Decisions with if and switch · Java tutorial

if, else if, else#

int marks = 72;
String grade;

if (marks >= 90) {
    grade = "A";
} else if (marks >= 75) {
    grade = "B";
} else if (marks >= 60) {
    grade = "C";
} else {
    grade = "F";
}

Only the first true branch runs, so the order of checks matters. Java also checks that grade is definitely given a value on every path before you use it; remove the final else and the compiler will point that out.

Always use braces, even for one line, so a second line cannot quietly escape the if.

Combining conditions#

if (age >= 18 && hasTicket) { ... }
if (day.equals("Saturday") || day.equals("Sunday")) { ... }
if (!found) { ... }

Note equals for strings.

The ternary operator#

int larger = (a > b) ? a : b;
String parity = (n % 2 == 0) ? "even" : "odd";

The classic switch#

switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 6:
    case 7:
        System.out.println("Weekend");
        break;
    default:
        System.out.println("Weekday");
}

Without break, execution falls through into the next case, which is almost always a bug. Java's switch works on integers, characters, enums and strings.

The switch expression#

Modern Java (14 and later) has a cleaner form that returns a value, uses arrows instead of break, and never falls through:

String type = switch (day) {
    case 1, 2, 3, 4, 5 -> "Weekday";
    case 6, 7 -> "Weekend";
    default -> "Unknown";
};

int points = switch (grade) {
    case "A" -> 4;
    case "B" -> 3;
    case "C" -> 2;
    default -> 0;
};

Prefer this form in new code: it is shorter, and a missing case is far easier to spot.

Comparing doubles#

Floating-point results are approximate, so compare with a tolerance:

double x = 0.1 + 0.2;
if (Math.abs(x - 0.3) < 1e-9) { ... }    // true

Handling special cases first#

static int digits(int n) {
    if (n == 0) return 1;
    int count = 0;
    while (n != 0) {
        n /= 10;
        count++;
    }
    return count;
}

Key points#

Only the first true branch of an if chain runs.
Compare strings with equals inside conditions too.
Classic switch needs break; the arrow-form switch expression does not, and returns a value.
Compare doubles with a tolerance.

Next: loops.