Modern Switch Statements with Arrow Labels

Many people who learn switch statements come away believing that every case must end with a break, as if the language demanded it the way it demands a semicolon. The belief is understandable. Most switch statements in older code do end each case with break. But the rule is not part of the syntax itself. This article tests that belief, works out what the syntax really requires, and then looks at the behaviors that make the traditional form tricky.

Hold your prediction for the rest of the article. The output above is the answer to the question. The next sections explain exactly why the program behaves that way, and why the missing break is the reason for the extra line of output.

Trace the program with n equal to 1. The switch evaluates n once and gets 1. It compares that value with the constants in the case labels and finds a match at case 1. Execution begins with the statement under case 1, which prints one. There is no break after it, so the computer does not leave the switch. It moves on to the statements under case 2, which prints two. That group ends with break, so execution jumps past the closing brace. The case 3 group and the default group are never executed.

The diagram shows the central idea of this article. Nothing in the jump to the matching label stops execution. Execution simply keeps reading statements in order until it meets a break or the closing brace. Here the case 1 group has no break, so control falls into the case 2 group.

A traditional switch statement begins with the keyword switch followed by an expression in parentheses. The expression is evaluated once. Its body is enclosed in braces and contains a series of case labels. Each case label is written as the keyword case, then a constant, then a colon. Under each label sits a group of statements. The colon at the end of the label marks the traditional form. The newer style replaces that colon with an arrow. Both forms serve the same purpose of choosing one group of statements to run.

The break statement is what tells the computer to leave. A break at the end of a case causes execution to jump past the closing brace of the switch, skipping every remaining case. The syntax does not force you to write one. A program with no break at all will still compile. Whether a break is present or absent is therefore a decision about the behavior you want, and that decision is where most switch bugs begin.

Fall through is the name for the continuation that happens when a case finishes without a break. Control falls from one case into the next, much as water spills over a step into the level below. The source notes that fall through is legal Java and is occasionally useful. Its most natural use is the grouping pattern shown next.

The default case is the fallback group of a switch. It is introduced by the keyword default followed by a colon, and it runs when none of the other case labels match the expression. The default case is optional. A switch with no default is perfectly valid. If no label matches in that situation, the switch does nothing and execution continues after the closing brace.

Placement matters for reading, though the source does not require any particular position. In the traditional examples the default group appears last, which is the most common convention. Its statements follow the same fall through rules as any other group. If the default group is last and has no break, the switch simply ends there. If you place the default in the middle, a missing break will cause it to fall into whatever comes next.

The title of this article refers to the arrow style, which the source describes as the newer way to write switch labels. The source says the arrow replaces the colon and serves the same purpose of choosing one group of statements to run. The source material for this article focuses on the traditional colon form, so the table below lists only what the source supports. It does not give arrow-form rules beyond the replacement of the colon.