Copyright © 2013
Robert W. Hasker

Java Basics

• Ch. 3 in textbook through Sect. 3.10.
• Basic elements of a program
• Variables, initialization
• Expressions

Java Programs

• A first program: Dream.java
  • Annotated versions: several terms, additional tokens.
    • To view the annotation, save the files to your computer and open them in a standard PDF reader (as opposed to a web browser).
  • Core program: public...}
  • Comment at top: "block" comment: /* to */
  • Comment at end: one-liner
    • Comment at top important, one at end not so helpful
    • Never write a comment if its negation is clearly false:

              System.out.println("I have a dream!"); // Print message  <- useless!
      

  • blank lines/whitespace: ignored
  • reserved words/keywords: public, class
• main: starting point - copy/paste
  • See textbook of explanation if you'd like more info
• braces: { to } - always in pairs
• text between quotes: a "string" - can be any text, including text in other languages
• identifiers: main, Dream, even System and println
  • Rule: sequence of letters, digits, underscores, and dollar signs where first character is not a digit
  • Coding style: rules that affect readability, not legality
    • Use just letters & numbers except in extreme cases
    • Be descriptive! (more on this later)
    • Run words together with capital letters: PrintMyMessage
• An alternative way to talk about the program: pseudocode
  • Pseudocode for this program: print I have a dream
  • pseudocode: almost code - core ideas, just don't worry about Java rules
  • By the end of the quarter, you'll have no problem going from pseudocode to code

Moving Beyond Dreams

• You're a pyramid salesperson; you want to compute the cubic meters of limestone needed to build a pyramid of a given height.
• Note: height of great pyramid: about 160 meters
  • formula: volume = (4/3) * (h³ / tan²(incline))
  • Derivation: volume of pyramid = hK/3 where h is height, K is area of base
  • tan incline = o / a, so K = (2 * (height / tan incline))²
  • for an incline of 52^o, we get volume = .814 * height³
• Algorithm:
  1. set height of pyramid
  2. compute volume for pyramid
  3. print pyramid's height, volume
• Program, assuming height is 10 meters: Pyramid10.java
  • Multiple print statements: executed in order
  • print vs. println
  • Java also has addition, subtraction, and division (/)
  • Issues: too many digits printed, not terribly useful, difficult to read
• Fixes:
  • Introduce a variable height to store the height:

            double height = 10.0;
            System.out.print(0.814 * height * height * height);
    

    • this is a variable because it's value can change while running the program
    • double is a keyword indicating we are setting up a variable
    • double means the value can hold high precision numbers like 1.41421356237 and 18.2e48 - essentially Real numbers
    • also known as a declaration - we are declaring the variable to hold doubly-large, floating point values (real numbers)
  • Issue: 0.814 is a bit obscure
    • Could treat that as a variable to give it a name, but the number should not change
    • Solution:

              final double VOLUME_CONVERSION = 0.814;
      
      ...
      
              double volume = VOLUME_CONVERSION * height * height * height;
      

      • Known as a named constant
      • General form: final double name = value;
        or final int name = value;
        
      • Style issue: name in all upper case
      • Should introduce a named constant for any non-trivial value in your program
      • Named constants make the code easier to read: "no magic/mystery numbers".
      • Advantage of named constants: can change the value in just one place.
      • For instance, the above factor to 6 digits is 0.813876.
• Finally, reading values from the user: use package Scanner
  • Challenge in Java: reading data from keyboard
  • Most programs don't do this, so Java optimized for other input methods
  • Solution we used in first lab:
    1. import java.util.Scanner;
    2. Scanner in = new Scanner(System.in);
    3. double height = in.nextDouble();
  • Issue: need to tell user to enter a number so user knows what to do!
  • Final program: PyramidVolume.java
• Note: this program and many others available on samples page

More on Java Programs

• Compiling and running from command line prompt
  • Note will need to set PATH variable - out of scope
  • javac
  • java
  • bytecode - open in Notepad++
  • Java Virtual Machine (JVM), Java Runtime Environment (JRE), Java Development Kit (JDK)
  • why bytecode makes programs portable
• Types of errors
  • Compilation errors
  • Execution errors - much harder to fix
• How we develop programs
  • Understand what problem is; esp: compute result by hand
  • Write pseudocode, probably for a portion of the system
  • implement and test that portion
  • repeat for the rest of the system
• Some general survival notes
  • Windows way to associate files with programs
  • How to turn on showing extensions
  • Might associate .java with Notepad++
  • Set indentation in IntelliJ to 3 or 4 characters
  • Make sure tabs are off
  • Make sure know how to change brightness

Review

• Program structure
• Introducing computation
• Simple input and output