University of Washington, CSE 142

Lab 3: Parameters, Graphics

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

Basic lab instructions

• Mouse over highlighted words if you're not sure what they mean!
• Talk to your classmates for help.
• 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 much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
• Feel free to complete problems in any order.
• Make sure you've signed in on the sign-in sheet before you leave!

Today's lab

Goals for today:

• gain a better understanding of how scope works
• learn the basics of declaring and passing parameters to methods
• learn to set a breakpoint with the jGRASP debugger to find out what is going on inside a program
• use the instructor-provided DrawingPanel and Java's Graphics and Color classes

Scope

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public static void main(String[] args) {
    int x = 5;
    for (int i = 1; i <= 5; i++) {
        int y = 10;
	System.out.println(x) // x is still in scope here!
    }
    System.out.println(x) // x is still in scope here, too!
}

• x is in scope between its declaration on line 2, and the curly brace that encloses it on line 8.
• y is in scope between its declaration on line 4 and the curly brace that encloses it on line 6.
• Loop variables are in scope between their for loop's { }. So, i is in scope between lines 3 - 6.

Note: Two variables with the same name cannot both exist within the same scope.

Exercise : Scoping 1

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public static void main(String[] args) { 
    mystery();
}

public static void mystery() {
    int x = 50;
    System.out.println(x);
}

50

Exercise : Scoping 2

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public static void main(String[] args) { 
   int x = 15;
   mystery();
}

public static void mystery() {
    int x = 50;
    System.out.println(x);
}

50

Exercise : Scoping 3

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public static void main(String[] args) { 
   int x = 15;
   mystery();
   System.out.println(x);
}

public static void mystery() {
   int x = 50;
   x = x + 5; 
   System.out.println(x);
}

55 
15

Parameters

A parameter allows you to pass in a value (an expression or a variable!) to a method as you call it.

Example:

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public static void main(String[] args) {
   squared(3);          // 3 times 3 is 9
   squared(8);          // 8 times 8 is 64  
   
   int x = 5;
   squared(x);          // 5 times 5 is 25

   squared(2 + 2)       // 4 times 4 is 16
}

public static void squared(int num) {
    System.out.println(num + " times " + num + " is " + (num * num));
}

Exercise : Parameters 1

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public static void main(String[] args) { 
   int x = 15;
   mystery(x);
}

public static void mystery(int y) {
   System.out.println(y);
}

15

Exercise : Parameters 2

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public static void main(String[] args) { 
   int x = 15;
   int y = 20;
   mystery(x);
}

public static void mystery(int y) {
   System.out.println(y);
}

15

Value Semantics

When passing around ints, doubles, chars, or Strings, we need to consider value semantics.

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public static void main(String[] args) {
   int x = 5;
   int y = x;
   x = 10;     // note that y still = 5. It does not change values as x changes.
}

continued on next slide...

Value Semantics (continued)

This concept persists within parameters. For example:

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public static void main(String[] args) {
    int x = 5;
    mystery(x);
    // x still equals 5.
}

public static void mystery(int x) {
    x = x + 5;     // the value of x in mystery is now 10. main's x is unchanged.
}

Exercise : Parameters 3

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public static void main(String[] args) { 
   int x = 15;
   mystery(x);
   System.out.println(x);
}

public static void mystery(int x) {
   x = x + 50;
   System.out.println(x);
}

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15

Parameter Ordering

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public static void main(String[] args) {
   mystery(5, 8, "abc");     // prints "5 8 abc
                             //         8 abc 5"
}

public static void mystery(int x, int y, String z) {
   System.out.println(x + " " + y + " " + z);
   System.out.println(y + " " + z + " " + x);
}

Exercise : String Parameters 1

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public static void main(String[] args) { 
   String x = "y";
   mystery(x);
}

public static void mystery(String x) {
   System.out.println(x);
}

y

Exercise : String Parameters 2

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public static void main(String[] args) { 
   String x = "y";
   mystery(x);
   mystery("x");
   mystery("y");
}

public static void mystery(String x) {
   System.out.println(x);
}

y
x
y

Graphics

Now we'll explore several exercises related to drawing graphics. (none)

• We'll use a provided class DrawingPanel that works with Java classes Graphics (a "pen" for drawing) and Color.
• Download DrawingPanel.java by right-clicking the link and selecting "save as".
• You can't open DrawingPanel by clicking on an icon. But if you make a DrawingPanel in Java (which we'll do soon!), a panel will open in a pop-up! You can interact with this window with your mouse, and can check the correctness of your drawings in DrawingPanel by clicking File, Compare to Web File....

Checkpoint: Congratulations!

Nice job making it this far--labs are tough! Feel free to work with the person next to you for the remaining slides. Labs are a unique opportunity (unlike homework) to collaborate directly on ideas, and practice peer programming.

These next problems get a little more challenging as we explore earlier concepts further.

We put a lot of problems in here so that you have plenty to refer back to later when working on homework. Don't feel bad if you don't finish all of them--Brett can't finish them all in a 50 minute lab, either! :)

Forest the cat says good job!

Parameterized methods and Graphics

When you want to divide a graphical program into multiple drawing methods, you must pass Graphics g as a parameter in addition to any other parameters. Example:

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public static void main(String[] args) {
    DrawingPanel panel = new DrawingPanel(400, 300);
    Graphics g = panel.getGraphics();
    ...
    drawStuff(g, 13, 52, 7);
}

public static void drawStuff(Graphics g, int a, int b, int c) {
    g.drawLine(a, 45, b, c);
    ...
}

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!