Commands, and input that doesn't fit
The command programs of module 25 trusted their users. Real users mistype a command, ask for something that isn't there, or type five where a number should be. A program shouldn't crash then, and it shouldn't quietly do something wrong either: it should say what's wrong and wait for the next command. Three kinds of bad input come up in almost every command program:
| Bad input | Example | The answer |
|---|---|---|
| An unknown command | lend |
Unknown command: lend |
| A name that isn't there | log, then Cleo, who isn't a volunteer |
No volunteer called Cleo |
| A number that isn't valid | five, or 40 hours in one shift |
Hours must be a whole number from 1 to 12 |
A food bank's volunteers log the hours they help out. The plan is step 1's: Volunteer for the data, FoodBank for the logic, and UserInterface for the commands, as in module 25. The volunteers are added in main, and the commands are:
log, then a name and a number of hours: adds the hours to that volunteer. A shift is 1 to 12 hours.total: prints the hours of all the volunteers added up.quit: ends the program.
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
FoodBank bank = new FoodBank();
bank.add(new Volunteer("Ada"));
bank.add(new Volunteer("Ben"));
UserInterface ui = new UserInterface(scanner, bank);
ui.start();
}
}public class Volunteer {
private String name;
private int hours;
public Volunteer(String name) {
this.name = name;
this.hours = 0;
}
public String getName() {
return this.name;
}
public int getHours() {
return this.hours;
}
public void addHours(int hours) {
this.hours += hours;
}
public String toString() {
return this.name + ": " + this.hours + " hours";
}
}import java.util.ArrayList;
public class FoodBank {
private ArrayList<Volunteer> volunteers;
public FoodBank() {
this.volunteers = new ArrayList<>();
}
public void add(Volunteer volunteer) {
this.volunteers.add(volunteer);
}
public Volunteer find(String name) {
for (Volunteer volunteer : this.volunteers) {
if (volunteer.getName().equals(name)) {
return volunteer;
}
}
return null;
}
public int totalHours() {
int total = 0;
for (Volunteer volunteer : this.volunteers) {
total += volunteer.getHours();
}
return total;
}
}import java.util.Scanner;
public class UserInterface {
private Scanner scanner;
private FoodBank bank;
public UserInterface(Scanner scanner, FoodBank bank) {
this.scanner = scanner;
this.bank = bank;
}
public void start() {
while (true) {
System.out.print("> ");
String command = this.scanner.nextLine();
if (command.equals("quit")) {
System.out.println("Thank you!");
break;
}
if (command.equals("log")) {
String name = this.scanner.nextLine();
int hours = this.toNumber(this.scanner.nextLine());
Volunteer volunteer = this.bank.find(name);
if (volunteer == null) {
System.out.println("No volunteer called " + name);
} else if (hours < 1 || hours > 12) {
System.out.println("Hours must be a whole number from 1 to 12");
} else {
volunteer.addHours(hours);
System.out.println(volunteer);
}
} else if (command.equals("total")) {
System.out.println("Total: " + this.bank.totalHours() + " hours");
} else {
System.out.println("Unknown command: " + command);
}
}
}
private int toNumber(String text) {
try {
return Integer.valueOf(text);
} catch (Exception e) {
return -1;
}
}
}log
Ada
4
log
Cleo
3
log
Ben
five
log
Ben
5
lend
total
quit> Ada: 4 hours
> No volunteer called Cleo
> Hours must be a whole number from 1 to 12
> Ben: 5 hours
> Unknown command: lend
> Total: 9 hours
> Thank you!Each kind of bad input has its own place:
- An unknown command falls through every
ifandelse ifto the lastelse. Without thatelse, a typo gets no answer at all. - A name that isn't there: the logic class says so with its return value.
findreturnsnull, and the user interface turns that into a message. FoodBank still doesn't print anything. - A number that isn't valid: turning typed text into a number is the user interface's job, so it checks the number too, before the logic gets it.
Read the whole command first
log takes three lines: the command, the name and the hours. The user interface reads all of them before it checks anything. Suppose it checked the name first and skipped the rest of the command when the name was wrong. The hours, 3, would still be waiting in the input, and the next round would read them as a command: Unknown command: 3. Every command reads the same number of lines, whatever is wrong with them.
Is it a number?
Integer.valueOf("five") throws a NumberFormatException, and without a catch the program would crash. Module 20's try and catch turn the crash into an answer. The helper method toNumber returns the number, or -1 when the text isn't a whole number:
private int toNumber(String text) {
try {
return Integer.valueOf(text);
} catch (Exception e) {
return -1;
}
}- When
Integer.valueOfsucceeds,returnhands the number back right away. When it throws, Java jumps to thecatchblock, and that returns-1. -1works as a signal because no valid number of hours is negative. The range check after it catches-1together with0,40and every other number outside 1 to 12, so one message covers them all.- The method is
private(module 18): it's a helper for the user interface itself, and no other class needs it.