Motivating examples for switch/case in Java

Question

Students first see switch/case as a second syntax for if/else, but eventually they should see its strength in creating cascades. I know of the traditional example of printing a cumulative song, like "Twelve Days of Christmas", and below I've placed some code I use to illustrate the difference in use-case between if statements and switch cases. It nicely prints out all 108 lines of the song Alouette in 44 lines of code.

The only other motivating example I can think of is a graduated tax calculator (based on the US Federal Income Tax), where cascades can be a nice way to approach the problem.

These examples are fine, but it's not enough to get the lightbulb to flick on for most of my students. What I need is a few more motivating examples that really show students why switch/case is useful. What other examples show this clearly?

    System.out.println("Alouette, gentille alouette,\n"
            + "Alouette, je te plumerai.\n");

    for(int verse = 1; verse <= 8; verse++){
        if(verse == 1){
            System.out.println("Je te plumerai la tête.\nJe te plumerai la tête.");
        } else if (verse == 2){
            System.out.println("Je te plumerai le bec.\nJe te plumerai le bec.");
        } else if (verse == 3){
            System.out.println("Je te plumerai les yeux.\nJe te plumerai les yeux.");
        } else if (verse == 4){
            System.out.println("Je te plumerai le cou.\nJe te plumerai le cou.");
        } else if (verse == 5){
            System.out.println("Je te plumerai les ailes.\nJe te plumerai les ailes.");
        } else if (verse == 6){
            System.out.println("Je te plumerai les pattes.\nJe te plumerai les pattes.");
        } else if (verse == 7){
            System.out.println("Je te plumerai la queue.\nJe te plumerai la queue.");
        } else if (verse == 8){
            System.out.println("Je te plumerai le dos.\nJe te plumerai le dos.");
        }

        switch(verse){
            case 8:
                System.out.println("Et le dos! Et le dos!");
            case 7:
                System.out.println("Et la queue! Et la queue!");
            case 6:
                System.out.println("Et les pattes! Et les pattes!");
            case 5:
                System.out.println("Et les ailes! Et les ailes!");
            case 4:
                System.out.println("Et le cou! Et le cou!");
            case 3:
                System.out.println("Et les yeux! Et les yeux!");
            case 2:
                System.out.println("Et le bec! Et le bec!");
            case 1: 
                System.out.println("Et la tête! Et la tête!");
            default: 
                System.out.println("Alouette! Alouette!\n"
                        + "Ah-ah-ah-ah\n");
        }

        System.out.println("Alouette, gentille alouette,\n"
                + "Alouette, je te plumerai.\n");

Answer 1

Consider why you might prefer to use a switch in a production environment:

1. Fall through processing. The blackjack example given by @Peter is a good example of this because it mirrors how we think about the problem: all of the face cards fall into the same case & the code follows suit. Be careful with this though, it's easy to abuse this to get terser code at the expense of readability
2. Default handlers. Personally, I find this most useful not in a 'single moment of code', but over a series of refinements. For instance, it's common to not have all your logic to handle multiple situations in place when you start coding, but it's also easy to miss a special case that you meant to add later. A default handler is a convenient & concise way to stub out code that you haven't yet gotten to. Default handlers for that throw an exceptions or log the event can help avoid silent failures & are less likely to get 'coded out' by an mistakenly adding or removing an if or else.
3. Maintainability. I find it safer to add & remove code from a switch statement than an if / else if / else block as I'm more likely to accidentally omit an else from an else if than I am to omit a break from a switch.

4. As per Joshua Bloch's Effective Java: "Switches on enums are good for augmenting external enums types with constant-specific behavior." For example if you cannot change the source for an Operation enum, but need add support for an inverse method:

   public static Operation inverse(Operation op){
      switch(op){
         case PLUS:   return Operation.MINUS;
         case MINUS:  return Operation.PLUS;
         case TIMES:  return Operation.DIVIDE;
         case DIVIDE: return Operation.TIMES;
      }
   }
   

5. Readability. I find a moderately sized switch statement easier to grok than an equivalent if / else if / else block.

When possible, avoid contrived examples for motivating usage. Instead, show where you (or someone else) used a switch & discuss why it was preferable.

Answer 2

The example that instantly comes to mind is Blackjack. See the code excerpt below for an extended example in C (even though it's C, the syntax will pretty much hold true for Java as well):

int cardValue(char card)
{
    int aceValue = -1;

    if (card == 'A')
    {
        do
        {
            printf("What value would you like Ace to be?\n");
            printf("Type 1 or 11: ");
            aceValue = GetInt();
        } while (aceValue != 1 && aceValue != 11);
    }

    switch (card)
    {
        case '2':
            return 2;
        case '3':
            return 3;
        case '4':
            return 4;
        case '5':
            return 5;
        case '6':
            return 6;
        case '7':
            return 7;
        case '8':
            return 8;
        case '9':
            return 9;
        case 'T':
        case 'J':
        case 'Q':
        case 'K':
            return 10;
        case 'A':
            return aceValue;
        default:
            return -1;
    }
} 

This function takes in a character corresponding to the card's value. I used a single char as the data type for each card, so I needed a function that took in a card's name (i.e. letter) and returned its value. Were I to design this again, I would probably reduce it down to one return statement and have a variable represent something like cardValue (and in that case I'd need to use break statements, which I currently omit thanks to return). That said, the switch statement is a clear example where you don't want a bunch of if-else statements. For the face cards that would be redundant. There's a nice mix of those cases that "fall through" and those that don't.

Using card games where cards share the same value is a fun, but realistic example for students to see the value of a switch statement.

I'll share one other short example where students have to implement a command-line calculator in C and used switch to identify which operation to perform. Here's the switch from there:

    switch (argv[2][0]) {
        case 'x':
            printf("%f\n", numOne * numTwo);
            break;
        case '/':
            printf("%f\n", numOne / numTwo);
            break;
        case '+':
            printf("%f\n", numOne + numTwo);
            break;
        case '-':
            printf("%f\n", numOne - numTwo);
            break;
        case '%':
            mod = numOne - (numTwo * modNum);
            printf("%f\n", mod);
            break;
        default:
            printf("Usage: <operation> must be x, /, +, -, or %%\n");
            return 1;
    }

With this example the clarity of the code is made much stronger by virtue of the case statements for each operation. From a readability perspective, it makes it clear how the operations are separated out by showing so visually.