Test your loop expressions in Practice-It! using the checkmark icon above. Use your expressions in the loop tests of the inner loops of your code.
Exercise : SlashFigure
Write a Java program in a class named SlashFigure to produce the following output with nested for loops. Use a loop table if necessary to figure out the expressions.
A class constant is a global value that cannot be changed.
public static final typename = expression;
Example:
public static final int DAYS_PER_WEEK = 7;
public static final double TAX_RATE = 0.10;
for loop expressions w/ constant
When adding a class constant to a loop expression, it affects the constant that must be added in the expression.
Suppose we have the two loop expressions below for figure sizes of 5 and 9. The third line of the table shows the general formula that would be used if we turned our figure's size into a constant named SIZE.
size
expression
relationship
5
8 * line + 16
16 = 3 * 5 + 1
9
8 * line + 28
28 = 3 * 9 + 1
SIZE
8 * line + (3 * SIZE + 1)
continued on the next slide ...
Exercise : for loop table w/ constant
You already found loop expressions for the slash figure at size 6. Now make a table at size 4 and use the two to generalize the loop expression in terms of a constant for the figure size.
Breakpoints help you debug programs (tutorial). Copy/paste this code into jGRASP:
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public class Numbers {
public static void main(String[] args) {
int number = 42;
for (int i = 1; i <= 1000; i++) {
number = number * 37 % 103;
}
int number2 = number * number;
System.out.println("result = " + number2);
}
}
Set a breakpoint on the statement inside the for loop (line 5). Do this by moving your cursor to the beginning of that line until you see a
stop-sign icon
and then clicking.
continued on the next slide...
Exercise - jGRASP Debugger
Now you can debug the program by clicking on the debug icon
(looks like a ladybug).
It will stop every time it hits line 5.
Then you can see the values of variables i and number.
To continue running, press the top-left play
or step
buttons.
Using this approach, fill in the table below indicating what value number has when i has the given value.
Keep in mind that you are figuring out what value number has just before it executes this line of code.
i = 1, number =
42
i = 2, number =
9
i = 3, number =
24
i = 4, number =
64
continued on the next slide...
Exercise - jGRASP Debugger
Click on the stop-sign
a second time to get rid of it and then set a new stop point on line number 7.
This will allow you to find out the value of number after the loop is done executing.
Again press "play"
to start the programming running again.
At line 7, what is the value of number?
69
Variable Scope
A variable's scope is the part of a program in which it exists. In Java, the scope of a variable starts when it is declared and ends when the closing curly brace for the block that contains it is reached. A variable is said to be in scope where it is accessible.
public class Example {
public static void main(String[] args) {
performTest();
}
public static void performTest() {
int count = 12;
for (int i = 1; i <= 12; i++) {
runSample();
System.out.print(count);
}
}
public static void runSample() {
System.out.print("sample");
}
}
In which of these two blocks is the variable count in scope?
Parameters
A parameter allows you to pass in a value to a method as you call it.
public static void name(typename) { // declare
methodName(expression); // call
Example:
public static void squared(int num) {
System.out.println(num + " times " + num + " is " + (num * num));
}
...
squared(3); // 3 times 3 is 9
squared(8); // 8 times 8 is 64
Exercise : Parameters output
What two values are printed by each call below? Write them in the boxes provided.
public class MysteryNums {
public static void main(String[] args) {
int x = 15;
sentence(x, 42); //1542
int y = x - 5;
sentence(y, x + y); //1025
}
public static void sentence(int num1, int num2) {
System.out.println(num1 + " " + num2);
}
}
Solving "Parameter Mystery" problems
Note the values passed, the order in which they are passed, and the order the method prints them.
public class MysterySoda {
public static void main(String[] args) {
String soda = "coke";
String pop = "pepsi";
String pepsi = "soda";
// #1 = "coke", #2 = "pepsi", #3 = "soda"carbonated(soda, pop, pepsi);
}
// #1 #2 #3
public static void carbonated(String coke, String soda, String pop) {
// #2 #3 #1
System.out.println("say " + soda + " not " + pop + " or " + coke);
}
}
output: // say#2pepsi not#3soda or#1coke
Exercise : Parameter Mystery
Fill in the boxes with the output that each method call would produce:
public class MysterySoda {
public static void main(String[] args) {
String soda = "coke";
String pop = "pepsi";
String coke = "pop";
String pepsi = "soda";
String say = pop;
carbonated(soda, pop, pepsi); // say pepsi not soda or coke
carbonated(coke, soda, pop); //say coke not pepsi or pop
carbonated(pop, pepsi, pepsi); //say soda not soda or pepsi
carbonated("pop", pop, "koolaid"); //say pepsi not koolaid or pop
carbonated(say, "say", pop); //say say not pepsi or pepsi
}
public static void carbonated(String coke, String soda, String pop) {
System.out.println("say " + soda + " not " + pop + " or " + coke);
}
}
Exercise : Parameter Mystery
Fill in the boxes with the output that each method call would produce:
public class MysteryNumbers {
public static void main(String[] args) {
String one = "two";
String two = "three";
String three = "1";
int number = 20;
sentence(one, two, 3); // three times two = 6
sentence(two, three, 14); // 1 times three = 28
sentence(three, three, number + 1); // 1 times 1 = 42
sentence(three, two, 1); // three times 1 = 2
sentence("eight", three, number / 2); // 1 times eight = 20
}
public static void sentence(String three, String one, int number) {
System.out.println(one + " times " + three + " = " + (number * 2));
}
}
Exercise : Parameter Mystery
Fill in the boxes with the output that each method call would produce:
public class Mystery {
public static void main(String[] args) {
String hear = "bad";
String song = "good";
String good = "hear";
String walk = "talk";
String talk = "feel";
String feel = "walk";
claim(feel, song, feel); // to walk the walk is good
claim(good, hear, song); // to hear the good is bad
claim(talk, "song", feel); // to feel the walk is song
claim("claim", talk, walk); // to claim the talk is feel
}
public static void claim(String hear, String good, String song) {
System.out.println("to " + hear + " the " + song + " is " + good);
}
}
Exercise : Syntax errors
The following Java program has 10 errors. Can you find all of them?
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public class Parameters {
public static void main() {
double bubble = 867.5309;
double x = 10.01;
printer(double x, double y);
printer(x);
printer("barack", "obama");
System.out.println("z = " + z);
}
public static void printer(x, y double) {
int z = 5;
System.out.println("x = " + double x + " and y = " + y);
System.out.println("The value from main is: " + bubble);
}
}
Copy and paste the code into jGrasp and see if you can
fix the errors.
Exercise - answer
line 2: method main requires parameter String[] args
line 5: cannot use variable y without declaring and
initializing it
line 5: cannot declare the type of x and y in
the method call
line 6: cannot call printer without the correct number of
parameters (2, in this case)
line 7: cannot call printer by passing the incorrect type
of parameters (double, in this case)
line 8: cannot refer to the variable z: it is in scope
inside printer, not main
line 11: must provide a type for x
line 11: the name and type for parameter y are in the
wrong order (should be type, then name)
line 13: cannot declare the type of x in
the println
line 14: cannot refer to the variable bubble: it is in
scope inside main, not printer
Exercise - Corrected
version
Here is a corrected version of the program:
public class Parameters {
public static void main(String[] args) {
double bubble = 867.5309;
double x = 10.01;
double y = 5.3;
printer(double x, double y);
printer(x, 0.0);
printer("barack", "obama");
int z = 5;
System.out.println("z = " + z);
}
public static void printer(double x, y double double y) {
int z = 5;
System.out.println("x = " + double x + " and y = " + y);
System.out.println("The value from main is: " + bubble);
}
}
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:
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);
...
}
Exercise : Syntax errors
The following program contains 12 mistakes! What are they?
Once you think you've found them all, compile/run your corrected code.
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import Java.*;
public class PrettyPicture {
public static void main(String[] args) {
DrawingPanel panel = DrawingPanel(220, 150);
setBackgroundColor(Color.YELLOW);
Graphics g = panel.Graphics();
panel.setColor(new Color.BLUE);
g.drawRectangle(50, 25);
g.setColor("RED");
g.fillEllipse(130, 25, 42.1, 40.5);
}
}
answer on next slide...
Exercise - answer
line 1: incorrect import statement; should import java.awt.*
line 5: missing word new before 2nd occurrence of DrawingPanel
line 6: method name should be setBackground
line 6: missing panel. before setBackground
line 8: method name should be getGraphics
line 9: the setColor method is part of object g, not panel
line 9: should not write new before Color.BLUE
line 10: method name should be drawRect
line 10: missing two parameters from drawRect (width and height)
line 11: color should be specified as Color.RED, not "RED"
line 12: method name should be fillOval
line 12: wrong parameter types; width/height must be integers
Exercise - corrected version
Here is a corrected version of the program:
import java.awt.*;
public class PrettyPicture {
public static void main(String[] args) {
DrawingPanel panel = new DrawingPanel(220, 150);
panel.setBackground(Color.YELLOW);
Graphics g = panel.getGraphics();
g.setColor(Color.BLUE);
g.drawRect(50, 25, 10, 10);
g.setColor(Color.RED);
g.fillOval(130, 25, 42, 40);
}
}
Exercise : Face
Write a complete Java program that draws the following output:
window size: 220 x 150 px
overall face circle: 100 px diameter; top-left corner at (10, 30)
eyes: blue circles, 20 px diameter; top-left at (30, 60) and (70, 60)
mouth: red line from (40, 100) to (80, 100)
Exercise - answer
import java.awt.*;
public class Face1 {
public static void main(String[] args) {
DrawingPanel panel = new DrawingPanel(220, 150);
Graphics g = panel.getGraphics();
g.drawOval(10, 30, 100, 100); // face outline
g.setColor(Color.BLUE);
g.fillOval(30, 60, 20, 20); // eyes
g.fillOval(70, 60, 20, 20);
g.setColor(Color.RED); // mouth
g.drawLine(40, 100, 80, 100);
}
}
Exercise : Stairs loop table
Consider the output at right.
The first stair's top-left corner is at position (5, 5).
The first stair is 10x10 px in size.
Each stair is 10px wider than the one above it.
Fill in the table below with the coordinates and sizes of the first five stairs.
Note which values change and which ones stay the same.
stair
x
y
width
height
1
5
5
10
10
2
5
15
20
10
3
5
25
30
10
4
5
35
40
10
5
5
45
50
10
Exercise : Stairs
Write a complete Java program to draw the stairs.
Copy/paste the code template below into jGRASP and fill in your own expressions or values for each stair's x, y, width, and height.
Use your table from the previous slide to help you find the correct expressions.
The values that change for each stair should become expressions in terms of the loop counter variable, i.
import java.awt.*;
public class Stairs1 {
public static void main(String[] args) {
DrawingPanel panel = new DrawingPanel(110, 110);
Graphics g = panel.getGraphics();
for (int i = 0; i < 10; i++) {
g.drawRect(x, y, width, height);
}
}
}
Exercise : Stairs 2
Modify your stairs program to draw one (or all) of the following outputs.
Modify only the body in your for loop.
(You may want to make a new table to find the expressions for x, y, width, and height.)
→
Exercise - answer
To get each output, change the for loop body to the following:
Suppose you have an existing program that draws the "face" figure at right.
Let's modify the program using methods and parameters so that we can draw several faces at different locations.
Modify the Face program to draw the following output.
Write a parameterized method that draws a face at different positions.
window size: 320 x 180 px
faces' top-left corners at (10, 30) and (150, 50)
Exercise - answer
import java.awt.*;
public class Face2 {
public static void main(String[] args) {
DrawingPanel panel = new DrawingPanel(320, 180);
Graphics g = panel.getGraphics();
drawFace(g, 10, 30);drawFace(g, 150, 50);
}
public static void drawFace(Graphics g, int x, int y) {
g.setColor(Color.BLACK);
g.drawOval(x, y, 100, 100);
g.setColor(Color.BLUE);
g.fillOval(x + 20, y + 30, 20, 20);
g.fillOval(x + 60, y + 30, 20, 20);
g.setColor(Color.RED);
g.drawLine(x + 30, y + 70, x + 70, y + 70);
}
}
Exercise : Face 3
Modify your previous Java program to draw the following output.
Use a for loop with your parameterized method to draw faces at different positions.
window size: 520 x 180 px
faces' top-left at (10, 30), (110, 30), (210, 30), (310, 30), and (410, 30)
Exercise - answer
import java.awt.*;
public class Face3 {
public static void main(String[] args) {
DrawingPanel panel = new DrawingPanel(520, 180);
Graphics g = panel.getGraphics();
for (int i = 0; i < 5; i++) {
drawFace(g, 10 + i * 100, 30);
}
}
public static void drawFace(Graphics g, int x, int y) {
g.setColor(Color.BLACK);
g.drawOval(x, y, 100, 100);
g.setColor(Color.BLUE);
g.fillOval(x + 20, y + 30, 20, 20);
g.fillOval(x + 60, y + 30, 20, 20);
g.setColor(Color.RED);
g.drawLine(x + 30, y + 70, x + 70, y + 70);
}
}
-->
Exercise : Number lines, part 1
Write some nested for loops to produce the
following output:
Write a method named printGrid that accepts two integers representing a number of rows and columns and prints a grid of integers from 1 to (rows*columns) in column-major order.
Check your answer using Practice-It from the check-mark icon above.
Exercise : printSquare
Write a method printSquare that accepts min and max integers and prints a square of lines of increasing numbers. The first line should start with the minimum; each line that follows should start with the next-higher number. The numbers on a line wrap back to the minimum after it hits the maximum. For example, the call:
printSquare(3, 6);
should produce the following output:
3456
4563
5634
6345
If the maximum passed is less than the minimum, the method produces no output. Check your answer using Practice-It from the check-mark icon above.
Hint: remember the mod operator or think of each lines as having two parts...
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!
and then clicking.
(looks like a ladybug).
or step
buttons.
Face.java
