TCU CoSc 10403 Introduction to Programming (with Java)

1. TCU CoSc 10403 Introduction to Programming (with Java) Variables and Methods

2. Java Identifiers • Identifiers are the words a programmer uses to identify particular items in a program. • They may be used to name a variable (e.g. studentName), a class (e.g., HelloWorld or Lab2), or a method (e.g., paint or init). • Most identifiers have no predefined meaning except as specified by the programmer. • Rules & Naming Conventions • There are rules and naming conventions that are discussed in great detail in Chapter 6 • Suffice it to say that when you create (instantiate) an object or create a variable to do arithmetic with – it MUST be declared. • Java allows the creation of variables to hold integer values, real values, boolean values, and characters (these are called primitive data types). ***Note: Java is case sensitive. The identifier xyz is not the same as XYZ.

3. Declaration includes the data type and a variable name. Variables • DeclarationGraphics g; int numCourses; // integer values double salesTax; // floating pt number Color myColor = new Color(50,100,150); • Initialization – assignment for the first time • Assignment name = “Cookie Monster”; numCourses = 4; salesTax = 4.75; • Variable Reference String myName; myName = “Java Guru”; g.drawString( myName, 0, 12 ); • Objects Student tom = new Student(“Tom Jones”); or Student tom; tom = new Student(“Tom Jones”); Assignment changes the value of a variable.. Slide created by: Professor Elizabeth Boese, “An Introduction to Programming with Java Applets”, Jones and Bartlett Publishers.

4. Scope: Instance Variable vs. Local Variable import javax.swing.*; import java.awt.*; public class ColorEx extends JApplet { String favColor; String ski; public void paint( Graphics g ) { favColor = “Favorite color”; ski = “Love 2 ski”; g.setColor( Color.RED ); g.drawString( favColor, 30, 45 ); g.setColor( new Color( 12,34,52) ); g.drawString( ski, 30, 53 ); } } import javax.swing.*; import java.awt.*; public class ColorEx extends JApplet { public void paint( Graphics g ) { String favColor = “Favorite color”; String ski = “Love 2 ski”; g.setColor( Color.RED ); g.drawString( favColor, 30, 45 ); g.setColor(new Color( 12,34,52) ); g.drawString( ski, 30, 53 ); } } Instance Variable vs. Local Variable Instance variable: declared inside the class Local variable : declared inside a method Slide created by: Professor Elizabeth Boese, “An Introduction to Programming with Java Applets”, Jones and Bartlett Publishers.

5. Real World Example of a Method • If you have ever used a recipe to prepare a meal – you may have advanced to a step in the recipe only to find that you need to refer to another recipe for something that you need for the meal (say a sauce that is explained on a different page). • This way of writing has an import analogue in programming – called methods in Java. • The nice thing about this is that important details about the sauce (i.e., ingredients, preparation time, etc.) can be hidden from the current recipe. • The same is true for programming – regardless of the language being used.

6. Methods • In the context of programming - a method is a list (sequence) of statements to be performed. • Methods are used in all programming languages. They are called different names (e.g., procedures, subroutines, functions, subprograms, etc.). In Java – they are methods. • But they all do the same thing – namely, when they are called (invoked) they run and execute the statements that are enclosed within them. (e.g., the paint() method). • They are provided in programming languages as a means of decomposing large programs of statements into smaller ones – ones that are more easy to manage and to develop.

7. return type of void designates the method doesn’t return anything Methods paint is the name of the method • Example public void paint( Graphics g ) { . . . //executable statements } • Has it’s own scope braces { } • Statements in method only run when the method is called/invoked • Parameters (none or multiple) listed in parenthesis • Each must specify the data type and a variable name • Separate multiple parameters with a comma paint ( Graphics g ) Student (“John Doe”, 20, “Freshman”); Color (50, 100, 150); g is called a “parameter”. Its type is a Graphicsobject, which has methods such as drawString that we can call

8. Method Types Method Types: • There are two slightly different forms of methods in Java. • The first receives whatever arguments it is passed, executes the statements in its body, and returns control of the program where it was first invoked – nothing is passed back! These take the form: voidmethodName(list of arguments) { list of declarations, statements, and inner classes } (2) The second receives whatever arguments it is passed, executes the statements in its body, and returns control of the program where it was first invoked – but passes back some information. These take the form: returnTypeName methodName(list of arguments) { list of declarations, statements, and inner classes return expression //must be same type as returnTypeName }

9. Method – Flow of Control • You will recall that the init()method is invoked by the system when you submit the bytecode to the interpreter • A call to a method places the “called” method into execution. • When the “called” method reaches the end (or a return statement) – control is passed by to the “calling” method (just as though there was no interruption in the execution flow of the invoking method).

10. Method call that returns a value The # 2 gets copied into variable num1, the # 7 gets copied into the variable num2 The value9gets returned to where the program called this method import java.awt.*; import javax.swing.*; public class Calculate extends JApplet { public void paint (Graphics g ) { int addition = getAdd( 2, 7 ); String added = "2 + 7 = " + addition; g.drawString ( added, 0, 12 ); String subtracted = "2 - 7 = " + getSubtract( 2, 7 ); g.drawString ( subtracted, 0, 24 ); } public int getAdd( int num1, int num2 ) { return num1 + num2; } public int getSubtract( int n1, int n2 ) { int diff; diff = n1 - n2; return diff; } } The # 2 gets copied into variable n1, the # 7 gets copied into the variable n2 The value -5gets returned to where the program called this method Slide created by: Professor Elizabeth Boese, “An Introduction to Programming with Java Applets”, Jones and Bartlett Publishers.

11. Why Use Methods? • Code is easier to read: • Program decomposition – • step-wise-refinement • top-down design • Repeated code: • For access by other objects: • Events: (we’ll discuss later)

12. // Code Easier to read: with methods import java.awt.*; import javax.swing.*; public class HouseMethods extends JApplet { int WINDOW_WIDTH = 20; int WINDOW_HEIGHT = 30; public void paint (Graphics g ) { paintHouse( g ); paintLandscape( g ); } public void paintHouse( Graphics grph ) { grph.setColor( Color.pink ); grph.fillRect ( 100,100,200,200 ); grph.setColor( Color.black ); Polygon poly = new Polygon(); poly.addPoint(100,100); poly.addPoint(200,50); poly.addPoint(300,100); grph.fillPolygon(poly); grph.setColor( Color.blue ); grph.fillRect ( 200,230,40,70); paintWindow( grph, 120, 150 ); paintWindow( grph, 150, 150 ); paintWindow( grph, 200, 150 ); paintWindow( grph, 230, 150 ); } public void paintWindow( Graphics gp, int x, int y ) { gp.setColor( Color.blue ); gp.fillRect ( x, y, WINDOW_WIDTH, WINDOW_HEIGHT ); } public void paintLandscape( Graphics g ) { g.setColor( Color.black ); // tree g.fillRect ( 400,130,30,170 ); g.setColor( Color.green ); g.fillOval( 370,80,100,100 ); g.fillRect ( 0,295,500,5 ); // grass } } Why Have Methods? Which code is easier to figure out how to add a new window? import java.awt.*; import javax.swing.*; public class House extends JApplet { public void paint (Graphics g ) { g.setColor( Color.pink ); g.fillRect ( 100,100,200,200 ); g.setColor( Color.black ); Polygon poly = new Polygon( ); poly.addPoint(100,100); poly.addPoint(200,50); poly.addPoint(300,100); g.fillPolygon(poly); g.setColor( Color.blue ); g.fillRect ( 200,230,40,70); g.fillRect ( 120,150,20,30); g.fillRect ( 150,150,20,30); g.fillRect ( 200,150,20,30); g.fillRect ( 230,150,20,30); g.setColor( Color.black ); g.fillRect ( 400,130,30,170 ); g.setColor( Color.green ); g.fillOval( 370,80,100,100 ); g.fillRect ( 0,295,500,5 ); } }

13. Why Have Methods? // Repeated code using a method: import javax.swing.*; import java.awt.*; public class NeedMethods2 extends JApplet { public void paint( Graphics g ) { drawRows( g, 20, 20 ); drawRows( g, 30, 30 ); } public void drawRows( Graphics graphics, int x, int y ) { graphics.fillRect( x,y, 10,10 ); graphics.fillRect( x+20, y, 10, 10 ); graphics.fillRect( x+40, y, 10, 10 ); graphics.fillRect( x+60, y, 10, 10 ); graphics.fillRect( x+80, y, 10, 10 ); } } import javax.swing.*; import java.awt.*; public class NeedMethods extends JApplet { public void paint( Graphics g ) { // row 1 g.fillRect( 20,20, 10,10 ); g.fillRect( 40,20, 10,10 ); g.fillRect( 60,20, 10,10 ); g.fillRect( 80,20, 10,10 ); g.fillRect( 100,20, 10,10 ); // row 2 g.fillRect( 30,30, 10,10 ); g.fillRect( 50,30, 10,10 ); g.fillRect( 70,30, 10,10 ); g.fillRect( 90,30, 10,10 ); g.fillRect( 110,30, 10,10 ); } } Which program would be easier to use to create a full-size checkerboard?

14. Why Have Methods? • Events • Events in Java are triggered when: • User selects a button/checkbox/item in list/etc. • User moves/drags the mouse • User types a key • Timer expires • More… • Each event automatically calls a particular method to handle the type of event that occurred. • We’ll discuss events in more detail later

15. Summary • Variables • Scope: instance variables vs. local variables • Method Structure • Purpose of methods