Building Java Programs

Lab: Return

Except where otherwise noted, the contents of this document are Copyright 2019 Stuart Reges and Marty Stepp.

Lab goals

Goals for this problem set:

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 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.