Monday, November 17, 2014

The Old Switcheroo

Let's talk about the 'switch' statement for a bit.  This is a form of conditional that is better suited to some tasks than if/else.

First, let's look at the structure of a switch:

switch(variable) {
    case value1:
        //Code to execute for value1
        break;

    case value2:
        //Code to execute for value1 or value2

    case value3:
        //Code to execute for value3
        break;

    default:
        //Code for any value other than the three above

}
// Done with switch

In detail.

If variable is equal to value1, we execute the code under 'case value1', then we 'break', which means we exit the current block (jump to 'Done with switch').

If variable is equal to value2, we execute the code under 'case value2', then continue to execute the code under 'case value3' before we hit the 'break' and head out to 'Done with switch'.

If variable has any other value, we execute the code under 'default'.

This is basically equivalent to:

if (variable == value1 || variable == value2) {
    if (variable == value1) {
        //Code to execute for value1;
    }
    //Code to execute for value1 or value2
}
else if (variable == value3) {
    //Code to execute for value3
}
else {
    Code for any value other than the three above
}

But I think in this case the switch is a bit easier to follow.

Sadly, the switch works with only a limited number of data types.  Basically it will work for atomic types that represent integer values, the objects that wrap those atomic types (like Integer or Long), and String.  It will also work with Enumerations, but I haven't written about those yet...  I guess I should get to those soon.

So, let's combine a for loop with a switch statement to get some output, shall we?

        for (int switchValue = 0; switchValue < 5; switchValue ++) {
            switch (switchValue) {
                case 0:
                    System.out.println("Just starting out.");
                    break;

                case 1:
                    System.out.println("Making progress.");
                    break;

                case 4:
                    System.out.println("All done.");
                    break;

                case 5:
                    System.out.println("We will never get here.");
                    break;

                default:
                    System.out.println("Getting there...");
                    break;
            }
        }


This runs a simple loop that goes from 0 to 4.  Along the way, we run a switch to execute various blocks of code based on that value.  The output from this block of code looks like this:

Just starting out.
Making progress.
Getting there...
Getting there...
All done.

If you follow the code, I hope this makes sense to you.  The 'default' statement catches anything that isn't 0, 1, 4 or 5, so it runs a couple of times in the middle.  If this is not clear, drop me a line and I'll try to improve it.