Computer Science 101

1
Computer Science 101

• More Java Programming

2
All About Java

• Was developed at Sun Microsystems in the early
  1990s
• A general-purpose language but
• Programs could run in small devices (embedded
  systems)
• Internet-ready and Web-ready
• Standard support for multimedia applications

3
Compiling and Running a Java Program

• A Java compiler translates a Java program to an
  equivalent program in byte code
• A Java virtual machine (JVM) is a program that
  interprets a Java byte code program to execute it

User inputs
Java code
Program outputs
byte code
Compiler
JVM
4
Portability

• A Java program translates to a standard byte code
• A Java byte code can run on any JVM
• A JVM, and thus a Java program, can run
• In a PDA
• In a desktop computer
• In a Web browser

5
Resources

• Suns Java Web site
• http//java.sun.com/j2se/
• Documentation for Java
• http//java.sun.com/j2se/1.4.2/docs/api/index.html
  
• Java programming environment
• http//www.bluej.org/

6
A Design Template

• Most simple problems have the form
• Get input data from the user
• Use algorithms to process the data
• Output the results to the user

7
Example Circle Area
public void buttonClicked (JButton buttonObj)
double radius radiusField.getNumber() //
Input radius double area 3.14 radius
radius // Compute area
areaField.setNumber (area) //
Output area areaField.setPrecision (2)
// Format output
Input gets data from a field Computation
calculates the area Output sends data to a field
8
Input
double radius radiusField.getNumber() //
Input radius

• There are three types of fields
• IntegerField for ints (whole numbers)
• DoubleField for doubles (decimal point numbers)
• TextField for Strings (names, etc.)
• Use the method getNumber() to obtain the number
  stored in a numeric field.
• Use the method getText() to obtain the string
  stored in a text field.

9
Output
areaField.setNumber (area) //
Output area areaField.setPrecision (2)
// Format output
Use the method setNumber(lta numbergt) to display a
number in a numeric field. Use the method
setText(lta stringgt) to display a string in a text
field. Use the method setPrecision(ltan integergt)
to set a DoubleFields precision limit.
10
Customer Request
Modify the circle area program so that it can
also compute the radius from the area.
11
Analysis

• The program uses
• to compute the radius from the area
• The program displays results with the maximum
  allowable precision

12
Proposed Interface
13
Design

• Because we display the results with the maximum
  allowable precision, we use the most precise
  value of ? supported by the programming language

The constant Math.PI represents ? to 15 places of
precision. The method Math.sqrt(lta numbergt)
returns the square root of a number.
14
Pseudocode for buttonClicked
if the user selects compute area button get
the radius area Math.PI radius radius
display the area else get the area radius
Math.sqrt(area / Math.PI) display the radius
15
Implementation
public void buttonClicked(Button buttonObj)
double radius, area if (buttonObj
areaButton) radius radiusField.getNumber(
) area Math.PI radius radius
areaField.setNumber(area) else
area areaField.getNumber() radius
Math.sqrt(area / Math.PI)
radiusField.setNumber(radius)
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Syntax of Compound Statements
ltstatement-1gt ltstatement-ngt
A compound statement is a sequence of zero or
more statements.
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Syntax of Selection Statements
if (ltBoolean expressiongt) // One-way
decision ltstatementgt if (ltBoolean
expressiongt) // Two-way decision
ltstatementgt else ltstatementgt
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Semantics of Selection Statements
false
if (ltBoolean expressiongt) ltstatementgt if
(ltBoolean expressiongt) ltstatementgt else
ltstatementgt
?
true
statement
false
?
true
statement
statement