Building Java Programs

Lab 1: Ch. 1: Java Basics, Static Methods

Except where otherwise noted, the contents of this document are Copyright 2012 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp and Stuart Reges

Today's lab

Goals for today:

• learn about the jGRASP editor software
• gain familiarity with syntax errors and debugging
• learn about various useful online tools and resources
• practice using static methods to capture structure and redundancy

Exercise : Compile and run a program in jGRASP

Recall from lecture: A Java program must be compiled, or translated into binary instructions. Then it can be executed or run. When you run a program, it displays output messages to the user in a text window called a console.

For this exercise, let's compile and run a short program that we will provide to you. (See the following slides.) If you get stuck, ask a classmate or TA for help.

Exercise - run jGRASP and create file

• Run the jGRASP editor from the Start Menu of the lab computer (or run it from your own laptop, if you brought one).
• In jGRASP, create a new Java program: Click File → New → Java from the top menu. An empty white text window should appear in jGRASP.
  

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Exercise - copy/paste

• Copy and paste the following program into jGRASP:

  public class MyFirstProgram {
      public static void main(String[] args) {
          System.out.println("Hello, world!");
          System.out.println();
          System.out.println("This is my very first program.");
          System.out.println("Hooray!");
      }
  }
  

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Exercise - save

• Save the program: Click the Save toolbar button, or click File → Save from the top menu, or press the hotkey Ctrl-S (Command-S on a Mac).
• The file name must end with .java and must match the 'public class' name from the Java program. Name this file MyFirstProgram.java.

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Exercise - compile

• Compile the program (translate it to binary instructions): Press the toolbar button that looks like a green plus (+) sign. Or press hotkey Ctrl-B. You should see "jGRASP: operation complete".
  

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Exercise - run

• Run the program: Press the toolbar button that looks like a running man. Or press hotkey Ctrl-R. You should see the program's output in the bottom console area.
  

Exercise : Modify an existing program

Modify your MyFirstProgram file to produce the following console output. Note the blank lines; you should include those in your output.

Hello, world!
I am learning to program in Java.
I hope it is a lot of fun!

I hope I get a good grade!

Maybe I'll change my major to computer science.

Exercise : Syntax errors

• The following program contains 11 errors! What are they?

  1 2 3 4 5 6 7 8 9

  public class Tricky public static main(String args) { System.out.println(Hello world); system.out.Pritnln("Do you like this program"?); System.out.println() System.println("I wrote it myself."; { }

• Once you think you've found the errors, create/compile/run a corrected version of this program.

answer on next slide...

Exercise - answer

1. line 1: missing { after Tricky
2. line 2: missing void before main
3. line 2: missing [] after String
4. line 3: missing " marks around Hello world
5. line 4: system should be System (uppercase S)
6. line 4: Pritnln should be println (lowercase P and fixed spelling)
7. line 4: ? should be before "
8. line 5: missing semicolon after ()
9. line 7: missing ) after "
10. line 8: System.println should be System.out.println
11. line 8: { should be }

Exercise - corrected version

• Here is a corrected version of the program:

  public class Tricky {
      public static void main(String[] args) {
          System.out.println("Hello world");
          System.out.println("Do you like this program?");
          System.out.println();
  
          System.out.println("I wrote it myself.");
      }
  }
  

Exercise : Exploring syntax errors

Discover what error messages the compiler produces when you make each of the following mistakes. How many unique error messages are you able to cause the compiler to produce?

• Naming your file incorrectly, then compiling.
• Forgetting a keyword such as void or class
• Forgetting a quotation mark "
• Forgetting a parenthesis ( or )
• Forgetting a dot .
• Using too many or too few braces { or }

Notice that the error messages don't always make it obvious what is wrong. But they usually tell you the right line number to fix.

Exercise : What's the output?

• How many lines of output are produced (including blank lines)?

  public class Tricky {
      public static void main(String[] args) {
          System.out.println("Testing, testing,");
          System.out.println("one two three.");
          System.out.println();
          
          
          System.out.println("How much output");
          System.out.println();
          System.out.println("will there be?");
      }
  }
  

• Answer: 6 lines. The blank lines in the code don't count, but the System.out.println(); statements do.

Escape sequences

An escape sequence inserts a special character into a println statement.

Example:

System.out.println("I said \"hello\" to Fred.");

Exercise : What's the output?

• What output is produced by the following code?

  System.out.println("Shaq is 7'1");
  System.out.println("The string \"\" is an empty message.");
  System.out.println("\\'\"\\\\\"");
  

  (Try to figure it out without running the code.
  If you give up, paste it into jGRASP and run it.)

  Note: when you see a purple check mark as in the upper-right corner of this page, that means that this problem is also available in PracticeIt by clicking on the check mark.

• Answer:

  Shaq is 7'1
  The string "" is an empty message.
  \'"\\"
  

Static methods

Recall the syntax for writing a static method. Methods are useful for representing a program's structure and capturing common code to avoid redundancy:

public static void name() {
    statements;
}

Example:

public static void song() {
    System.out.println("This is the song that never ends,");
    System.out.println("Yes, it goes on and on, my friends.");
}

Exercise : FightSong

Go, team, go!
You can do it.

Go, team, go!
You can do it.
You're the best,
In the West.
Go, team, go!
You can do it.

Go, team, go!
You can do it.
You're the best,
in the West.
Go, team, go!
You can do it.

Go, team, go!
You can do it.

The following program produces the output at left, but it has poor structure and redundancy. Download it and open it in jGRASP, then add at least two static methods.

• FightSong.java

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Exercise : FightSong, cont'd

Go, team, go!
You can do it.

Go, team, go! 
You can do it.
You're the best,
In the West.
Go, team, go!
You can do it.

Go, team, go! 
You can do it.
You're the best,
In the West.
Go, team, go!
You can do it.

Go, team, go!
You can do it.

Did you choose your methods well? Avoid the following pitfalls:

• Does the same println statement occur more than once in your code?
• Are there any println statements in the main method?
• Is your main method a good summary of the overall program? (If main is only 1-2 lines long, it is not a good summary.)
• Do you have any patterns of method calls that are repeated in the code?

Please ask your TA or neighbor to check out your methods if you're not sure whether you made good choices.

Exercise : MuchBetter

Write a complete Java program named MuchBetter that produces the following output (note the blank line):

A "quoted" String is
'much' better if you learn
the rules of "escape sequences."

Also, "" represents an empty String.
Don't forget: use \" instead of " !
'' is not the same as "

(You can check your output on the Output Comparison Tool web page.)

Exercise : Spikey

• Write a complete program named Spikey that produces the following output:

    \/
   \\//
  \\\///
  ///\\\
   //\\
    /\
  

  (Use only the material we have learned so far. You can check your output on the Output Comparison Tool web page.)

Exercise : Lanterns

• Write a complete program named Lanterns that produces the following output. Use static methods to capture structure and remove redundancy. (Check your output on the Comparison Tool.)

      *****
    *********
  *************
  
      *****
    *********
  *************
  * | | | | | *
  *************
      *****
    *********
  *************
  
      *****
    *********
  *************
      *****
  * | | | | | *
  * | | | | | *
      *****
      *****
  

Exercise : Spell your name

• Write a complete program that outputs your name in giant letters like the following:

  M     M    IIIII     SSSSS     PPPPPP
  MM   MM      I      S     S    P     P
  M M M M      I      S          P     P
  M  M  M      I       SSSSS     PPPPPP
  M     M      I            S    P
  M     M      I      S     S    P
  M     M    IIIII     SSSSS     P
  

  The giant letters should flow vertically. Use methods to avoid redundancy when the same giant letter appears multiple times in your name. (Use only the material from Chapter 1.)

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!