The switch statement

Figure 155. Better: Using switch Slide presentation 
...
switch(number) {
    case 1: System.out.println("Monday"); break;
    case 2: System.out.println("Tuesday"); break;
    case 3: System.out.println("Wednesday"); break;
    case 4: System.out.println("Thursday"); break;
    case 5: System.out.println("Friday"); break;
    case 6: System.out.println("Saturday"); break;
    case 7: System.out.println("Sunday"); break;

    default: System.out.println("Invalid number " + number); break;
} ...
Enter a weekday number (1=Monday, 2=Tuesday,...) : 6
Saturday

Figure 156. switch Syntax Slide presentation 
switch(expression) {
[case value_1 :
    [statement]*
    [break;] ]
[case value_2 :
    [statement]*
    [break;] ]
  ...
[case value_n :
    [statement]*
    [break;] ]
[default:
    [statement]*
    [break;] ]
}

exercise No. 57

Why “break”?

Q:

Do we need break statements in Figure 155, “Better: Using switch ”? Rewrite Figure 154, “Numbers to day's names: The hard way ” replacing all conditions by switch / case statements without using break. What do you observe on execution?

A:

Figure 154, “Numbers to day's names: The hard way ” can be rewritten as: 

final Scanner scan = new Scanner(System.in);
System.out.print("Enter a weekday number (1=Monday, 2=Tuesday,...) : ");
final int number = scan.nextInt();
switch(number) {
  case 1: System.out.println("Monday");
  case 2: System.out.println("Tuesday");
  case 3: System.out.println("Wednesday");
  case 4: System.out.println("Thursday");
  case 5: System.out.println("Friday");
  case 6: System.out.println("Saturday");
  case 7: System.out.println("Sunday");

  default: System.out.println("Invalid number " + number);
}

Entering a day's number 5 yields:

Enter a weekday number (1=Monday, 2=Tuesday,...) : 5
Friday
Saturday
Sunday
Invalid number 5

So the break statements effectively prohibit “fall-through” towards subsequent case labels. Moving “7 / Sunday” to the beginning this becomes even clearer: 

...
switch(number) {
   case 7: System.out.println("Sunday"); // Deliberately moved to the top
   case 1: System.out.println("Monday");
   case 2: System.out.println("Tuesday");
   case 3: System.out.println("Wednesday");
   case 4: System.out.println("Thursday");
   case 5: System.out.println("Friday");
   case 6: System.out.println("Saturday");
   default: System.out.println("Invalid number " + number);
} ...

This time entering a value of 5 again we no longer see "Sunday" on output: 

Enter a weekday number (1=Monday, 2=Tuesday,...) : 5
Friday
Saturday
Invalid number 5

Thus fall-through does not depend on numerical case label ordering but solely on their order of appearance within code.

exercise No. 58

Extending to month days

Q:

Consider January 2052:

Monday Tuesday Wednesday Thursday Friday Saturday Sunday
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31

Write an application converting a given day's number within January 2052 (this may be generalized to arbitrary months) to its name e.g.: 

Enter a day's number:>23
Tuesday

Tip

The modulo operator “%” is your friend.

A:

With respect to Figure 155, “Better: Using switch ” two modification are being required: 

...
switch(day % 7) {
   case 1: System.out.println("Monday"); break;
   case 2: System.out.println("Tuesday"); break;
   ...
   case 0: System.out.println("Sunday"); break;
}
Figure 157. Switching on strings Slide presentation 
String month, season;  
...
// Since Java 7: String based case labels
switch(month) {
    case "March": case "April": case "May":
       season = "Spring"; break;

    case "June": case "July": case "August":
        season = "Summer"; break;

    case "September": case "October": case "November":
       season = "Autumn"; break;

    case "December": case "January": case "February":
       season = "Winter"; break;
    }
}

exercise No. 59

Converting day's names to numbers.

Q:

Consider the reverse problem to Figure 153, “Converting numbers to day's names ”: We want to map a given day's name to its number as in the following example: 

Enter a weekday (Monday to Sunday): Friday
5

Issue an error message in case an inappropriate text is being entered: 

Enter a weekday (Monday to Sunday): July
Unknown day name 'July'

Tip

  1. Starting from Java™ 7 String based switch statements are allowed.

  2. You may read strings into your application using Scanner.next().

A:

 
final Scanner scan = new Scanner(System.in));
System.out.print("Enter a weekday (Monday to Sunday):>");
final String day = scan.next();
switch(day) {
  case "Monday": System.out.println(1); break;
  case "Tuesday": System.out.println(2); break;
  case "Wednesday": System.out.println(3); break;
  case "Thursday": System.out.println(4); break;
  case "Friday": System.out.println(5); break;
  case "Saturday": System.out.println(6); break;
  case "Sunday": System.out.println(7); break;
  default: System.out.println("Unknown day name '" + day + "'");
      break;
}

exercise No. 60

Day categories.

Q:

We want to group working days into categories:

Day Category
Monday Start of work week
Tuesday Midweek
Wednesday
Thursday
Friday End of work week
Saturday Weekend
Sunday

Example execution:

Enter a weekday (Monday to Sunday):>Wednesday
Midweek

Tip

Sometimes omitting break statements allowing for fall-through is your friend.

A:

 
final Scanner scan = new Scanner(System.in));
System.out.print("Enter a weekday (Monday to Sunday):>");
final String day = scan.next();
switch(day) {
  case "Monday": System.out.println("Start of work week"); break;

  case "Tuesday":
  case "Wednesday":
  case "Thursday": System.out.println("Midweek"); break;

  case "Friday": System.out.println("End of work week"); break;

  case "Saturday":
  case "Sunday": System.out.println("Weekend"); break;

  default: System.out.println("Unknown day name " + day); break;
}

exercise No. 61

Roman numerals, using switch

Q:

Re-implement Roman numerals using a switch statement rather then an if ... else if ... else.

A:

 
final Scanner scan = new Scanner(System.in));

System.out.print("Enter a number:>");
final int number = scan.nextInt();

switch (number) {
  case 1: System.out.println("I"); break;
  case 2: System.out.println("II"); break;
  case 3: System.out.println("III"); break;
  case 4: System.out.println("IV"); break;
  case 5: System.out.println("V"); break;
  case 6: System.out.println("VI"); break;
  case 7: System.out.println("VII"); break;
  case 8: System.out.println("VIII"); break;
  case 9: System.out.println("IX"); break;
  case 10: System.out.println("X"); break;

  default:System.out.println("Decimal value " + number + " not yet implemented");
    break;
}
Figure 158. Allowed types for switch statements Slide presentation

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