1. Chapter 2 Programming by Example

2. A Holistic Perspective Three sections separated by blank lines. Program comment File: FileName.java -------------------------- What the program does Imports Library packages Main class

3. Program Comment Format /* * File: HelloProgram.java * ----------------------------------- * Describe what this program does. Inputs and outputs * if any. The user can run this program without reading * the code. * * Author: E. Roberts, 123456 */

4. Main Class Format public class HelloProgram extends GraphicsProgram { public void run() { add(new GLabel(“hello, world”, 100, 75)); } Indentations (3 spaces each level) Positions of “{“ and “}” ClassName and methodName

5. Main Class public class HelloProgram extends GraphicsProgram { body of the class definition } Key words: public, class, extends Main class name must be the same as file name Main class HelloProgram is a subclass of GraphicsProgram, defined in acm.program package (must be imported).

6. Body of Main Class public void run() { add(new GLabel(“hello, world”, 100, 75)); } run: method name run(): no arguments passed to run add: method defined in acm.graphics, one argument (an object of GLabel class) GLabel: a class defined in acm.graphics, requires three arguments

7. Out put: Hello, world 100 75

8. Add Two Integers /* * File: Add2Integers.java * ------------------------------- * This program prompts for two integers, adds them, and prints their sum. * * Author: E. Roberts, 123456 */ Import acm.program.*; public class Add2Integers extends ConsoleProgram { public void run() { println(“This program adds two integers.”); int n1 = readInt(“Enter n1: “); int n2 = readInt(“Enter n2: “); int total = n1 + n2; println(“The total is “ + total + “.”); }

9. Questions Add2Integers is a subclass of what class? Where is ConsoleProgram (handles console input and output) class defined? Where are the methods println and readInt defined? run() is a method in which class?

10. Console Output println: displays the argument string then returns to next line println(“This program adds two integers.”) Displays: This program adds two integers. println(“The total is “ + total + “.”) Displays, when the value of total is 42, The total is 42. +: concatenation operator

11. Console Input readInt: displays argument string, prompts an integer entered from keyboard, returns the entered integer. readInt(“Enter n1: “) Displays Enter n1: prompts an integer from keyboard, if 17 is typed, Enter n1: 17 returns 17

12. HelloProgram Revisited Split add(new GLabel(“hello, world”, 100, 75)); Into GLabel msg = new GLabel(“hello, world”, 100, 75); add(msg); Why? Control font, size, color, etc, in between. How?

13. Sending Messages Setting font-size and color public void run() { GLabel msg = new GLabel(“hello, world”, 100, 75); msg.setFont(“Helvetica-24”); msg.setColor(Color.RED); add(msg); } hello, world

14. GRect and GOval /* * File: GRectPlusGOval.java * ------------------------------------ * This program draws a red rectangle and a black * outlined and green filled oval */ import acm.graphics.*; import acm.program.*; import java.awt.*;

15. GRect and GOval (cont.) public class GRectPlusGOval extends GraphicsProgram { public void run() { GRect rect = new GRect(100, 50, 125, 60); rect.setFilled(true); rect.setColor(Color.RED); add(rect); GOval oval = new GOval(100, 50, 125, 60); oval.setFilled(true); oval.setColor(Color.GREEN); add(oval); }

16. position: ( 100, 50) width: 125 height: 60 unit: pixel