I. Running Java
 A. Getting it
------------------------------------------
             GETTING JAVA

Already on department Linux machines, in

   /opt/java/j2sdk1.4.1_01/

For home, go to http://java.sun.com
and get J2SDK 1.4.1_01

What you get:

    javac    - compiler
    java     - interpreter (virtual mach.)
    jar      - archiver
    javadoc  - HTML page creation
    ...

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 B. Hello World
------------------------------------------
    SUMMARY OF COMILATION AND RUNNING

    HelloWorld.java
        |
        | javac HelloWorld.java
        v
    HelloWorld.class
        |
        | java HelloWorld
        v
    (output)

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 C. setting your CLASSPATH
   Who can explain what the PATH does in Unix?
------------------------------------------
          THE CLASSPATH (ON LINUX)

On Linux, cygwin

  $ printenv CLASSPATH
  .:...
  $ java HelloWorld

What if "." is  not in the CLASSPATH?

  $ export CLASSPATH=""
  $ java HelloWorld

Using multiple directories:

  $ ls applogic userinterface
  $ cd applogic
  $ export \
  CLASSPATH="$HOME/classes/cs362/lectures/objects-classes/applogic"
  $ javac Message.java
  $ export \
  CLASSPATH=".:$HOME/classes/cs362/lectures/objects-classes/applogic"
  $ javac Message.java
  $ cd ../userinterface
  $ javac HelloWorld.java
  $ export \
  CLASSPATH="$HOME/classes/cs362/lectures/objects-classes/applogic"
  $ javac HelloWorld.java
  HelloWorld.java:6: cannot resolve symbol
  ...
------------------------------------------
     So can you summarize how the CLASSPATH works so far?
 D. packages in Java
------------------------------------------
                 PACKAGES

Goals:
   prevent name conflicts
   support large scale development

For each name N in package P
   can refer to N by P.N
   from outside package

Packages contain:
     classes, interfaces,
     packages

Example:

  userinterface
      HelloWorld
  applogic 
      Message
------------------------------------------
------------------------------------------
          EXAMPLE

// Add the following line of code
// to put this class in package applogic.
package applogic;

/** This class knows the message to print.
 */
public class Message {

    /** the message */
    private String msg;

    /** Initialize this Message object. */
    public Message() {
        msg = "Hello, class!";
    }

    /** Return the message. */
    public String get() {
        return msg;
    }
}
------------------------------------------
        What do we have to do to make the code in HelloWorld work
        with this?
        Can you summarize how java and javac use CLASSPATH?
 E. Eclipse
------------------------------------------
        ECLIPSE EDITOR

Free, IDE for Java

Get it from:
    http://eclipse.org

Already installed on Linux:
    $ eclipse
------------------------------------------
II. Exception Handling
       Have you used exceptions in C++?
 A. motivation
------------------------------------------
     MOTIVATION FOR EXCEPTION HANDLING

How to make a "bullet-proof" abstraction?

Returning a status code:
  - inefficient in the normal case
  - users who forget aren't warned
    ==> insecure
  - checking status gets in the way
    ==> hard to read programs

Exit the program if problems found:
  - inflexible ==> poor reuse

Require callers to check preconditions:
  - doesn't work for untrusted clients
    ==> insecure
  - hard for clients to check sometimes
    ==> inefficient, hard to read

Better: exceptions, try/catch statements
------------------------------------------
        So when should you use exceptions in a design?
 B. exception mechanism
  1. terms
------------------------------------------
     EXCEPTION HANDLING MECHANISMS

A convenient form of passing blocks

Terms:
   An exception is an object that may be


   The code that is executed in response


------------------------------------------
 C. in Java
  1. exception hierarchy
------------------------------------------
   EXCEPTION TYPES IN JAVA

All exceptions are subclasses of Throwable

Unchecked exceptions are subclasses of
either Error or RuntimeException

Throwable
   Error
     AWTError
     LinkageError
     ThreadDepthError
     VirtualMachineError
       OutOfMemoryError
       ...
   Exception
     AWT Exception
     ClassNotFoundException
     CloneNotSupportedException
     IOException
       EOFException
       FileNotFoundException
       ...
     RuntimeException
       ArithmeticException
       ArrayStoreException
       ClassCastException
       SecurityException
       IndexOutOfBoundsException
       NullPointerException
       IllegalArgumentException
       ...
------------------------------------------
        Advantages of a hierarchy?
  2. declarations in Java of what exceptions a method can throw
------------------------------------------
  DECLARING EXCEPTIONS THROWN BY METHODS
               IN JAVA

Syntax for declaring exceptions
that may be thrown by a method:

   throws <type-name> [, <type-name>] ...

Example:

    char readChar() throws IOException {
        ...readByte() ...
    }

    byte readByte() throws IOException {
        ...
    }
------------------------------------------
        Why?
  3. throwing exceptions
------------------------------------------
        THROWING AN EXCEPTION

To throw an exception you have to
   - create an exception object,
   - use "throw" to throw it.

Syntax:

    throw <expression>;

Example:

    throw new IOException("bad file");
------------------------------------------
  4. catching exceptions
------------------------------------------
       CATCHING AN EXCEPTION

Syntax:

   try
       <block>
   [catch (<exception type> <identifier>)
       <block>] ...
   [finally
        <block>]


Example:

    try {
    } catch (IOException ioe) {
       System.err.println(e.getMessage());
    } catch (NullPointerException npe) {
       System.err.println(npe);
       throw npe;
    } finally {
       myFile.close();
    }

------------------------------------------
        What else needs to be said?