The Line Class

1. The Line Class Suppose you are involved in the development of a large mathematical application, and this application needs an object to represent a Line. You might proceed in this manner: Attributes of a Line: x coordinate of endpoint 1 y coordinate of endpoint 1 x coordinate of endpoint 2 y coordinate of endpoint 2 the length of the line

2. Behaviors (actions) of a Line: change point 1 change point 2 report length report equation **there are many more behaviors that a Line could have .. We’ll keep it simple We can describe the class that will be need for this class in a UML class diagram.

3. UML (unified modeling language) is a language of diagrams, which can be used to describe a design. A UML class diagram has 3 parts:

5. Putting together the UML diagram involved determining which data types will be used to for storing each of the attributes. Additionally, it was necessary to think of what information needed to be provided for each behavior to take place (parameters). Once this design activity has been completed, it is relatively easy to code the class skeleton….

6. public class Line{ private int xcoord1 = 0; //pt 1 default (0,0) private int ycoord1 = 0; private int xcoord2 = 1; // pt 2 default (1,0) private int ycoord2 = 0; private double len = 1.0; // initial/default values have been given to each attribute public Line() { // method implementation } public Line (int x1,int y1, int x2, int y2){ // method implementation } public void setPt1(int xpos, int ypos){ // method implementation } public void setPt2(int xpos, int ypos){ // method implementation } public double length () { // method implementation } public String equation () { // method implementation } }

7. Instance Fields (Variables) • access specifier (such as private) • type of variable (such as int) • name of variable (such as xcoord1) eg: private int xcoord1; Each object created from this class (each instance) gets its own copy of this variable. Private indicates that this class member is not accessible from other classes Initial values may be provided for instance variables, which are stored prior to the execution of constructor statements.

8. Accessing Instance Fields The length method of the Line class can access the private instance field len: public double length(){ return len; } But methods outside the Line class cannot access the variable len!!

9. Coding the Constructors • A constructor initializes the instance variables • Constructor name = class name • Constructor has NO return type • Constructor is only method called with ‘new’ public Line ( ){ } //this constructor makes no changes to // default values assigned in declarations public Line (int x1,int y1, int x2, int y2){ xcoord1 = x1; ycoord1 = y1; xcoord2 = x2; ycoord2 = y2; len = Math.sqrt((x2 – x1) *( x2 – x1) + (y2 – y1) * (y2 – y1) ); }

10. Coding the Mutators * mutators are methods used from outside the class to change the value of one of more instance variables (they ‘mutate’ the object, or change its state) public void setPt1(int xpos, int ypos){ xcoord1 = xpos; ycoord1 = ypos; len = Math.sqrt((xcoord2 – xcoord1) *( xcoord2 – xcoord1) + (ycoord2 – ycoord1) * (ycoord2 – ycoord1) ); } public void setPt2(int xpos, int ypos){ xcoord2 = xpos; ycoord2 = ypos; len = Math.sqrt((xcoord2 – xcoord1) *( xcoord2 – xcoord1) + Math.pow (ycoord2 – ycoord1, 2) ); }

11. Explicit and Implicit Parameters public void setPt1(int xpos, int ypos){ xcoord1 = xpos; ycoord1 = ypos; …… etc …. } *xpos and ypos are clearly parameters to this method, however there is a ‘hidden’ parameter … the object which was used to invoke the method Assume myline is a Line object, and the call: myline.setPt1(6,7); Within the setPt1 code: xcoord1 refers to xcoord1 of the object to the left of the dot ycoord1 refers to ycoord1 of the object to the left of the dot: Means: myline.xcoord1 = xpos; //however, this syntax would not myline.ycoord1 = xpos; //be permitted

12. Coding an Access Method * an access method is a method which returns the value of instance data, which cannot be directly accessed outside the class public double length () { return len; }

13. One Additional Method * This method uses the instance data to return a printable representation of the object … in this case, the line’s equation in y=mx + b form public String equation () { double m = slope(); double b; if (xcoord1 == xcoord2) //vertical line return “x = “ + xcoord1; else if (ycoord1 == ycoord2) // horizontal return “y = “ + ycoord1; else{ b = ycoord1 – ( m * xcoord1); return “y = “ + m + “* x + “ + b; } A slope method must be added to this class for this method to function properly.

14. Building a Test Program 1. Make a new subfolder for your program. 2. Make one file for each class. 3. Compile all files. 4. Run the test program.

15. public class LineApp{ //create a Line, change its position, report it’s length and output its position public static void main (String [] args){ Line myline = new Line( 2,2,6,7); Line anotherline = new Line(); myline.setPt1(1,2); System.out.println(“The length of my line is” + myline.length() ); System.out.println(“The line is: “ + myline.equation() ); System.out.println(“The length of my line is” +anotherline.length() ); System.out.println(“The line is: “ + anotherline.equation() ); } }

16. Access public class LineApp{ //create a Line, change its position, report it’s length and output its position public static void main (String [] args){ Line myline = new Line( 2,2,6,7); NOTE: it would be illegal to try to access instance variables from this class !! myline.xcoord1 = 1; //error myline.ycoord1 = 2; //error System.out.println(“The length of my line is” + myline.length(); System.out.println(“The line is: “ + myline.equation(); } } The instance data is ENCAPSULATED by the object!!!! This preserves the integrity of the data!!

17. Variable Types • Instance fields (xcoord1 in Line) • Local variables (myline in LineApp, ) • Parameter variables (xpos in setPt1)