Conditions

Conditions let a program choose which code to run. Without them a program would always do the same thing: show a discount to everyone or let anyone into the site without a password.

if

The code in curly braces after if runs only when the condition in parentheses is true. Code in curly braces is called a block:

void main() {
    int age = 20;
 
    if (age >= 18) {
        IO.println("Access granted");
    }
}

Output:

Access granted

In Java a condition must be a boolean. You cannot put a number or a string there — the compiler reports an error.

else

The else block runs when the condition is false. Exactly one of the two blocks always runs:

void main() {
    int age = 15;
 
    if (age >= 18) {
        IO.println("Access granted");
    } else {
        IO.println("Access denied");
    }
}

Output:

Access denied

else if

When there are more than two options, add else if between if and else. Conditions are checked top to bottom, and the first matching block runs — the rest are skipped:

void main() {
    int score = 75;
 
    if (score >= 90) {
        IO.println("Excellent");
    } else if (score >= 70) {
        IO.println("Good");
    } else {
        IO.println("Needs work");
    }
}

Output:

Good

Order matters: if the >= 70 check came first, a score of 95 would also get "Good".

Compound conditions

Several checks are combined with the logical operators && (and) and || (or). Java evaluates them in short-circuit fashion: if the answer is clear from the left side, the right side is not checked:

void main() {
    int age = 25;
    boolean hasTicket = true;
 
    if (age >= 18 && hasTicket) {
        IO.println("Enjoy the show");
    }
}

Output:

Enjoy the show

The ternary operator

When you need to pick one of two values, the ternary operator condition ? value1 : value2 is handy. It returns the first value if the condition is true and the second if it is false:

void main() {
    int stock = 0;
    String status = stock > 0 ? "In stock" : "Out of stock";
    IO.println(status);
}

Output:

Out of stock

The ternary operator is good for short choices. If there are more branches or several actions in them, write a regular if.

Common mistake

An extra semicolon after the condition cuts the block off from it. The if now controls the empty statement ;, and the block in braces always runs:

void main() {
    int balance = 0;
 
    if (balance > 100); {
        IO.println("Purchase completed");
    }
}

Output:

Purchase completed

The compiler does not treat this as an error, so it is easy to miss. The { must come right after the condition's ).