Multiway If Statements in Java

Many programs need to pick one of several actions depending on the value of a single variable. A student might want one message for the number 1, another message for the numbers 2, 4, and 8, and a different message for every other number. Java offers two common ways to write this kind of decision. One is a chain of if, else if, and else statements, which this article calls a multiway if statement. The other is the switch statement, which lists the possible values as cases. The two forms are closely related, and this article looks at how they relate and how to decide which one fits a given problem.

The multiway if statement and the switch statement solve the same kind of problem. In both, a program examines a value and runs exactly one group of statements based on what that value is. The difference lies in how each form expresses the decision. A multiway if statement is a sequence of separate tests, each one a condition that is either true or false. A switch statement names the variable once and then lists the values it might hold as cases.

Because both forms make the same kind of choice, the source states that a multiway if statement can have exactly the same effect as a switch statement that handles the same cases. The source also notes that any switch can be replaced by a multiway if. Understanding why these two forms match is the key to reading either one with confidence.

The best way to read a multiway if statement is as a sequence of questions asked in order. Each question is a test on the variable. The first test that is true decides which statements run. The final else catches every value that no earlier test accepted. The tests run in order and stop at the first success, so once a branch is chosen, the remaining else if tests and the final else are skipped.

Notice that each test compares the same variable, N, with a constant. That shared pattern is exactly what a switch statement is built to express. The diagram below shows the decisions this chain makes. Each diamond is one test, and each box is the output that test leads to when it succeeds.

A switch statement names the variable once, in the parentheses after the keyword switch. Then each case label lists one value that the variable might hold. When a case label matches, control enters the statements beneath it. A break statement ends the switch so that control does not fall through into the next case. Cases that share one set of statements can be stacked, with no statements between their labels. That stacking is how the switch form covers 2, 4, and 8 together.

Set the two versions side by side and the correspondence is direct. The variable named once in the switch appears in every test of the chain. Each stacked case label corresponds to one comparison in an or group of the chain. The default label corresponds to the final else. For the value 2, both versions reach the same branch and print the same line. This is why the source can say the two forms have the same effect on the same cases.

The source gives two reasons a programmer might prefer one form. The first is readability. In the multiway if chain, the variable N is written out again in every test, and the value groups are expressed as repeated comparisons joined by or. In the switch form, the variable is named once and each possible value sits on its own labeled line, so a reader can scan the case labels to see the full list of handled values.

The second reason is efficiency. The source says a switch statement might be more efficient because the computer can jump directly to the correct case, instead of working through a series of tests to reach it. In a multiway if chain, a value must be compared against each earlier test. The source uses the word might, so this is a possibility that depends on how the program is compiled and run, not a guaranteed speed gain.

Because the tests run in order and stop at the first success, the order of the tests is part of the meaning of the chain. The mistakes below come from forgetting that, or from forgetting how the switch form differs.