University of Washington, CSE 142

Lab 1: Static Methods; Expressions

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

Basic lab instructions

Talk to your classmates for help. You can even work on the lab with a partner if you like.
You may want to bring your textbook to future labs to look up syntax and examples.
Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
Complete as many problems as you can within the allotted time. You don't need to keep working on these exercises after you leave the lab.
Feel free to complete problems in any order.
Before you leave today, make sure to check in with one of the TAs in the lab to get credit for your work.

Today's lab

Goals for today:

meet some of your classmates
learn about the jGRASP editor software
gain familiarity with syntax errors and debugging
learn about various useful CSE 142 online tools and resources
practice using static methods to capture structure and redundancy (Ch. 1)
use expressions for numeric computations (Ch. 2)

Exercise : Meet your neighbors

In this exercise, you will meet your neighbors and write a program that displays facts about them.

Create a complete Java program in a class named Neighbors.

Introduce yourself to two people around you (get up to do so if you have no immediate neighbors). Ask them for their full name, including their middle initial. Make sure you know how to spell their name correctly! Ask them for an interesting fact about themselves.

Your program should output the name and interesting fact of two of your neighbors. You may put all of your code in the main method.

Your output should look like the following:

Talib K. Greene's mother is an English professor in New York.
Jermaine L. Cole was born in Germany.

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.

continued on the next slide...

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

continued on the next slide...

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.

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

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

Download a program from lecture and 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.

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

continued on the next slide...

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?

Compare your method choice with your neighbor's. Discuss any different decisions you made and ask a TA if you have questions about them.

Exercise : Practice verifying output

Your homework must match expected output exactly. You can use our Output Comparison Tool web page to check your output.

In this exercise, we will check whether a program produces correct output. Download the program and open it in your jGRASP editor.

WrongOutput.java

continued on the next slide...

Exercise : Practice verifying output, cont'd

Go to the CSE 142 Labs page. Under Lab 1, click Output Comparison Tool.
In the Output Comparison page, select WrongOutput.java from the list.
Run jGRASP and use it to edit WrongOutput.java. Copy/paste the program's output from jGRASP's console into the "Actual Output" box of the Output Comparison web page.
Click Compare on the page. (Also click Add Line Numbers on the web page.)

Which output line number does not match?	4
What should be the output for that line?	"Four score and seven years ago,
Which expected blank line number is missing?	8

Expressions

Recall that Java has expressions to represent math and other computations. Expressions may use operators, which are evaluated according to rules of precedence. Every expression produces a value of a given type.

Type	Description	Example	Result
`int`	integers (up to 2³¹ - 1)	`3 + 4 * 5`	`23`
`double`	real numbers (up to 10³⁰⁸)	`3.0 / 2.0 + 4.1`	`5.6`
`String`	text characters	`"hi" + (1 + 1) + "u"`	`"hi2u"`

Exercise : Expressions (2.1)

Write the results of each of the following expressions. If you're stuck, ask a TA or neighbor.

`12 / 5 + 8 / 4`		4
`3 * 4 + 15 / 2`		19
`-(1 + 2 * 3 + (1 + 2) * 3)`		-16
`42 % 5 + 16 % 3`		3
`2.5 * 2 + 17 / 4`		9.0
`4.5 / 3 / 2 + 1`		1.75

Exercise : More expressions (2.1)

Write the results of each of the following expressions.

`5 * 6 / 4 % 3 - 23 / (14 % 6)`		-10
`30 % 9 + 5 % 8 - 11 % 4 % 2`		7
`1 + 9 / 2 * 2.0`		9.0
`46 / 3 / 2.0 / 3 * 4/5`		2.0
`50 / 9 / 2.0 + 200 / 10 / (5.0 / 2)`		10.5

jGRASP Interactions Pane

jGRASP has a useful feature called the Interactions Pane that allows you to type in Java expressions or statements one at a time and instantly see their results.
To use it, run jGRASP and then click the "Interactions" tab near the bottom.

continued on the next slide...

Exercise : Using jGRASP Interactions Pane

In this exercise, you'll use the Interactions Pane to quickly discover the result of some expressions that would be difficult to evaluate by hand. Copy/paste each expression below into the Interactions Pane to evaluate it, then input the answer into this slide.

`123 * 456 - 789`		55299
`3.14 + 1.59 * 2.65`		7.3535
`2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2`		1024
`2 + 2 + "xyz" + 3 + 3`		"4xyz33"

(For the last expression, the Interactions Pane doesn't put "" quotes around Strings when displaying results, so you must add those yourself if needed. For example, if the Interactions Pane gives you a result of abc123, it should be written here as "abc123".)

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.

Sequence	Special character
`\n`	new-line (goes to the next line)
`\t`	tab (indents output by roughly 8 spaces)
`\"`	quotation mark
`\\`	backslash

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.)

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!

University of Washington, CSE 142

Lab 1: Static Methods; Expressions

University of Washington, CSE 142

Lab 1: Static Methods; Expressions

Basic lab instructions

Today's lab

Exercise : Meet your neighbors

Exercise : Practice indentation

Exercise : Practice indentation, cont'd

Exercise : Practice indentation, cont'd

Exercise : Syntax errors

Exercise - answer

Exercise - corrected version

Exercise : Exploring syntax errors

Static methods

Exercise : FightSong

Exercise : FightSong, cont'd

Exercise : Practice verifying output

Exercise : Practice verifying output, cont'd

Expressions

Exercise : Expressions (2.1)

Exercise : More expressions (2.1)

jGRASP Interactions Pane

Exercise : Using jGRASP Interactions Pane

Exercise : What's the output?

Escape sequences

Exercise : What's the output?

Exercise : MuchBetter

Exercise : Spikey

If you finish them all...

Exercise : `FightSong`

Exercise : `FightSong`, cont'd

Exercise : `MuchBetter`

Exercise : `Spikey`