Java Control Flow with if-else Statements

A switch statement lets a program choose among several blocks of code based on the value of one variable. Instead of writing a long chain of separate checks against the same value, you name the variable once and list the possible values as cases. This article works through a traditional Java switch, then asks you to predict its behavior before explaining the mechanism behind it. It then covers the if-else statement, which chooses between two alternatives using a boolean expression.

Keep the variable N in mind as the running example. The switch is written in parentheses after the keyword switch, and each case label is followed by a value and a colon. The statements after that colon are the code for that case. Read the structure once for its shape. You will trace it step by step in the sections that follow.

If your prediction was different, that gap is useful. The next sections explain how the switch chooses where to start, what it runs once it gets there, and where it stops. The same rules govern the trace you will follow.

A switch statement has a fixed structure. The keyword switch is followed by a parenthesized expression, here the variable N. Inside a pair of braces, the switch contains multiple case labels. Each label is followed by a value and a colon, and each case is associated with a specific value. The statements after that colon are the code for that case. A switch may also include one optional default label, which covers values that no case handles.

When a switch runs, the program evaluates the variable in parentheses and compares its value against the case labels. When one label matches, execution jumps to the statements under that label. The statements under other labels are skipped. Matching decides only where execution starts. It is important to separate the choice of entry point from what happens afterward.

The diagram shows one route through the switch. The variable is compared to a label, and a match sends execution into that label's statements. The break then sends execution out of the switch entirely. Labels that were checked and did not match are never run. Read the diagram as a route, not as a list of every line in the code.

Stacked labels show that several values can share one block. Labels written one after another with no statements between them all lead to the same group of statements. In the generated example above, case 2:, case 4:, and case 8: sit above one println, so any of those three values prints the same message.

The default label is optional. When it is present, its block runs only when the switch variable matches none of the case values. Default is typically used for unexpected or unhandled values. The switch first checks the case labels, and default is used only if none of them matched. Its position in the code does not change this matching rule.

In a switch that handles the values 1 through 9 with special cases, any value outside the listed cases falls to default. That includes values such as 7 that are in range but not listed, as well as values outside the range 1 to 9. Because the listed cases form the explicit choices, default catches everything else.

The break keyword is what ends a case's block. A case statement does not have a break by default, so the programmer must write one. If a case's statements do not end with break, execution does not stop at the end of that case. It continues into the statements of the next case below it. This is called fall-through, and it is one of the most common sources of unexpected output in a switch.

An if-else statement gives a program two possible paths. It has a boolean expression, an if block, and an else block. The boolean expression determines which path is selected. When the expression is true, the statement or block after if runs. When it is false, the statement or block after else runs. The two alternatives form one conditional choice.

The source example separates cases for more than one year and for one year. The condition years > 1 controls which message block runs. A final formatting statement placed after the closing brace of the if-else is outside the conditional choice, so it runs regardless of which alternative was chosen.

Either alternative can be a block containing multiple statements, and braces make the boundaries of that block visible. A block groups statements so that the condition controls them together. In the source swapping example, the block after if contains a declaration and several assignments. If x is greater than y, those statements work together to exchange the values of x and y. The braces show that all of these statements belong to the if alternative.

To trace an if-else, work through three locations in order. First evaluate the boolean expression. Second, identify which braces enclose the selected alternative. Third, follow the statements after the closing brace, which run afterward regardless of the condition's result. This is the same procedure whether the condition is true or false.