JETT Workshop - Java Basics

Computer Science (CSCI) Department
College of Engineering, Computer Science, & Technology (ECST)

JETT Workshops
Java Basics

Fall 2004

Control Structures and Relational Operators
(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann.
Visit www.wiley.com/college/horstmann for information on the current edition of this excellent text.)

from the AP CS Java Subset
- The control structures if, if/else, while, for, return are part of the AP Java subset.
- The do/while, switch, plain and labeled break and continue statements are not in the subset.

Investment with Compound InterestInvest $10,000, 5% interest, compounded annually

Year	Balance
0	$10,000
1	$10,500
2	$11,025
3	$11,576.25
4	$12,155.06
5	$12,762.82

When will the balance be at least $20,000?

while statement

while (condition)
   statement;

repeats the statement while the condition is true

while (balance < targetBalance)
{
   year++;
   double interest = balance * rate / 100;
   balance = balance + interest;
}

note that "statement" is a block statement

Investment Example

Flowchart for while Loop

Common Error: Infinite Loop

Loops run forever--must kill program

while (year < 20)
{  
   balance = balance + balance * rate / 100;
   // oops - forgot to decrement year! 
}

while (year > 0)
{  
   year++; // oops, meant --
   . . .
}

Common Error: Off-by-1 Error

Should year start at 0 or 1?

Should the test be < or <=?

year = 0;
while (balance < targetBalance)
{
   year++;
   double interest = balance * rate / 100;
   balance = balance + interest;
}
System.out.println("Reached target after " + year + " years.");

Avoiding Off-by-1 Error

Run through a simple example:
target balance = $20,000, interest rate 50%
after one year: balance = $15,000
after two years: balance = $22,500
Therefore: year must start at 0
interest rate 100%
after one year: balance = $20,000
loop should stop
Therefore: must use <

do Statement

Executes loop body at least once
do
statement
while (condition);

Example: Validate input

double value;
do
{
   String input = JOptionPane.showInputDialog(
      "Please enter a positive number");
   value = Integer.parseInt(input);
}
while (input <= 0);

Flowchart for do Loop

for statement

for (initialization; condition; update)
   statement

Example:

for (int i = 1; i <= n; i++)
{
   double interest = balance * rate / 100;
   balance = balance + interest;
}

Equivalent to

initialization;
while (condition)
{ 
   statement; 
   update; 
}

Flowchart for for Loop

Investment Example
- Investment.java
- InvestmentTest.java

Common Errors: Semicolons

A semicolon that shouldn't be there

sum = 0;
for (i = 1; i <= 10; i++); // oops!
   sum = sum + i;
System.out.println(sum);

A missing semicolon

for (i = 1; i <= 10; sum = sum + i++)
   System.out.println(sum);

Nested Loops

Create triangle pattern
[]
[][]
[][][]
[][][][]

Loop through rows

for (int i = 1; i <= n; i++)
{
   // make triangle row
}

Make triangle row is another loop

for (int j = 1; j <= i; j++)
   r = r + "[]";
r = r + "\n";

Put loops together => Nested loops

Triangle Example
- Triangle.java
- TriangleTest.java

Reading Input Values

General pattern: "Loop and a half"

boolean done = false;
while (!done)
{
   String input = read input;
   if (end of input indicated)
      done = true;
   else
   {
      process input
   }
}

DataSet.java

InputTest.java

String Tokenization

StringTokenizer breaks up string into tokens
By default, white space separates tokens
"4.3 7 -2" breaks into three tokens:
"4.3", "7", "-2"

For example

StringTokenizer tokenizer 
   = new StringTokenizer();
while (tokenizer.hasMoreTokens())
{  
   String token = tokenizer.nextToken();
   process token
}

InputTest.java

Traversing Characters in String

s.charAt(i) is the ith character of the string s

for (int i = 0; i < s.length(); i++)
{
   char ch = s.charAt(i);
   process ch
}

Example: Count vowels

int vowelCount = 0;
String vowels = "aeiouy";
for (int i = 0; i < s.length(); i++)
{
   char ch = s.charAt(i);
   if (vowels.indexOf(ch) >= 0)
      vowelCount++;
}

s.indexOf(ch) is the index of the first occurrence of ch in s, or -1 if ch doesn't occur in s

Random Numbers

Random number generator

Random generator = new Random();
int n = generator.nextInt(a);      // 0 <= n < a
double x = generator.nextDouble(); // 0 <= x < 1

Throw die (random number between 1 and 6)
int d = 1 + generator.nextInt(6);
Die.java
DieTest.java

Exercise - you know what to do - play with these...