Introduction to Switch Statements

Imagine a program that must respond differently to each of several possible values, such as a menu choice or a grade. One approach is a long chain of separate comparisons, each checked in turn. Java offers a different approach: the switch statement. It tests one expression and jumps to the position that matches its value, so the possible positions are listed together in one place.

Each possible position is marked by a case label. At most one position can be marked as the default, which is used when no case label matches. The arrangement replaces a long chain of separate comparisons with one list of positions, and the value of the expression decides which position is reached.

Running a switch is rarely the hard part. Most of the difficulty lies in reading one. When a switch produces a result you did not expect, the useful question is which position control jumped to, and why that position was chosen. The rest of this article gives you the vocabulary and rules to answer that question yourself.

A switch statement accepts only expressions of certain types. The expression may be one of the primitive integer types int, short, or byte. It may be the primitive char type. It may be String. It may also be an enum type.

An enum type is a type whose values are named constants. The source points forward to a later introduction to enum types, so for now it is enough to treat an enum value as one of a fixed set of named choices that a switch can test.

A case label takes the form case followed by a constant list. The constant list contains one or more literals of the same type as the expression, separated by commas. After the label comes the arrow symbol, made of a hyphen and a greater-than character, and then a single statement. That statement may be a subroutine call statement, a throw statement, or a block statement, and a block can contain several nested statements.

Tracing means following control step by step from the switch expression to the position it reaches. The diagram shows the entries of the first example. The expression is compared against each label in order until a match is found. The matching entry runs its statement, and control then leaves the switch.

Now change the value to 7. No case label contains 7, so no entry matches. The default entry is then the position control jumps to, and the program prints other. This is the point where the unexpected result begins for many readers: they assume a label matched when none did. Checking each label against the value, in order, locates the position reliably.

Java has two syntaxes for switch statements. The original syntax was modeled on the C programming language, and the source describes it as error-prone and somewhat awkward to read. It is still common, because a great deal of existing code uses it, so you need to be able to read it. The newer syntax requires Java 17 and improves on the original. The source advises using the new syntax when you write new code. Both forms rest on the same idea: a value selects a jump point, so learning the case label rules helps with either form.

Because the arrow form attaches exactly one statement or block to each entry, a reader can see where each case body ends without searching for a break. The rest of this article follows the new syntax, and the case label rules described above apply to both forms.

Programs do not always perform every instruction. Sometimes they must choose a path. An if statement creates that choice by examining a boolean-valued expression. The expression is either true or false, and that result determines which action the computer takes. The decision is made at the decision point, and only one of the possible results is carried out for a given run.

An if statement does not always need an else clause. The one-way form, called here a simple if, is useful when the program should perform an action only for a true condition. If the expression is false, the contained statement is skipped and control continues with whatever follows the if statement. An if-else statement, by contrast, always chooses one of two alternatives: exactly one of them executes.

When an if statement produces an unexpected result, inspect the decision in order. First identify the boolean expression. Next determine whether its value is true or false. Finally check which statement belongs to that result. In an if-else statement, exactly one alternative executes. In a simple if statement, a false result means the contained action is skipped and execution moves on.