Except where otherwise noted, the contents of this document are Copyright 2012 Stuart Reges and Marty Stepp.
lab document created by Marty Stepp, Stuart Reges, Whitaker Brand and Hélène Martin
Goals for today:
return
values to send data between methodsif/else
statements to select between multiple code actionsScanner
to create interactive programs that read user inputclass mean for a1 (i.e., average score for assignment 1)  / 10 
9.19 
class mean for lab1 (i.e., average score for lab 1)  / 2 
1.95 
A return value is when a method sends a value back to the code that called it.
public static type name(parameters) { // declare
...
return expression;
}
variableName = methodName(parameters); // call
Example:
public static double fToC(double tempF) { return (tempF  32) * 5.0 / 9.0; } ... double bodyTemp = fToC(98.6); // bodyTemp stores 37.0 double freezing = fToC(32); // freezing stores 0.0
Method  Description  Example 

Math.abs

absolute value 
Math.abs(308) returns 308

Math.ceil

ceiling (rounds upward) 
Math.ceil(2.13) returns 3.0

Math.floor

floor (rounds downward) 
Math.floor(2.93) returns 2.0

Math.max

max of two values 
Math.max(45, 207) returns 207

Math.min

min of two values 
Math.min(3.8, 2.75) returns 2.75

Math.pow

power 
Math.pow(3, 4) returns 81.0

Math.round

round to nearest integer 
Math.round(2.718) returns 3

Math.sqrt

square root 
Math.sqrt(81) returns 9.0

Write the results of each expression.
Use the proper type (such as .0
for a double
).
Note that a variable's value changes only if you reassign it using the =
operator. Discuss any errors you make with your neighbor.
double grade = 2.7; Math.round(grade); // grade = 2.7 grade = Math.round(grade); // grade = 3.0 double min = Math.min(grade, Math.floor(2.9)); // min = 2.0 double x = Math.pow(2, 4); // x = 16.0 x = Math.sqrt(64); // x = 8.0 int count = 25; Math.sqrt(count); // count = 25 count = (int) Math.sqrt(count); // count = 5 int a = Math.abs(Math.min(1, 3)); // a = 3
Consider the following method for converting milliseconds into days:
// converts milliseconds to days public static double toDays(double millis) { return millis / 1000.0 / 60.0 / 60.0 / 24.0; }
Write a similar method named area
that takes as a parameter the radius of a circle and that returns the area of the circle.
For example, the call area(2.0)
should return 12.566370614359172
.
Recall that area can be computed as π times the radius squared and that Java has a constant called Math.PI
.
Write a method named pay
that accepts two parameters: a real number for a TA's salary, and an integer for the number of hours the TA worked this week.
The method should return how much money to pay the TA.
For example, the call pay(5.50, 6)
should return 33.0
.
The TA should receive "overtime" pay of 1 ½ normal salary for any hours above 8.
For example, the call pay(4.00, 11)
should return (4.00 * 8) + (6.00 * 3) or 50.0
.
if/else
StatementsAn if/else statement lets your program choose between 2 or more options.
if (test) { statement(s); } else { statement(s); }
Example:
if (gpa >= 2.0) { System.out.println("Welcome to Mars University!"); } else { System.out.println("Please apply again soon."); }
if
/else
mysteryConsider the following Java code. Fill in the boxes with the output produced by each of the method calls.
public static void mystery(int n) { System.out.print(n + " "); if (n > 10) { n = n / 2; } else { n = n + 7; } if (n * 2 < 25) { n = n + 10; } System.out.println(n); } 

Write a method named numUnique
that accepts three integers as
parameters and that returns the number of unique integers among the three.
For example, the call numUnique(18, 3, 4)
should return 3
because the parameters have 3 different values. By contrast, the
call numUnique(6, 7, 6)
would return 2 because there are only
2 unique numbers among the three parameters: 6 and 7.
Compare your solution to your neighbors'. Did you all solve it the same way?
Write a method quadrant
that accepts a pair of real numbers x and y and returns the quadrant for that point:
For example, quadrant(2.3, 14.2)
returns 2. If the point falls directly on either axis, return 0.
Scanner
Method name  Description 

nextInt()

reads and returns the next token as an int , if possible

nextDouble()

reads and returns the next token as double , if possible

next()

reads and returns a single word as a String

nextLine()

reads and returns an entire line as a String

Example:
import java.util.*; // so you can use Scanner ... Scanner console = new Scanner(System.in); System.out.print("How old are you? "); // prompt int age = console.nextInt(); System.out.println("You typed " + age);
Write a complete program DevryAdmit
with the behavior shown below.
Use the Scanner
to read user input for a student's grade point average and SAT exam score.
A GPA below 1.8 will cause the student to be rejected; an SAT score below 900 will also cause a rejection. Otherwise the student is accepted.
Devry University admission program What is your GPA? 3.2 What is your SAT score? 1280 You were accepted!
Check your answer using PracticeIt from the checkmark icon above.
int sum = 0;
for (int i = 1; i <= 100; i++) {
sum = sum + i;
}
System.out.println(sum); // 5050
Scanner
sumCopy and paste the following code into jGrasp.
public class SumNumbers { public static void main(String[] args) { int low = 1; int high = 1000; int sum = 0; for (int i = low; i <= high; i++) { sum += i; } System.out.println("sum = " + sum); } }
continued on next slide...
Scanner
sum
Modify the code to use a Scanner
to prompt the user for the values of low
and high
. Below is a sample execution in which the user asks for the same values as in the original program (1 through 1000):
low? 1 high? 1000 sum = 500500
Below is an execution with different values for low
and high
:
low? 300 high? 5297 sum = 13986903
You should exactly reproduce this format.
Use the Output Comparison Tool to check your work.
Write a method named pow
that accepts a base and an exponent
as parameters and returns the base raised to the given power. For example,
the call pow(3, 4)
returns 3 * 3 * 3 * 3 or 81. Do not
use Math.pow
in your solution; use a cumulative algorithm
instead. Assume that the base and exponent are nonnegative. See ch4 lecture
slides on cumulative sums for a hint.
if
/else
factoringif
/else
. For example:
if (x < 30) { a = 2; x++; System.out.println("CSE 142 TAs are awesome! " + x); } else { a = 2; System.out.println("CSE 142 TAs are awesome! " + x); }
else
went away!)
a = 2; if (x < 30) { x++; } System.out.println("CSE 142 TAs are awesome! " + x);
if
/else
Factoringmain
and run it to make sure it works properly.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 
public class IfOops { public static void main(String[] args) { int a = 7, b = 42; minimum(a, b); if {smaller = a} { System.out.println("a is the smallest!"); } } public static void minimum(int a, int b) { // returns which int is smaller if (a < b) { int smaller = a; } else (a => b) { int smaller = b; } return int smaller; } } 
if
statement should use ()
parentheses, not {}
brackets=
should be ==
smaller
is out of scope herevoid
should be int
=>
should be >=
(or better
yet, no if
test is needed)int
when
returning itint smaller
is out of scope here (declare
outside if
or return directly)public class IfOops { public static void main(String[] args) { int a = 7, b = 42; int smaller = minimum(a, b); if (smaller == a) { System.out.println("a is the smallest!"); } } public staticvoidint minimum(int a, int b) { // returns which int is smaller int smaller; if (a < b) {intsmaller = a; } elseif (a >= b){intsmaller = b; } returnintsmaller; } }
public class AgeCheck { public static void main(String[] args) { int myAge = 19; // I am 19; let me see if I can drive message(myAge); } // Displays message about driving to user based on given age public static void message(int age) { if (myAge >= 16) { System.out.println("I'm old enough to drive!"); } if (myAge <= 16) { System.out.println("Not old enough yet... :*("); } } }
if
and else
in a clumsy way.
Improve the style of the code.
public class AgeCheck { public static void main(String[] args) { int myAge = 19; // I am 19; let me see if I can drive message(myAge); } // Displays a message about driving to user based on given age public static void message(int age) { if (age >= 16) { System.out.println("I'm old enough to drive!"); } else { System.out.println("Not old enough yet... :*("); } } }
AgeCheck
program's message
method with:
// Possibly prints some message(s) to the user based on the given age public static void message(int age) { if (age >= 21) { System.out.println("I can legally purchase alcohol!"); } else if (age >= 17) { System.out.println("I can purchase a ticket to an Rrated movie."); } else if (age >= 16) { System.out.println("I can get my driver's license!"); } }
if
s
and else
s in this method to behave properly.
You're thinking about going with your friends to a
movie. Write a Java method seeMovie
that
accepts two parameters: the cost of a ticket in dollars,
and the rating number of stars the movie received out of
5. The method should print how interested you are (very, sortof, or not).
Use the following criteria:
Write a Java program that draws the following output using a for
loop.
import java.awt.*; public class Spiral { public static void main(String[] args) { DrawingPanel panel = new DrawingPanel(170, 170); Graphics g = panel.getGraphics(); for (int i = 0; i < 8; i++) { g.drawLine( 10*i, 10 + 10*i, 160  10*i, 10 + 10*i); // top g.drawLine(160  10*i, 10 + 10*i, 160  10*i, 160  10*i); // right g.drawLine( 10 + 10*i, 160  10*i, 160  10*i, 160  10*i); // bottom g.drawLine( 10 + 10*i, 20 + 10*i, 10 + 10*i, 160  10*i); // left } } }
import java.awt.*; public class Spiral { public static void main(String[] args) { DrawingPanel panel = new DrawingPanel(170, 170); Graphics g = panel.getGraphics(); int x = 0, y = 10; int len = 160; for (int i = 0; i < 8; i++) { g.drawLine(x, y, x + len, y); // right x = x + len; len = len  10; g.drawLine(x, y, x, y + len); // down y = y + len; g.drawLine(x, y, x  len, y); // left x = x  len; len = len  10; g.drawLine(x, y, x, y  len); // up y = y  len; } } }
If you finish all the exercises, try out our PracticeIt 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!