Welcome to Lesson 16!


Learning Objectives

By the end of today's class, you should know...
  • What is a void method?
  • What is the difference between a void and non-void method?
  • How do you call a void method (compared to a non-void method)?
  • When do you use a void method?

Lesson 15 Practice Exam Questions

Write a proper Javadoc comment for the below method:

Name: lowerCaseCounter:
  • It takes a String parameter
  • It returns an int for the total number of lower case letters contained in the String


Void Methods

  • Previously we looked at methods that returned one value
  • Methods returning a value use a return statement return result;
  • A method that returns no value is called a void method
  • In Java, void methods are defined like methods that return a value
  • However, the keyword void replaces the return type
  • For example, what do you notice that is different about the following?
public static void displayDegrees(double degreeFarenheit) {
double degreeCelsius = 5.0 / 9 * (degreeFarenheit - 32);
System.out.println(degreeFarenheit + " degrees Fahrenheit is equivalent to "
         + degreeCelsius + " degrees Celsius.");
return;
}

  • There are only two differences between definitions for void methods and other methods:
  1. void return type
  2. return statement is optional and does not specify a value if used
  • If no return type is specified, the method returns after executing the last statement
  • Here is an example program using the void method shown above

Example Program With a void Method

public static void displayDegrees(double degreeFarenheit) {
double degreeCelsius = 5.0 / 9 * (degreeFarenheit - 32);
System.out.printf(degreeFarenheit
    + " degrees Fahrenheit is equivalent to "
    + "%.1f degrees Celsius.\n\n", degreeCelsius);
return; //optional
}

public static void main(String[] args) {
Scanner input = new Scanner(System.in);
double fTemperature;

System.out.print("Enter a temperature in Fahrenheit: ");
fTemperature = input.nextDouble();

//Notice method call without assigning result to variable
displayDegrees(fTemperature);
}

When to Write void Methods

  • When we use a non-void method, we are asking a question
  • The method returns a value in response to our question
double square_root = Math.sqrt(9.0);

  • When we use a void method, we are giving the computer a command
displayDegrees(212);

  • We do not expect or receive an answer

Common Errors With void Methods
  • Note that we cannot call a void method from a System.out.print statement
  • For example, the following causes a compile error:
System.out.println(displayDegrees(fTemperature)); // NO!

  • The reason is that a void methods does not return a value and System.out.println has nothing to print
  • Similarly, we cannot call a void method in an assignment statement:
double temp = displayDegrees(fTemperature); // NO!

  • There is nothing to assign to the variable temp


Activity 16.1: Into the Void (10 pts)
  • Void methods are useful for printing out information in a particular format.
  • Let's consider dates and times.
  • In America, we use the 12 hour clock, but in Europe, the 24 hour clock is used.
  • For example, in America, 8:30 at night is represented as 8:30pm, while in Europe, it is represented as 20:30.
  • In America, we write dates in this format MM-DD-YYYY. In Europe, dates are often written as DD.MM.YYYY
  • Let's write a program that uses void methods to format dates and times.
  • We will print each date and time in both the American and European formats for our user.
  • Open up Eclipse and create a new Java class named DateTime
  • Copy and paste the starter code below into your file:
/**
 * @author
 * CIS 36A, Activity 16.1
 */
import java.util.Scanner;
public class DateTime {

    public static void main(String[] args) {
        int year;
        int day;
        int month;
        int hour;
        int minutes;
        String dayEve;
        Scanner input = new Scanner(System.in);

        System.out.println("Welcome! This program will print dates and times "
            + "in both the American and European styles!\n");

        System.out.println("First, let's print a formatted date.\n");
  
        System.out.print("Please enter the current year: ");
        year = input.nextInt();
        System.out.print("Please enter the current month: ");
        month = input.nextInt();
        System.out.print("Please enter the current day: ");
        day = input.nextInt();
        System.out.println();
 
        //call to the formatDateAmerican method here
        //call to the formatDateEuropean method here
  
        System.out.println("\nNow, let's print a formatted time.\n");
  
        System.out.print("Please enter the current hour: ");
        hour = input.nextInt();
        System.out.print("Please enter the current minutes: ");
        minutes = input.nextInt();
        System.out.print("Please enter whether it is \"morning\" or \"evening\": ");
        dayEve = input.next();
        System.out.println();

  
        //call to the formatTimeAmerican method here
        //call to the formatTimeEuropean method here

        System.out.println("\nHave a good day!");
    }
   
}
  • Now, you need to write four methods as follows:
formatDateAmerican
takes as input three integer parameter, one for the year, one for the month and one for the day
prints a formatted version of the date to the console, using the format m/d/yyyy
returns nothing
formatDateEuropean
takes as input three integer parameters, one for the year, one for the month and one for the day
prints a formatted version of the date to the console, using the format d.m.yyyy
returns nothing
formatTimeAmerican
takes as input two integer parameters, one for the hour, one for the minutes, and a string parameter that contains either "morning" or "evening"
prints a formatted version of the time to the console, using the format H:MMam or H:MMpm
returns nothing
formatTimeEuropean
takes as input two integer parameters, one for the hour, one for the minutes, and a string parameter that contains either "morning" or "evening"
prints a formatted version of the time to the console, using the 24 hour clock. Note that there is no am or pm in this format.
returns nothing
  • These methods should be written outside of main (above or below is fine!).
  • When you are finished writing your methods, call them in main in the place indicated by the comments.
  • Upload your dateTime.java file to Canvas.
Your output should look identical the output below when you are finished:


More Method Practice


Activity 16.2: More Method Practice (10 pts)
  • Copy and paste the starter code into a new file called Methodical.java
  • Write the required methods as described by the comments (i.e. you fill in the body of the method).
  • Then, run the code when you are finished to see if you wrote the methods correctly.
  • Check the test results and make any alterations to your methods as necessary.
  • When all of the tests pass, upload your code to Canvas.


/*
 * @author
 * CIS 36A, Activity 16.2
 */

public class Methodical {
    /*
     * Given two int values, returns their sum.
     * Unless the two values are the same, then return double their sum.
     * sumDouble(1, 2) → 3
     * sumDouble(3, 2) → 5
     * sumDouble(2, 2) → 8
     */
   
    public static int sumDouble(int a, int b){
        return 0;
    }
   
    /*
     * Given 2 ints, a and b, return true if one of them is 10
     * or if their sum is 10.
     * makes10(9, 10) → true
     * makes10(9, 9) → false
     * makes10(1, 9) → true
     */

    public static boolean makes10(int a, int b) {
        return false;
    }
    /*
     * We have two monkeys, a and b, and the parameters
     * aSmile and bSmile indicate if each is smiling.
     * We are in trouble if they are both smiling
     * or if neither of them is smiling.
     * Return true if we are in trouble.
     * monkeyTrouble(true, true) → true
     * monkeyTrouble(false, false) → true
     * monkeyTrouble(true, false) → false
     */

    public static boolean monkeyTrouble(boolean aSmile, boolean bSmile) {
        return false;
    }
   
    /*Return true if the given non-negative number is a multiple of 3
    * or a multiple of 5. Use the % "modulus" operator
    * or35(3) → true
    * or35(10) → true
    * or35(8) → false
    * */

    public static boolean or35(int a) {
        return false;
    }

    /*
     * Given a String, return a new string where "not " has been added to the front.
     * However, if theString already begins with "not", return the string unchanged.
     * notString("candy") → "not candy"
     * notString("x") → "not x"
     * notString("not bad") → "not bad"
     */
   
    public static String notString(String str) {
        return "";
    }

    /*
     * Given a string, return a new string where the first and last chars have been exchanged
     * frontBack("code") → "eodc"
     * frontBack("a") → "a"
     * frontBack("ab") → "ba"
     */
   
    public static String frontBack(String str) {
        return "";
    }
   
    /*
     * We'll say that a number is "teen" if it is in the range 13..19 inclusive.
     * Given 3 int values, return true if 1 or more of them is/are teen.
     * hasTeen(13, 20, 10) → true
     * hasTeen(20, 19, 10) → true
     * hasTeen(20, 10, 13) → true
     */

    public static boolean hasTeen(int num1, int num2, int num3) {
        return false;
    }
   



    public static void main(String[] args)
    {
        int result;
        boolean answer;
        String value;
        System.out.println("***Testing sumDouble***\n");
        result = sumDouble(1, 2);
        System.out.println("Should print 3: " + result);
        result = sumDouble(3, 2);
        System.out.println("Should print 5: " + result);
        result = sumDouble(2, 2);
        System.out.println("Should print 8: " + result +"\n");
      
        System.out.println("***Testing makes10***\n");
        answer = makes10(9, 10);
        System.out.println("Should be true: " + answer);
        answer = makes10(9, 9);
        System.out.println("Should be false: " + answer);
        answer = makes10(1, 9);
        System.out.println("Should be true: " + answer + "\n");
          
        System.out.println("***Testing monkeyTrouble***\n");
        answer = monkeyTrouble(true, true);
        System.out.println("Should be true: " + answer);
        answer = monkeyTrouble(false, false);
        System.out.println("Should be true: " + answer);
        answer = monkeyTrouble(true, false);
        System.out.println("Should be false: " + answer + "\n");
      
        System.out.println("***Testing or35***\n");
        answer = or35(3);
        System.out.println("Should be true: " + answer);
        answer = or35(10);
        System.out.println("Should be true: " + answer);
        answer = or35(8);
        System.out.println("Should be false: " + answer + "\n");
      
        System.out.println("***Testing notString***\n");
        value = notString("candy");
        System.out.println("Should be not candy: " + value);
        value = notString("x");
        System.out.println("Should be not x: " + value);
        value = notString("not bad");
        System.out.println("Should be not bad: " + value + "\n");
      
        System.out.println("***Testing frontBack***\n");
        value = frontBack("code");
        System.out.println("Should be eodc: " + value);
        value = frontBack("a");
        System.out.println("Should be a: " + value);
        value = frontBack("ab");
        System.out.println("Should be ba: " + value + "\n");
      
      
        System.out.println("***Testing hasTeen***\n");
        answer = hasTeen(13, 20, 10);
        System.out.println("Should be true: " + answer);
        answer = hasTeen(20, 19, 10);
        System.out.println("Should be true: " + answer);
        answer = hasTeen(20, 10, 13) ;
        System.out.println("Should be true: " + answer);
        answer = hasTeen(20, 10, 45) ;
        System.out.println("Should be false: " + answer + "\n");
      
        System.out.println("***End of Tests***");
      
    }
   
}

Wrap Up

Q1: Write a proper Javadoc comment for the below method:

  • Name: displayPrice

    • takes in one double parameter for a price
    • Prints the price to the console with a dollar sign and to two decimal places
    • returns nothing


    Q2: Write the below method:

    • Name: displayPrice
      • takes in one double parameter for a price
      • Prints the price to the console with a dollar sign and to two decimal places
      • returns nothing

    Q3: Now call the method using the starter code below:

    public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    double number;
    System.out.print("Enter the price: ");
    number = input.nextDouble();
    System.out.print("The formatted price is: ");
    //call the method here!
    }

    Upcoming Assignments

    • Activity 16.1 and 16.2 due Tuesday at 11:59pm
    • Assignment 15 due Tuesday at 11:59pm
    • Assignment 16 due Friday at 11:59pm
    • Quiz 8 due Friday at 11:69pm

    ~Have a Great Day!~