Building Java Programs, 4th edition

Lab: Java Basics

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Today's lab

Goals for today:

• learn about the jGRASP editor software
• gain familiarity with syntax errors and debugging

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 : 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.
  \'"\\"
  

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 : Practice indentation

Programs should be indented properly to make them easier to read:

• { brace → increase indent of following lines by one tab
• } brace → decrease indent of that line and following lines by one tab

Example:

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

Make sure that your Neighbors program has good indentation.

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Exercise : Practice indentation, cont'd

Our Indenter Tool web page can fix a program's indentation.

In this exercise, we will use the Indenter Tool to fix the following program that has poor indentation. Download it and open it in jGRASP:

• Icky.java

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Exercise : Practice indentation, cont'd

• Go to the CSE 142 Labs page. Under Lab 1, click Indenter Tool.
• Copy/paste the Icky.java code into the Indenter's text box.
• Click Fix Indentation. (Indent by 3 spaces.)
  Paste the properly indented code in the box below.

public class Icky {
   public static void main(String[] args) {
      System.out.println("Well-indented programs");
      System.out.println("look much better.");
      System.out.println("Please fix me");
      System.out.println("so I look nicer");
   }
}

Exercise : Syntax errors

• The following program contains 11 errors! Work with a partner to find them all. You may want to run the program and jGRASP and see what kinds of error messages are shown for each kind of mistake.

  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 : Readings

Read one or more of the following articles and discuss with your neighbors:

• What is computer science?
• Intel Seeks to Eliminate Computer Passwords With Wave of Hand
• Talking cars help each other see around blind corners
• Computer program can identify rough sketches