The if and else Statement in Java
This is the moment a program stops being a list of instructions and starts making choices. Everything that makes a game feel alive comes from here.
What you will be able to do
- Write an if statement with a condition and a body
- Add an else branch and predict which one runs
- Spot the stray semicolon that silently breaks an if
Words you will need
- if statement
- In plain words Do this, but only when that is true.
- More precisely A control structure that executes its body when its condition is true.
- Branch
- In plain words One of the paths the program can take.
- More precisely A block of statements reached only under a particular condition.
- Block
- In plain words The code between a pair of braces.
- More precisely A brace-delimited group of statements treated as one unit.
Build it step by step
Every step below leaves a scenario that compiles and runs. If you stop halfway you will have something that works, not something broken. Follow along in Greenfoot rather than reading straight through.
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Step 1
if (score > 100) { Greenfoot.playSound("win.wav"); }Read it as one sentence: IF score is greater than 100, play the sound.
The condition goes in parentheses. The body goes in curly braces. If the condition is false, the whole body is skipped and the program carries straight on.
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Step 2
if (lives > 0) { move(4); } else { Greenfoot.stop(); }else is the other path. EXACTLY ONE of the two bodies runs, always. Never both, never neither.
Notice else has no condition of its own. It means "in every other case", so it does not need one.
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Step 3
public void act() { if (isAtEdge()) { turn(180); } move(3); }Now it is doing real work. Every frame: check the edge, turn if needed, then move.
Look at where `move(3)` sits. It is OUTSIDE the braces, so it runs every frame no matter what the condition said. Moving it inside the braces would mean the actor only moves when it is at the edge, which is a completely different game and a very common accident.
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Step 4
if (score > 100); // <-- this semicolon ends the if { Greenfoot.playSound("win.wav"); }This compiles. It runs. It is wrong, and it will not tell you.
The semicolon ends the if statement immediately, so the if now controls nothing at all. The braces below become an ordinary block that always runs, and your sound plays every single frame regardless of the score.
There is no error message for this, ever. If an if seems to always run, look for this semicolon first.
Mistakes almost everyone makes here
These are the wrong ideas students actually build at this point. Read them even if you think you understand, because a wrong idea you have not noticed is the expensive kind.
Common wrong idea else runs after the if body.
What is actually true Exactly one branch runs. else is the alternative, not the continuation.
Why it matters Expecting both to run makes every branching program behave inexplicably.
Common wrong idea A semicolon after `if (...)` is harmless punctuation.
What is actually true It ends the if, so the block below always runs.
Why it matters It produces no error and no warning. It is the hardest bug in this unit to see.
Common wrong idea Code after the closing brace only runs when the condition was true.
What is actually true It runs either way. Only what is INSIDE the braces is conditional.
Why it matters This is the difference between an actor that patrols and one that never moves.
Common wrong idea Braces are optional so it is fine to leave them off.
What is actually true Java permits it, but adding a second line later silently breaks it.
Why it matters Only the first line is controlled by a brace-less if. The second always runs.
Check your understanding
Answer these before moving on. They are graded instantly and you can retry.
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1. score is 50. What happens? if (score > 100) { move(4); } turn(10);
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2. How many branches of an if/else run?
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3. An if seems to run every single time even when the condition should be false. What would you check first?
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4. Does else need a condition of its own?
Fill in the code
The player should lose a life when it touches a spike, and otherwise keep moving. Fill in the missing pieces.
(isTouching(Spike.class))
{
lives = lives - 1;
}
{
move(3);
}Stuck? Open a hint for each blank
- Blank 1: The keyword that starts a decision.
- Blank 2: The keyword for the other path. It takes no condition.
Lesson quiz
This one counts toward your progress. Take it when the section above makes sense.
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1. What is wrong with `if (lives > 0);`?
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2. lives is 0. Which runs? if (lives > 0) { move(4); } else { Greenfoot.stop(); }
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3. Where does the condition of an if statement go?
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4. Why does this course always use braces even for a one-line if?
The short version
- Condition in parentheses, body in curly braces.
- Exactly one branch of an if/else runs.
- Code after the closing brace runs either way.
- A semicolon after the condition silently breaks the if. No error, ever.
If you get stuck
These pages cover the problems that come up most often in this lesson. Opening one is not cheating and it is not counted against you.
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