Programming by Example

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

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Out put
75
Hello, world
100
Hello, world
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?

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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

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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

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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

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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.

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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)

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• position ( 100, 50)
• width 125 height 60 unit pixel