Exercise 1 of 4
Your First If
With if, code only runs when a condition is true:
if (age >= 18) {
IO.println("Adult");
}
The code checks whether number is greater than 10. Change number so the text gets printed.
Starter code
void main() {
int number = 5;
if (number > 10) {
IO.println("Greater than 10!");
}
}
Background
How the compiler handles conditions
In bytecode, an if turns into a conditional jump. The CPU checks a comparison and either jumps into the "then" block or skips right over it.
Modern CPUs do branch prediction — they guess which branch is more likely and start preparing that code early. If the CPU guesses wrong, the work gets thrown away (pipeline flush).
Exercise 2 of 4
If-Else
With else you say what should happen when the condition is not true:
if (condition) {
// if true
} else {
// if false
}
The println lines are already there. Just fill in the condition of the if: check whether temperature is greater than or equal to 20.
Starter code
void main() {
int temperature = 22;
if (???) {
IO.println("Warm");
} else {
IO.println("Cold");
}
}
Background
The ternary operator — if-else in short
Java has a one-liner version of if-else: result = condition ? ifTrue : ifFalse;
Example: String s = temperature >= 20 ? "Warm" : "Cold"; — same decision as the if-else block here, just more compact.
Exercise 3 of 4
Comparing
Java has these comparison operators:
== equal to!= not equal to< less than> greater than<= less than or equal to>= greater than or equal to
Watch out: checking for equality is == (double!), not = (that's assignment).
Fill in the condition: check whether password has the value 1234.
Starter code
void main() {
int password = 1234;
if (???) {
IO.println("Access granted");
} else {
IO.println("Wrong password");
}
}
Background
== vs. .equals() — the most important Java trap
For primitive types (int, double, boolean), == compares the values — that's exactly right.
For objects (String, arrays, your own classes), == compares the reference (is it the very same object in memory?), not the content! For strings and most objects you need .equals(). Arrays are special: their .equals() also just compares references — use Arrays.equals() instead. This is one of the most common beginner mistakes in Java.
Exercise 4 of 4
Else-If
For multiple conditions, use else if:
if (grade == 1) {
// ...
} else if (grade == 2) {
// ...
} else {
// everything else
}
Add the missing else if block: if grade equals 2, print Good.
Starter code
void main() {
int grade = 2;
if (grade == 1) {
IO.println("Very good");
// Add here: else if grade == 2 -> "Good"
} else {
IO.println("Not bad");
}
}
Background
Else-if chains vs. switch
Long else if chains get messy fast. From about 4 cases on, a switch is often easier to read. Since Java 14 there are even switch expressions with arrow syntax: case 1 -> "Very good";
The compiler turns a switch into a jump table (the tableswitch / lookupswitch bytecode instructions) — so with many cases, a switch is usually faster than a long if-else chain, too.