Building Java Programs

Lab 3: Ch. 3: Parameters, Return

Except where otherwise noted, the contents of this document are Copyright 2012 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp and Stuart Reges

Today's lab

Goals for today:


A parameter allows you to pass in a value to a method as you call it.

public static void name(type name) {   // declare
methodName(expression);                // call


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

Solving "Parameter Mystery" problems

Exercise : Parameter Mystery practice-it

Exercise : Syntax errors

Exercise - answer

  1. line 5: cannot use variable y without declaring and initializing it
  2. line 5: cannot declare the type of y in the method call
  3. line 6: cannot call printer without the correct number of parameters (2, in this case)
  4. line 7: cannot call printer by passing the correct type of parameters (double, in this case)
  5. line 8: cannot refer to the variable z: it is in scope inside printer, not main
  6. line 11: must provide a type for x
  7. line 11: must provide a variable name for the second parameter
  8. line 12: must refer to the parameters using the exact same spelling
  9. line 13: cannot refer to variables in main that were not passed into printer as a parameter

Exercise - Corrected version

Exercise : printSquare practice-it

Exercise : printGrid practice-it

Exercise : Parameter Mystery practice-it

Exercise : Parameter Mystery practice-it

Returning Values

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


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

Math expression syntax

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

Exercise - Math expressions

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 re-assign it using the = operator.

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

Exercise : area practice-it

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.

Exercise : pay practice-it

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.

String methods

Method name Description
charAt(index) character at given index
indexOf(str) index where the start of the given String appears in this string (-1 if not found)
length() number of characters in this String
replace(str1, str2) a new string with all occurrences of str1 changed to str2
substring(index1, index2)
or substring(index1)
the characters in this string from index1 (inclusive) to index2 (exclusive); if index2 is omitted, grabs till end of string
toLowerCase() a new string with all lowercase letters
toUpperCase() a new string with all uppercase letters

Exercise : String expressions

Write the results of each expression with Strings in "quotes" and characters in single quotes ('a')

//       index 0123456789012345
String str1 = "Frodo Baggins";
String str2 = "Gandalf the GRAY";
"o Baggins"
str2.substring(3, 14)
"dalf the GR"
str2.replace("a", "oo")
"Goondoolf the GRAY"
str2.replace("gray", "white")
"Gandalf the GRAY"
"str1".replace("r", "range")

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!