if Statements

Vocabulary

English	Meaning
if statement/ɪf ˈsteɪtmənt/	A statement that executes a block or chooses between branches according to a Boolean condition.

Running code conditionally

An if statement 条件语句 runs a block only when its condition is true.
if (score >= 60) { System.out.println("Pass"); }
If the condition is false, the block is skipped entirely.
This is selection — the program chooses whether to run the block.

Adding else

An else clause runs alternative code when the condition is false.
if (score >= 60) { print("Pass"); } else { print("Fail"); }
Exactly one of the two branches runs — never both, never neither.
else is optional; a bare if just skips its block when false.

Traffic lights: selection chooses which branch runs, just as if statements choose code paths

Braces group the block

Braces { } group the statements that belong to the if or else.
Without braces, only the single next statement is controlled by the if.
Always use braces, even for one line — it prevents a common bug.
Indent the block so the structure is visible at a glance.

Tracing the branch

To trace an if-else, evaluate the condition, then run the matching branch.
With score = 75: 75 >= 60 is true → run the if block → prints "Pass".
With score = 40: 40 >= 60 is false → run the else block → prints "Fail".
Only the code in the chosen branch executes.

Watch out

Without braces, an if controls only the ONE statement right after it. So if (x > 0) a(); b(); always runs b() — the indentation lies. Always wrap the block in { }. And in an if-else, exactly one branch runs; you never fall through into both, unlike separate if statements.

Example

Grading with if-else:

if (score >= 90) { grade = "A"; } else { grade = "B"; }
score = 95 → condition true → grade is "A".
score = 80 → condition false → grade is "B".

Key idea

An if statement runs a block only when its condition is true; an optional else runs alternative code when it's false — exactly one branch executes. Use braces { } to group each block (without them, only the next single statement is controlled). Trace by evaluating the condition, then running the matching branch.

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