if / else if / else

Programs make decisions with if, else if, and else:

int temperature = 31;

if (temperature > 30) {
    System.out.println("Stay hydrated");
} else if (temperature > 20) {
    System.out.println("Perfect weather");
} else {
    System.out.println("Bring a jacket");
}

The conditions are tested top to bottom, and only the FIRST true branch runs — once one matches, the rest are skipped entirely. That ordering is part of your program's logic: testing temperature > 20 first would make the > 30 branch unreachable.

The parentheses hold a boolean expression; the braces hold the code to run. Braces are never optional in this course, even for one statement:

if (active) System.out.println("on");   // legal, but a maintenance trap

Without braces, only the next single statement belongs to the if — the famous "Apple goto fail" bug class. Always write the braces.

Conditions can nest, but deep nesting is a smell:

if (user != null) {
    if (user.isActive()) {
        if (user.hasPermission("write")) {
            // three levels deep — hard to follow
        }
    }
}

The same logic, flat and readable, using the && from Module 2:

if (user != null && user.isActive() && user.hasPermission("write")) {
    // one clear gate
}

Scope note: variables declared inside a branch disappear at its closing brace. Declare result variables before the branches if all branches contribute to them.

Next: switch — decisions by value.