2.3 if Statements

What you need to know

• if (condition) { statements } — runs the block only when the condition is true. This is one-way selection.
• if (condition) { A } else { B } — runs exactly one of A or B. Two-way selection.
• The condition must be a boolean. if (x) where x is an int doesn't compile. if (x = 5) doesn't compile either (assignment isn't a boolean).
• Braces are optional for a single statement, but omitting them is a classic trap: only the first statement is controlled by the if. Always use braces.
• A semicolon right after the condition — if (x > 0); — ends the if with an empty body, so the next block always runs.
• After the if/else completes, execution continues with the next statement regardless of which branch ran.

Worked example

int score = 72;
if (score >= 70)
    System.out.println("Pass");
    System.out.println("Congrats");   // NOT part of the if — always prints
if (score > 90)
{
    System.out.println("Honors");
}
else
{
    System.out.println("Standard");
}
// Output: Pass  Congrats  Standard

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• The if body without braces is exactly one statement. A second line, no matter how it's indented, is outside the if. The exam writes deliberately misleading indentation to test this. Always mentally add the braces.
• if (cond); — a semicolon right after the condition — is an empty if. The block that follows runs unconditionally. It compiles fine, which is why it's dangerous.
• The else pairs with the nearest unmatched if. With braces, no ambiguity. Without, the dangling-else rule applies (2.4).
• A condition that's a constant — if (true) — always runs. Rarely written on purpose, but appears in "which of these is redundant" questions.
• The if statement itself has no value. x = if (…) isn't Java. To choose between values, use if/else with two assignments.
• Two-way selection covers every input: one branch or the other. One-way selection has a "do nothing" case. That difference is what "how many lines are printed" questions probe.

Mistakes that cost points

• Attaching the second unbraced statement to the if. Only the first belongs. The indentation lies.
• Missing the stray semicolon. if (x > 5); means the next block always runs.
• Using a non-boolean condition. if (count) doesn't compile. Java isn't C.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

int n = 4;
if (n > 5)
    System.out.print("A");
    System.out.print("B");
System.out.print("C");