Lab 6 (Feb 18-19)

Creating Packages.

Packages in Java allow a collection of classes to be grouped together under a single name. Other classes can make use of the package by referencing the package name.

To create a package the programmer must do two things:

• the first is to identify all the files and classes that belong to the package

• the second is to collect all the compiled version of the classes into a subdirectory associated with the package name

package package-name;

class class-name {
  ...
}

class-name

package-name

• The package clause must be located at the top of the file.

• All classes in the file are added to the package.

• Classes that are public are available to every other class.

• Classes that with no modifier are available to any other class in the same package.

• The package phrase can be repeated on as many files as is necessary to group a series of classes into one package.

• If there is no package declaration in a file, then the classes in that file belong to a default, unnamed package.

The snapshots are taken on tucotuco.

You should be able to do the same thing in Cafe.

First we create a directory where we will keep the package:

tucotuco.cs.indiana.edu% pwd
/nfs/paca/home/user2/dgerman/A202/lab6
tucotuco.cs.indiana.edu% ls -l
total 0
tucotuco.cs.indiana.edu% mkdir SimpleIO
tucotuco.cs.indiana.edu% ls -l
total 2
drwxr-xr-x   2 dgerman  students     512 Feb 18 00:29 SimpleIO
tucotuco.cs.indiana.edu% cd SimpleIO
tucotuco.cs.indiana.edu% pwd
/nfs/paca/home/user2/dgerman/A202/lab6/SimpleIO
tucotuco.cs.indiana.edu%

We will try to emulate the ccj package, or the BreezyGUI package, in that we will be interested in getting information from the user (as input) and printing information to the user (as output).

Our package will be very small:

• It will have only two classes:
  1. Reader
  2. Writer
• Reader will define
  • a method to read integer values
  • our pause method
• Writer will define
  • a method to write integer values
  • a method to write_lines (strings of characters)

tucotuco.cs.indiana.edu% pwd
/nfs/paca/home/user2/dgerman/A202/lab6/SimpleIO
tucotuco.cs.indiana.edu% vi Reader.java
tucotuco.cs.indiana.edu% cat Reader.java
package SimpleIO;
 
import java.io.*; 
 
public class Reader {
  public static int read() throws IOException {
    BufferedReader stdin = new BufferedReader(
                             new InputStreamReader(
                               System.in)); 
    String         value = stdin.readLine(); 
    return         Integer.parseInt(value);  
  } 
  public static String pause() throws IOException { 
    System.out.println("Press Enter to continue..."); 
    BufferedReader stdin = new BufferedReader(
                             new InputStreamReader(
                               System.in));     
    return         stdin.readLine(); 
  }
} 
 
tucotuco.cs.indiana.edu%

tucotuco.cs.indiana.edu% pwd
/nfs/paca/home/user2/dgerman/A202/lab6/SimpleIO
tucotuco.cs.indiana.edu% vi Writer.java
tucotuco.cs.indiana.edu% cat Writer.java
package SimpleIO;
 
public class Writer {
  public static void write(int value) {
    System.out.print(value); 
  } 
  public static void write_line(String line) { 
    System.out.print(line); 
  }
} 
 
tucotuco.cs.indiana.edu%

tucotuco.cs.indiana.edu% ls -l
total 4
-rw-r--r--   1 dgerman  students     597 Feb 18 00:41 Reader.java
-rw-r--r--   1 dgerman  students     205 Feb 18 00:44 Writer.java
tucotuco.cs.indiana.edu% javac *.java
tucotuco.cs.indiana.edu% ls -l
total 8
-rw-r--r--   1 dgerman  students     779 Feb 18 00:45 Reader.class
-rw-r--r--   1 dgerman  students     597 Feb 18 00:41 Reader.java
-rw-r--r--   1 dgerman  students     510 Feb 18 00:45 Writer.class
-rw-r--r--   1 dgerman  students     205 Feb 18 00:44 Writer.java
tucotuco.cs.indiana.edu%

Using Packages

You've been using BreezyGUI and ccj.

It should be a piece of cake to use our SimpleIO package.

Let's write a program that asks the user for two numbers then adds them up and prints the sum, to the great satisfaction of the user:

tucotuco.cs.indiana.edu% pwd       
/nfs/paca/home/user2/dgerman/A202/lab6
tucotuco.cs.indiana.edu% vi Calculator.java
tucotuco.cs.indiana.edu% cat Calculator.java
import SimpleIO.*;
import java.io.IOException; 
 
public class Calculator {
  public static void main(String[] args) throws IOException {
    while (true) { 
      Writer.write_line("Give me an int: "); 
      int n = Reader.read();  
      Writer.write_line("Give me another int: "); 
      int m = Reader.read(); 
      Writer.write_line(n + " + " + m + " = " + (n + m) + "  "); 
      Reader.pause(); 
    } 
  } 
} 
tucotuco.cs.indiana.edu% javac Calculator.java
tucotuco.cs.indiana.edu% ls -l
total 6
-rw-r--r--   1 dgerman  students     895 Feb 18 01:22 Calculator.class
-rw-r--r--   1 dgerman  students     419 Feb 18 01:22 Calculator.java
drwxr-xr-x   2 dgerman  students     512 Feb 18 01:20 SimpleIO
tucotuco.cs.indiana.edu% java Calculator
Give me an int: 1
Give me another int: 1
1 + 1 = 2  Press Enter to continue...
 
Give me an int: 10
Give me another int: 345
10 + 345 = 355  Press Enter to continue...
 
Give me an int: 6
Give me another int: not an int see what happens...
java.lang.NumberFormatException: not an int see what happens...
        at java.lang.Throwable.(Compiled Code)
        at java.lang.Exception.(Compiled Code)
        at java.lang.RuntimeException.(Compiled Code)
        at java.lang.IllegalArgumentException.(Compiled Code)
        at java.lang.NumberFormatException.(Compiled Code)
        at java.lang.Integer.parseInt(Compiled Code)
        at java.lang.Integer.parseInt(Compiled Code)
        at SimpleIO.Reader.read(Compiled Code)
        at Calculator.main(Compiled Code)
tucotuco.cs.indiana.edu%

This represents an introduction to creating packages.

We'll get back to this when we do I/O and revisit exception handling.

Both BreezyGUI and the ccj package are similar to SimpleIO, except they are bigger.