Download 2. The J# Language - University of Illinois at Chicago

Lecture 3:
The J# Language
Objectives
“J# brings the Java programming language (1.4) to the .NET platform,
providing full access to the .NET Framework”
• The J# language by way of example…
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-2
2
Java and J#
• Using an example, let's take a tour of J#
• Goal?
- to demonstrate that J# can be viewed as pure Java…
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-3
3
Banking Application
• Neighborhood bank needs app to process daily transactions
– daily transactions reside in a file "transactions.txt"
– at night, transactions are applied to customer accounts stored
in "customers.txt"
transactions.txt
customers.txt
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
Banking App
3-4
4
Program design
• Design will consist of 4 classes:
– App, Customer, CustomersIO, Transactions
n
App
Customer
represents 1 Customer
1
main( )
CustomersIO
1
read(filename) : Customer[ ]
write(customers, filename)
1
Transactions
process(customers, filename)
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-5
5
Main program
• Coordinates transaction processing
1. read customer data into array of Customer objects
2. process transactions by updating objects
3. write objects back to file when done
public class App
{
public static void main(String[] args)
{
System.out.println("** Banking Application **");
Customer[] customers;
customers = CustomersIO.read("customers.txt");
Transactions.process(customers, "transactions.txt");
CustomersIO.write(customers, "customers.txt");
System.out.println("** Done **");
// keep console window open...
.
.
.
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-6
6
Exception handling
• Main program should deal with possibility of exceptions:
public class App
{
public static void main(String[] args)
{
try
{
System.out.println("** Banking Application **");
.
.
.
System.out.println("** Done **");
}
catch(Exception ex)
{
System.out.println(">> ERROR: " + ex.toString());
}
finally
{ // keep console window open...
System.out.println();
System.out.print("Press ENTER to exit...");
try { System.in.read(); } catch(Exception ex) { }
}
}//main
}//class
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-7
7
Build & run…
• Stub out the classes & methods,
build & run
– test normal execution…
– test throwing a java.io.IOException…
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-8
8
Customer
Customer class
• Represents 1 bank customer
– minimal implementation at this point…
public class Customer
{
public String firstName, lastName;
public int
id;
public double balance;
firstName : String
lastName : String
id : int
balance : double
Customer(fn, ln, id, balance)
toString( ) : String
// fields
public Customer(String fn, String ln, int id, double balance)
{
this.firstName = fn; // constructor
this.lastName = ln;
this.id
= id;
this.balance
= balance;
}
public String toString() // method
{
return this.id + ": " + this.lastName + ", " + this.firstName;
}
}
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-9
9
CustomersIO
CustomersIO class
read(filename) : Customer[ ]
write(customers, filename)
• Reads & writes the customer data
• File format:
– first line is total # of customers in file
– then firstname, lastname, id, balance for each customer…
7
Jim
Bag
123
500.0
Jane
Doe
456
500.0
.
.
.
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-10
10
CustomersIO.read( )
• First implement read( ), which returns an array of customers…
public class CustomersIO
{
public static Customer[] read(String filename) throws java.io.IOException
{
System.out.println(">> reading...");
java.io.FileReader
file
= new java.io.FileReader(filename);
java.io.BufferedReader reader = new java.io.BufferedReader(file);
Customer[]
customers;
String
fn,ln;
int
id,N;
N = Integer.parseInt(reader.readLine());
customers = new Customer[N];
for (int i = 0; i < N; i++)
{
fn = reader.readLine();
ln = reader.readLine();
id = Integer.parseInt(reader.readLine());
balance = Double.parseDouble(reader.readLine());
customers[i] = new Customer(fn, ln, id, balance);
}//for
reader.close();
return customers;
double
balance;
…
"Jane", …
"Jim", …
}
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-11
11
Build & run…
• Let's test the input code…
public class App
{
public static void main(String[] args)
{
try
{
System.out.println("** Banking Application **");
Customer[] customers;
customers = CustomersIO.read("customers.txt");
for (int i = 0; i < customers.length; i++)
System.out.println(customers[i]);
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-12
12
CustomersIO.write( )
• Now let's implement write( )…
…
"Jane", …
public class CustomersIO
{
.
.
.
"Jim", …
public static void write(Customer[] customers, String filename) throws java.io.IOException
{
System.out.println(">> writing...");
java.io.FileWriter file = new java.io.FileWriter(filename);
java.io.PrintWriter writer = new java.io.PrintWriter(file);
writer.println(customers.length);
for (int i = 0; i < customers.length; i++)
{
writer.println(customers[i].firstName);
writer.println(customers[i].lastName);
writer.println(customers[i].id);
writer.println(customers[i].balance);
}//for
writer.close();
}
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-13
13
Build & run…
• To test, just run it twice…
– first run outputs to "customers.txt"
– second run will re-input the new file &
check its format
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-14
14
Transactions
Transactions class
process(customers, filename)
• Reads the bank transactions & updates the customers
• File format:
– customer id, Deposit or Withdraw, then amount
– last transaction is followed by -1
123
D
100.0
456
W
100.0
.
.
.
-1
Introducing
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J#
in Visual Studio
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.NET
.NET
3-15
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Transactions.process( )
• Read Tx, find customer, update customer, repeat…
public class Transactions
{
public static void process(Customer[] customers, String filename) throws java.io.IOException
{
System.out.println(">> processing...");
java.io.FileReader
file
= new java.io.FileReader(filename);
java.io.BufferedReader reader = new java.io.BufferedReader(file);
String
action;
int
id;
double
amount;
Customer
c;
id = Integer.parseInt(reader.readLine());
while (id != -1) // for each transaction...
{
action = reader.readLine();
amount = Double.parseDouble(reader.readLine());
c = findCustomer(customers, id);
if (action.equals("D"))
c.balance += amount; // deposit
else
c.balance -= amount; // withdrawal
id = Integer.parseInt(reader.readLine()); // next Tx please...
}//while
reader.close();
}
Introducing
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J#
in Visual Studio
CS using
.NET
.NET
3-16
16
findCustomer( )
• Performs a linear search, returning first matching customer…
…
"Jane", …
.
.
.
"Jim", …
private static Customer findCustomer(Customer[] customers, int id)
{
for (int i = 0; i < customers.length; i++) // for each customer...
if (customers[i].id == id)
return customers[i];
// if get here, not found...
return null;
}
}//class
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-17
17
Build & run…
• Program should be complete at this point!
– but let's check with debugging output
– extend Customer to output balance…
public class Customer
{
.
.
.
public String toString()
{
return this.id + ": " + this.lastName
+ ", " + this.firstName
+ ": " + this.getFormattedBalance();
}
public String getFormattedBalance()
{
java.text.DecimalFormat formatter;
formatter = new java.text.DecimalFormat("$#,##0.00");
return formatter.format(this.balance);
}
}//class
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-18
18
J# is Java!
• Program we just developed is pure Java
– compiles with both Visual Studio .NET and Sun's JDK…
transactions.txt
customers.txt
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
Banking App
3-19
19
Compiling with Java:
• If you have Sun's JDK installed, you can compile and run the
app we just built:
– rename .jsl files to .java
– javac *.java
– create sub-directory called BankingApp (since files are part of
the package BankingApp)
– move .class files into BankingApp sub-directory
– copy "customers.txt" and "transactions.txt" into current
directory (not the BankingApp sub-directory)
– java BankingApp/App
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-20
20
JavaDoc comments
• Support for JavaDoc comments provided by Visual Studio .NET
– Tools menu, Build Comment Web Pages…
• @param and @return are supported; others are ignored
(@author, @version, and @see)
package BankingApp;
/**
* Main class for Banking Application.
*
* Author: Joe Hummel,
* Date:
April 2004
*/
public class App
{
.
.
.
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-21
21
Summary
• J# is Java on the .NET platform
– at least at the level of Java 1.4, with a subset of the class
libraries.
– eventually we'll see what else J# can do…
Introducing
Microsoft
J#
in Visual Studio
CS using
.NET
.NET
3-22
22