x + increment; } public void decreaseX(int decrement ) { x = x – decrement; } } p1 300 ... Give me an object p2 600 ... 300 300 x = 0.0 y = 0.0 public void increaseX(int increment) public void decreaseX(int decrement) x = 5.0 Define methods ... 600 Object create & use x = 0.0 y = 0.0 public void increaseX(int increment) public void decreaseX(int decrement) x = -10.0 public class TestPoint { public static void main( String[] args ) { Point p1 Point p2 = new Point(); p1.increaseX(5); p2.decreaseX(10); p1.decreaseX(3); } } Create Object new Point(); = ... Use methods

6. Define methods (1/3) Methods without return value public class Point { ….. public void printPoint( ) { System.out.print("The Point is :"); System.out.println("("+x+" , "+y+ ") "); } ..... } // There is no return value When the method has no return value, you should write ‘void’ <Memory> <Address> ... p3 700 ... public class TestPoint { public static void main( String[] args ) { ...... Point p3 = new Point(); p3.printPoint(); ...... } } 700 x = 0.0 y = 0.0 printPoint() { ... } ...

7. Define methods (2/3) Methods with a return value public class Point { ….. public doubleaddXAndY( ) { return x+y; } // There is a return value You should write the type of return value (int, double, …) here The type of return value is “double” <Memory> <Address> 3.0+2.0=5.0 5.0 ... p4 800 ... public class TestPoint { public static void main( String[] args ) { ...... Point p4 = new Point(); p4.x=3.0; p4.y=2.0; double z = p4.addXAndY(); ...... } } 800 x = 0.0 y = 0.0 addXAndY() { ... } x = 3.0 y = 2.0 ... z 5.0 ...

8. Define methods (3/3) Methods with parameters public int sum( int a, int b) { return a+b; } public void printSum() { int c = sum(3, 8); } parameter list 3+8 Type of parameters should be matched 11

9. p2 p1 … … … … … 1200 300 Console x y x y Memory Address … … … 1200 300 Practice– Class define & use • Look into “Point.java” in the “lab4.zip” and execute the “TestPoint.java”. • “Point.java” and “TestPoint.java” must be in the same directory. • public class Point { • public double x; • public double y; • public void printPoint() { • System.out.println ("("+x+" , "+y+")"); • } • public void increaseX(int increment) { • x = x+increment; • } • public void decreaseX(int decrement) { • x = x-decrement; • } • } • public class TestPoint { • public static void main(String[] args) { • Point p1 = new Point(); • Point p2 = new Point(); • p1.x = 0; • p1.y = 0; • p2.x = 1; • p2.y = 2; • p1.increaseX(5); • p2.decreaseX(10); • p1.decreaseX(3); • p1.printPoint(); • p2.printPoint(); • } • } : 5.0 : 0.0 : 2.0 : 1.0 : -9.0 (2.0 , 0.0) : 0.0 : 2.0 (-9.0 , 2.0)

10. public vs. private • private • Keyword to make instance variablewhich cannot be accessed from outside of the class • Need accessor method, mutator method to access private variable <Practice> Modify “Point.java” like the below, and run “TestPoint.java” public class Point { public double x; public double y; … } public class Point { private double x; private double y; … } • p1.x = 0; • p1.y = 0; • p2.x = 1; • p2.y = 2; <Error!> *Because variable x and y are private The field Point.x is not visible. // It is private variable The field Point.y is not visible. // It is private variable

11. p2 p1 … … … … … 500 100 x y x y Memory Address … … … 500 100 set, get method public void setX (double newX) { x = newX; } public void setY (double newY) { y = newY; } public void setXY (double newX, double newY) { x = newX; y = newY; } Method to change the Point’s private variable x and y into other value - newX and newY (mutator method) Method to access private variable of Point class – x, y (accessor method) public int getX() { return x; } public int getY() { return y; } x 10 <Example> Point p1= new Point(); Point p2= new Point(); p1.setX(10); p1.setY(5); P2.setXY(0, 0); int x = p1.getX(); : 10.0 : 0.0 : 5.0 : 0.0

12. The type of return value can be a class The type of parameter can be a class p 500 x y 500 Call by reference public Point findCenter(Point p1, Point p2) { Point p = new Point(); double x = (p1.getX() + p2.getX()) / 2; double y = (p1.getY() + p2.getY()) / 2; p.setX(x); p.setY(y); return p; } 30 301 (2.0+3.0)/2 (3.0+4.0)/2 500 p2 p3 p1 x y Point p1, p2, p3; p1 = new Point(); p2 = new Point(); p1.setXY(2,3); p2.setXY(3,4); p3 = findCenter(p1,p2) … 30 ? 2.5 … … 301 ? 500 ? 3.5 … … x y : 2.0 x y : 3.0 : 2.5 … … … : 3.0 : 4.0 : 3.5 30 301 Memory Address 500 30, 301 p3.getY(); 3.5

13. Practice (1/2) • Define SimpleElevatorclass

14. Practice (2/2) • SimpleElevator class • Instance variables • private int numOfFloor – from 1st floor to numOfFloorth floor • private int currentFloor – the elevator is currently at currentFloorth floor • private boolean doorOpened – whether the door of the elevator is opened • private int travelLength – total distance that the elevator has traveled. • public methods • SimpleElevator(int numFloor, int initCurrFloor) • Constructor. Use parameter to initialize field. Also set the doorOpened=false, travelLength =0 • void closeDoor() -Assign false to doorOpened • If the door is already closed, print “"** Error: The door is already closed." • void openDoor() - Assign true to doorOpened • If the door is already opened, print “"** Error: The door is already opened." • boolean isDoorOpened() – whether the door is opened. • int getCurrentFloor() - returns currentFloor • int getTravelLength() – returns travelLength • void resetTravelLength() – set travelLength to 0 • boolean goFloor(int destinFloor) • 1 <= destinFloor <= numOfFloor • If not, print “** Error: Invalid destination” and return false • doorOpened == false • if not, print “** Error: Please close the door first” and return false • else • increment travelLength by Math.abs(currentFloor – destinFloor) • set currentFloor to destinFloor, and return true • boolean isUsedMore(SimpleElevator e)- Compare travelLength

15. Run : TestSimpleElevator class public class TestSimpleElevator { public static void main(String[] args) { SimpleElevator elevator1 = new SimpleElevator(63, 1); SimpleElevator elevator2 = new SimpleElevator(15, 1); System.out.println("elevator1 is at " + elevator1.getCurrentFloor() + "th floor"); elevator1.goFloor(105); System.out.println("elevator1 is at " + elevator1.getCurrentFloor() + "th floor"); elevator1.openDoor(); elevator1.goFloor(63); System.out.println("elevator1 is at " + elevator1.getCurrentFloor() + "th floor"); elevator1.closeDoor(); elevator1.closeDoor(); elevator1.goFloor(63); System.out.println("elevator1 is at " + elevator1.getCurrentFloor() + "th floor"); System.out.println("elevator1's door is " + (elevator1.isDoorOpened() ? "opened" : "closed")); elevator1.goFloor(40); elevator1.goFloor(60); elevator1.goFloor(60); elevator1.goFloor(10); System.out.println("elevator1's travel length is " + elevator1.getTravelLength()); System.out.println("elevator1's is " + (elevator1.isUsedMore(elevator2) ? " " : " not ") + "used more"); elevator1.resetTravelLength(); System.out.println("After resetting, elevator1's travel length is " + elevator1.getTravelLength()); } <Execution Result> elevator1 is at 1th floor ** Error: Invalid destination elevator1 is at 1th floor ** Error: Please close the door first. elevator1 is at 1th floor ** Error: The door is already closed. elevator1 is at 63th floor elevator1's door is closed elevator1's travel length is 155 elevator1's is used more After resetting, elevator1's travel length is 0

16. Template: SimpleElevator class public class SimpleElevator { private int numOfFloor; private int currentFloor; boolean doorOpened; private int travelLength; public SimpleElevator(int numFloor, int initPosition) { numOfFloor = numFloor; if (initPosition > numFloor || initPosition < 1) currentFloor = 1; else currentFloor = initPosition; doorOpened = false; travelLength = 0; } public void openDoor() { } public void closeDoor() { } public boolean isDoorOpened() {} public int getCurrentFloor() { } public int getTravelLength() { } public void resetTravelLength() { } public boolean goFloor(int destinFloor) { } public boolean isUsedMore(SimpleElevator e) {} }