Solutions to the Try It Out Exercises in Java For Dummies, 8th Edition
by Barry Burd

Chapter 5: Controlling Program Flow with Decision-Making Statements

In this chapter:

Straight Talk

public class ElevatorFitter2 {

  public static void main(String[] args) {
    int weightOfAPerson = 150;
    int elevatorWeightLimit = 1400;
    int numberOfPeople = elevatorWeightLimit / weightOfAPerson;

    if (numberOfPeople >= 10) {
      System.out.println("You can fit all ten of the");
    } else {
      System.out.println("You can't fit all ten of the");
    }

    System.out.println("Brickenchicker dectuplets");
    System.out.println("on the elevator.");
  }
}

The Rule of Three

import javax.swing.JOptionPane;

public class Authenticator {

  public static void main(String[] args) {

    String username = JOptionPane.showInputDialog("Username:");
    String password = JOptionPane.showInputDialog("Password:");

    if (
        username != null && password != null &&
        (
          (username.equals("bburd") && password.equals("swordfish")) ||
          (username.equals("hritter") && password.equals("preakston")) ||
          (username.equals("ggow") && password.equals("ihaterr"))
        )
     )
    {
        JOptionPane.showMessageDialog(null, "You're in.");
    } else {
        JOptionPane.showMessageDialog(null, "You're suspicious.");
    }
  }
}

Out, Damn'd Not!

The program still works because username != null && password != null is logically equivalent to !(username == null || password == null). This is so because of DeMorgan's Law of logic. It's a kind of distributive law for the !, &&, and || operators.

Equal Bytes

The program crashes if the user clicks the Cancel button in one of the JOptionPane.showInputDialog calls. This happens because the condition !(username == null && password == null) isn't logically equivalent to the condition in the original program. Assume, for example, that the user types bburd in the Username: dialog box but clicks the Cancel button in the Password: dialog box. Then the value of username is bburd but the value of password is null (meaning that password doesn't have a valid String value). Then the condition !(username == null && password == null) is true because it's not the case that both username and password are null. So Java goes on to check if username.equals("bburd"), which it does. So then Java goes on to check if password.equals("swordfish"). But trying to call the equals method on the password variable causes the program to crash, because password isn't a valid String value. Instead, password is null.

I Changed My Mind

import javax.swing.JOptionPane;

public class Authenticator {

  public static void main(String[] args) {

    String username = JOptionPane.showInputDialog("Username:");
    String password = JOptionPane.showInputDialog("Password:");

    if (username == null || password == null) {
      JOptionPane.showMessageDialog(null, "Not enough information");
    } else if (
            (username.equals("bburd") && password.equals("swordfish")) ||
            (username.equals("hritter") && password.equals("preakston"))
       )
    {
        JOptionPane.showMessageDialog(null, "You're in.");
    } else {
        JOptionPane.showMessageDialog(null, "You're suspicious.");
    }
  }
}

Month to Month Resuscitation

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        var keyboard = new Scanner(System.in);

        System.out.print("Enter the name of a month: ");
        String month = keyboard.next();

        int numDays;

        switch (month) {
            case "January", "March", "May", "July", "August", "October",
                    "December" -> numDays = 31;
            case "February" -> numDays = 28;
            case "April", "June", "September", "November" -> numDays = 30;
            default -> numDays = 0;
        };

        System.out.print(month + " has ");
        System.out.print(numDays);
        System.out.println(" days.");
    }
}

Leaping to Conclusions

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        var keyboard = new Scanner(System.in);

        System.out.print("Enter the name of a month: ");
        String month = keyboard.next();

        int numDays;

        switch (month) {
            case "January", "March", "May", "July", "August", "October",
                    "December" -> numDays = 31;
            case "February" -> numDays = 28;
            case "April", "June", "September", "November" -> numDays = 30;
            default -> numDays = 0;
        };

        System.out.print("Is it a leap year (yes/no)? ");
        String isLeapYear = keyboard.next();

        if (month.equals("February") && isLeapYear.equals("yes")) {
            numDays = 29;
        }

        System.out.print(month + " has ");
        System.out.print(numDays);
        System.out.println(" days.");
    }
}

How About a Date?

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        var keyboard = new Scanner(System.in);

        System.out.print("Enter the name of a month: ");
        String month = keyboard.next();

        int numDays = switch (month) {
            case "January", "March", "May", "July", "August", "October",
                    "December" -> 31;
            case "February" -> 28;
            case "April", "June", "September", "November" -> 30;
            default -> 0;
        };

        System.out.print("Is it a leap year (yes/no)? ");
        String isLeapYear = keyboard.next();

        if (month.equals("February") && isLeapYear.equals("yes")) {
            numDays = 29;
        }

        System.out.print(month + " has ");
        System.out.print(numDays);
        System.out.println(" days.");
    }
}


import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        var keyboard = new Scanner(System.in);

        System.out.print("Enter the name of a month: ");
        String month = keyboard.next();

        int numDays = 0;

        switch (month) {
            case "January":
            case "March":
            case "May":
            case "July":
            case "August":
            case "October":
            case "December":
                numDays = 31;
                break;
            case "February":
                numDays = 28;
                break;
            case "April":
            case "June":
            case "September":
            case "November":
                numDays = 30;
                break;
        }

        System.out.print("Is it a leap year (yes/no)? ");
        String isLeapYear = keyboard.next();

        if (month.equals("February") && isLeapYear.equals("yes")) {
            numDays = 29;
        }

        System.out.print(month + " has ");
        System.out.print(numDays);
        System.out.println(" days.");
    }
}


    
The Yield App

     
    
Enter the name of a month: February
Leap year (true/false)? true
29 days