Just like Java provides multiple ways to iterate, it also provides multiple ways to branch. The switch-statement is a more powerful form of branching in Java, especially when branching based on the value of a variable. The switch-statement takes a little effort to master, and is expanding its functionality in the Java language. Here we’ll describe the original way switch- is used, at least for the time being. Watch this space.
Subsection6.4.1Parts of a switch-statement
The parts of a switch-statement can be a little confusing at first. Take a moment to study Figure 6.4.1 and pay special attention to the break keyword, because leaving it out is valid syntax, but leads to behavior that is often not what you intend. Leaving out a break when you meant to include it is a common mistake made by novice Java programmers.
Figure6.4.1.Parts of a switch-statement
There are four keywords to identify: switch, case, break, and default. The switch keyword is followed by parentheses containing a selector expression, and a code block delimited by brackets. The selector expression must evaluate to a number, String, or other constant. Each label following a case identifies the number or String to be matched against the value obtained from the evaluated selector expression. If a match is found with a caselabel, the statements following the case are executed. If/when a break is encountered, the switch statement is exited. If the default branch is encountered, it executes it code automatically. No match is required.
Subsection6.4.2Exiting a switch-Statement
It is important to understand that a switch-statement is exited only in one of two situations:
program control runs off the end of the code block, or
a break is encountered.
Caution.
If the statements after a case do not include a break, then program control proceed into the statements of the next case (if there is one), even if the next caselabel does not match the selector expression value.
Make sure you understand this behavior of a switch-statement. The switch- does not exit a case branch automatically when finished executing its statements like an if-statement does. It continues to flow into the statements of next case branch. Always include a break when you intend to exit a switch-statement.
Subsection6.4.3When to Use a switch-Statement
A switch-statement is very convenient when comparing an expression to a list of number or String constant values. For example, referring back to our DoWhileMenu.java example in Listing 6.2.2, we prompted the user for an integer, and then tested the user’s response against a list of integer options. We could have used a switch-statement instead of a multi-branch if-statement, which would have avoided the need to write test expressions for each branch of the if-statement. Below, we include the original multi-branch if-statement from Listing 6.2.2 on the left, and an updated version using a switch-statement on the right.
if (ans == 1) {
Oval o = new Oval();
o.setFillColor(r, g, b);
}
else if (ans == 2) {
Rectangle r1 = new Rectangle();
r1.setFillColor(r, g, b);
}
else if (ans == 3) {
RoundRect r2 = new RoundRect();
r2.setFillColor(r, g, b);
}
switch (ans) {
case 1:
Oval o = new Oval();
o.setFillColor(r, g, b);
break;
case 2:
Rectangle r1 = new Rectangle();
r1.setFillColor(r, g, b);
break;
case 3:
RoundRect r2 = new RoundRect();
r2.setFillColor(r, g, b);
break;
}
Adding a default: to the end of our switch-statement is similar to adding an else to the end of our if-statement. Forgetting one of the break statements would cause the statements of multiple case branches to be executed.
Subsection6.4.4When to Leave Out a break
Leaving out a break statement can be useful when you want to execute the same set of actions for multiple label value matches. A good example is when you are writing code that should handle the differences in behavior between different operating systems.
In Subsection 14.6.5 we described the keyboard events that the DoodlePad library can handle. In the BoxDriver.java program of Listing 14.6.11 we gave an example. Methods capable of handling keyboard events must have the following signature.
Unfortunately, the value of the keyText parameter has different values for certain special keys when your program is running on different operating systems. Table 6.4.2 lists differences for the arrow keys on macOS and Windows.
Table6.4.2.keyText value differences by operating system
Key
macOS
Windows
Left arrow
"←"
"Left"
Right arrow
"→"
"Right"
Up arrow
"↑"
"Up"
Down arrow
"↓"
"Down"
In our BoxDriver.java example we handled this difference by writing complex conditional tests for each if-statement branch in the key-press event handler. The following two code snippets show the original multi-branch if-statement on the left and an updated version rewritten as a switch-statement on the right. Note how the switch-statement has two case labels for each branch with no break between them. Statements under each pair of case labels will be executed when either label matches the value of keyText. The switch-statement on the right has much less clutter as compared to the if-statement on the left. More importantly, eliminating the complex conditionals required for each if- branch reduces the number of opportunities for a mistake.
if (keyText.equals("Left")
|| keyText.equals("←")) {
box.move(-10, 0);
} else if (keyText.equals("Right")
|| keyText.equals("→")) {
box.move(10, 0);
} else if (keyText.equals("Up")
|| keyText.equals("↑")) {
box.move(0, -10);
} else if (keyText.equals("Down")
|| keyText.equals("↓")) {
box.move(0, 10);
}
switch (keyText) {
case "Left":
case "←":
box.move(-10, 0);
break;
case "Right":
case "→":
box.move(10, 0);
break;
case "Up":
case "↑":
box.move(0, -10);
break;
case "Down":
case "↓":
box.move(0, 10);
break;
}
Subsection6.4.5What’s next for switch?
In a future version of Java, the switch-statement will get a major upgrade. Selector expressions will no longer be limited to Strings, numbers or constants. They will be permitted to evaluate to any valid type. Furthermore, case labels will be permitted to be patterns that are matched against the selector expression value. See JEP 433 1 if you are interested in learning how switch-statements will be enhanced in Java.
