Topics

Review

• Connecting to a Unix Machine
• Your Home Directory
• The Hierarchical Filesystem
• Moving To A Different Directory
• Always Know Where You Are
• Creating A New Directory
• Looking at the Contents of a File
• Compiled versus Interpreted Languages
• Compiled Languages
• Scripting Languages
• Java Works Differently

New Material

• A Simple Java Program
• Things That Make Java Hard
• Structure of Java Programs
• Copying A File in Unix
• The nano Text Editor
• Using nano in This Class
• Creating the Ex3.java for Today's Class Exercise
• Compiling the Source Code
• Running the Program

Homework

The first homework assignment is due this Sunday at 11:59 PM.

You will find the assignment here.

If you have a problem or a question, make a post on the Class Discussion Area.

Review

Connecting to a Unix Machine

• You can connect to Unix machine on which you have an account if you have software that implements the SSH (Secure Shell) protocol
• This software is called an ssh client
• If you have a PC, you need to download an SSH client
• A good free one is PuTTY which you can download here
• You will find instructions on using Windows machine to connect to a Unix machine here
• On a Mac the ssh client is already installed
• You do this by running the Terminal program
• Which is located in the Utilities directory inside the Applications directory
• You run the ssh client like this

  ssh YOUR_UNIX_USERNAME@users3.cs.umb.edu

  where you substitute your Unix username for YOUR_UNIX_USERNAME
• You find instructions on connecting to a Unix machine from a Mac here
• You will then have to enter your password

Your Home Directory

• When you first connect to the Unix machine you will be in your home directory
• A directory holds files and other directories
• It is called a folder on Windows and the Mac
• Every Unix account on our system has a home directory
• This directory belongs to you and you alone

The Hierarchical Filesystem

• Unix uses a hierarchical filesystem
• Which means that all files are kept in directories (folders)
• And directories live inside other directories
• With one special directory at the top called root
• Which is written as  / 

Moving To A Different Directory

• To move to a different directory you use the cd command
• cd stands for "change directory"
• The command has the following format

  
  cd  DIRECTORY_NAME

• The hierarchical filesystem can be a little confusing
• If you are ever confused by where you are when connected to Unix, there is a simple fix
• Use the cd command without an argument to go back to your home directory

Always Know Where You Are

• The hierarchical filesystem is like pyramid
• The root directory is at the top
• All other directories are inside this directory
• Or in sub-directories
• Or in sub-sub-Directories
• And so on
• The pyramid gets wider the deeper you go
• Many commands will not work properly if you are in the wrong directory
• Always know where you are in the hierarchical filesystem
• One way to learn your location is to use the pwd command

  $ pwd
  /home/olivia93

Creating A New Directory

• You create a directory using the mkdir command
• mkdir stands for "make directory"
• It has the following format

  
  mkdir DIRECTORY_NAME

Looking at the Contents of a File

• If you want to see the contents of a Unix file use the cat command
• cat stands for "concatenate"
• A term that will be defined later
• The format for this command is

  
  cat FILENAME

Compiled versus Interpreted Languages

• When you write a program in you create a text file
• This file consists of statements in the programming language
• This file is called the source code
• But the computer does not understand the language of the source code
• It only understand programs in its own binary machine language
• A program is needed to turn your source code into the binary code the machine understands
• There are two ways a program can do this
  • It can create a new file, translating the source code into machine language
  • It can run the program inside itself, translating the code line by line
• The programs that take the first approach are called compilers
• Programs that take the second approach are called interpreters
• The programming languages that take the first approach are compiled languages
• The programming languages that take the second approach are scripting languages

Compiled Languages

• Language like C and C++ are compiled languages
• You run a C compiler on a C source code file
• And it creates an executable file
• The executable file can then be run on the machine
• Unfortunately, different machines have different machine languages
• So an executable file for one machine will not run on a machine of a different type

Scripting Languages

• Languages like Perl, Python and PHP are scripting languages
• In a scripting language the source code is not compiled
• So no executable file is created
• Instead the source code is run inside a program called an interpreter
• Interpreters are written in a compiled language like C
• But they must be compiled for specific machines
• Programs written in a scripting language run more slowly than than compile programs

Java Works Differently

• Java takes a middle course between compiled and interpreted languages
• You take a Java source file and run it through the Java compiler, javac
• The source file must have the .java extension
• The compiler creates an object file with extension .class
• This object file does not contain machine language instructions
• Instead it contains what are called Java bytecodes
• So you cannot run this .class file directly on the machine
• Instead you must run it inside the Java interpreter, java
• The Java interpreter is also called the Java Runtime Environment (JRE)
• Or the Java Virtual Machine (JVM)
• Java interpreters have been written for all major platforms
• That means you can distribute a single set of .class files that will run on most machines
• Java gives developers most of the efficiency of a compiled language
• But the object files can be run almost anywhere

New Material

A Simple Java Program

• Let's see what a real java program looks like
• Here is the Hello program from the textbook

  public class Hello {
      public static void main(String[ ]  args)  {
          System.out.println("Hello  world!") ;
      }
  }

• Notice the name of the source file, Hello.java
• It is the same as the class name, Hello, with a .java extension
• To compile this program, I run

  $ javac Hello.java

• There are now two files whose name, before the extension, is Hello

  $ ls Hello*
  Hello.class  Hello.java

• To run the program, I use

  $ java Hello
  Hello World!

• Notice that the argument I gave to java was not the name of a file
• It was the name of the class

Things That Make Java Hard

• Java is an object oriented language
• This means that every Java program has to have a certain structure
• In a scripting language like Perl I could write the following to add the numbers from 1 to 1000

  $sum = 0;
  for ($i = 0; $i <= 1000; $i++) {
      $sum = $sum + $i;
  }
  print $sum;

• In Java, a program to the same thing would look like this

  public class  Sum_1000 {
      public static void main(String[ ]  args)  {
          int sum = 0;
          for (int i = 0; i < 1000; i++){
              sum += i;
          }
          System.out.println(sum);
  }

• I've highlighted in red all the extra things .that Java requires
• Why does Java do this?
• Different computer languages are designed for different purposes
• A scripting language like Perl and Python was designed to get something done fast
• Scripting languages aren't very fussy
• Which is great for doing something quickly
• Languages like Java were designed for writing big programs
• If you were writing software for a bank you wouldn't use Perl or Python
• You would use Java
• Why?
• Because all the fussiness built into Java makes it harder to make certain kinds of errors
• Java is a language for big projects
• Where many different programmers work together
• And they have to follow certain rules to avoid chaos

Structure of Java Programs

• Every program that you will write for this course will have a common structure
• The reason for this is beyond the scope of this course
• But don't worry about it
• Just think of it as something you have to do to make things work
• All Java programs must have the following format

  
  public class CLASS_NAME {
      public static void main(String[] args) {
          ...
      }
  }

• Once again CLASS_NAME is a placeholder
• You replace it with the name of your class
• You the Java statements to make the program work replace the ... above
• If your program does not have the lines above, it will not work
• To make life easier for you I have create a file you can use as a template
• The name of this file is xxx.java
• And you can find it in the directory /home/ghoffman/course_files/files_it114
• You will make a copy of this file for all assignments in this course
• You will then change the file to make it do what is required

Copying A File in Unix

• The Unix command to copy a file is cp which stands for "copy"
• It has the following format

  
  cp  ORIGINAL_FILE_NAME  NEW_FILE_NAME

• To make a copy of the template file creating a new file called Ex2.java I would write

  cp  /home/ghoffman/course_files/files_it114/xxx.java  Ex2.java

The nano Text Editor

• All your assignments for this class involve creating scripts on the Unix machine users3.cs.umb.edu
• Scripts are just text files that contain Python statements
• Test files only contain characters you type at the keyboard
• They do not contain fonts or styles like bold
• To create these files you need to use a Unix text editor
• There are many Unix text editors that you can use but I would suggest using nano
• nano is a simple text editor created as part of the GNU project
• In nano you run a command by holding down the Control key while pressing a letter key
• The Control key is on the lower left side of your keyboard and is often labeled "Ctrl"
• The control key is a modifier key
• If you hold down the Control key and press another key it makes something happen
• You can move to the beginning of a line of text by pressing Control A
• And move to the end of the line with Control E
• "Control A" means hold down the Control key while pressing A
• Although I used a capital A here I do not mean you should hold down the shift key
• I use capital letters when writing control key sequences because the capital letters are easier to read
• Some of the basic nano commands appear at the bottom of the page
• The ^ in this list of commands stands for the Control key so ^O means Control O
• The nano feature set is limited
• You can only work with one file at a time
• You can search for text but there is no search and replace feature
• nano does have a limited cut and paste feature
• If you press Control K the entire line will disappear
• Then if you go to another line and press Control U the line will be pasted back at that point
• When you want to save a file you press Control O
• And the name of the file will appear at the bottom of the screen
• You need to hit Enter to accept that name and complete the save process
• Control X will quit nano
• I have created a web page with instructions for using nano here
• There is a link to it on the class web page

Using nano in This Class

• There are two types of assignments in this class
  • Class Exercises
  • Homework Assignments
• In each case you will make a copy of the template file xxx.java to your current directory
• Then you will use nano to change the file
• To run nano use the following format

  
  nano  FILE_NAME

• You can use nano to create a new source code file
• Or to change a file that already exists
• Either way, we use the same format

  
  nano  FILE_NAME

Creating the Ex3.java for Today's Class Exercise

• In today's Class Exercise you are asked to create Ex3.java
• When you run this program it prints "Hello world!
• Following the instructions in today's Class Exercise you will create an ex3 directory inside you ex directory
• You will then cd into this directory and make a copy of xxx.java called Ex3.java

  cd ex3
  cp  /home/ghoffman/course_files/files_it114/xxx.java  Ex3.java
  

• Ex3.java will contain the following text

  public class CLASS_NAME {
      public static void main(String[] args) {
          # your code goes here
      }
  }

• Open the source code file in nano

  	
  nano  Ex3.java

• You need to make two changes
• Change CLASS_NAME in the first line to Ex3
• It must be capitalized this way or it won't work
• The first line should now look like this

  public class Hello {

• Next you need to remove the line that reads

  # your code goes here

• And replace it with

  System.out.println("Hello world!");

• Don't leave out the ; at the end of the line or it won't work
• Save your changes and quit

Compiling the Source Code

• Ex3.java is the source code
• You need to compile this to create the object file that you can run
• You do this by running javac the Java compiler

  javac Ex3.java

• If the compilation works you should see two files in your directory

  $ ls
  Ex3.class  Ex3.java

• Ex3.class is the object file
• It contains bytecodes that the Java interpreter understands
• If file does not compile you will get an error message
• Basically this means that you have made an error when you edited the file
• Often this means one of two things
• You made an error in capitalization
• Or you left out the ; at the end of the println statement

Running the Program

• users3 does not understand bytecodes
• To run your program you need to use the Java interpreter
• Which is also known as the Java Runtime Environment (JRE)
• The name of this program is java
• You need to tell it which object file to run
• But you don't use the full name of the file
• Instead you give it the filename without the .class extention

  $ java Ex3
  Hello world!
  

Class Quiz

Attendance

Class Exercise