CS 46A - Lecture 2

Pre-class reading

• Section 1.4 - 1.7
• Did you take the quiz?

The Java Programming Language

• Simple
  • Mostly—a few issues are vexing for beginners
• Safe
  • Virtual machine diagnoses program errors instead of causing random behavior
  • Great for learning
• Platform-independent (write once, run anywhere)
  • You can use Windows, Mac, Linux on your laptop
• Rich library
  • Many built-in features
  • Many third-party libraries
• Great tools
  • BlueJ, NetBeans, ...

Our First Program

public class HelloPrinter
{
   public static void main(String[] args)
   {
      // Display a greeting in the console window

      System.out.println("Hello, World!");
   }
}

Program Run:

• Hello, World!

The Structure of a Simple Program: Class Declaration

• Classes are the fundamental building blocks of Java programs:

  public class HelloPrinter

  starts a new class

• Every source file can contain at most one public class
• The name of the public class must match the name of the file containing the class:
  • Class HelloPrinter must be contained in a file named HelloPrinter.java

The Structure of a Simple Program: main Method

• Every Java application contains a class with a main method
  • When the application starts, the instructions in the main method are executed

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

  declares a main method

The Structure of a Simple Program: Comment

• The first line inside the main method is a comment:

  // Display a greeting in the console window

• Compiler ignores any text enclosed between // and end of the line
• Use comments to help human readers understand your program

public class HelloPrinter
{
   public static void main(String[] args)
   {
      // Display Hello World
      System.out.println("Hello");
      System.out.println("World");
   }
}

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

Lecture 2 Clicker Question 3

What is wrong with this program?

public class HelloPrinter
{
   public static void main(String[] args)
   {
      // Display Hello World!
      System.out.println("Hello World!")
   }
}

1. There should have been a semicolon at the end of the first line
2. There should have been a comma after Hello
3. There should have been a semicolon at the end of the sixth line
4. There is nothing wrong with the program

Lecture 2 Clicker Question 4

Which part of the program contains the first error that is reported when you try to compile this program?

public class Hell0Printer
{
   public static void main(String[] args)
   {
      // Display Hell0, W0rld!
      System.0ut.println("Hell0, W0rld!");
   }
}

1. public class Hell0Printer
2. // Display Hell0 W0rld!
3. System.0ut.println
4. "Hell0 W0rld!"

Lecture 2 Clicker Question 5

Which identifier is a cause of the first error that is reported when you try to compile this program?

public Class HelloPrinter
{
   public static void Main(string[] args)
   {
      // Display a greeting in the console window

      system.out.println("hello, world!");
   }
}

1. Class
2. Main
3. string
4. system

Tip: Download the Program Code

• It's on the book companion site (link on course page)
• Unzip somewhere so you can find it during homeworks/exams
• Copy/paste into BlueJ
• Don't type what you can copy/paste
• Not plagiarism since it's from the book

Tip: Understand the File System

• Folders (also called directories) contain files
• A file stores some data
• A Java program is a file (such as HelloWorld.java)
  • It must be inside some folder
  • You want to know where it is
• Windows hides folder structure to help naïve users
• Computer scientists are not naïve users

Tip: Know These Folders

• Home directory
  • c:\Users\Yourname, /Users/yourname, /home/yourname
• Download directory
  • Usually, Downloads inside the home directory
  • You can choose it in your browser configuration
• Desktop
  • Just kidding
  • Experts don't use the desktop because it is completely covered with the windows that we need for our work
  • If your browser downloads to the desktop, reconfigure it

Tip: Make These Folders

• Course directory
  • You need to make one
  • Good choice: sjsu/cs46a inside your home directory
  • That is, a folder sjsu inside your home directory, and another folder cs46a inside that
  • Use lowercase letters and NO SPACES in directory names
• Book code
  • Put it in sjsu/cs46a/book
• A separate directory for each assignment, lab, etc
  • sjsu/cs46a/lab1
  • sjsu/cs46a/hw1a
• In BlueJ
  • Use Project -> New Project and give project name (such as hw1a). BlueJ will make the folder
  • Use Project -> Open Non-BlueJ to open a book example

Lecture 2 Clicker Question 6

You want to start working on Homework 1. Into which folder should you put it?

1. On the desktop
2. Homework 1 inside My Documents
3. hw1 inside the home directory
4. hw1a (or hw1b or hw1c) inside cs46a