Building Java Programs, 3rd edition

Lab: Return

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

Lab goals

Goals for this problem set:

• use return values to send data between methods

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

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

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. 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

Exercise : area

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.

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Exercise : pay

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.

• (Try solving this problem in Practice-It by clicking the icon above.)