Umbrella or Not

Asking a question before acting

Everything you have written so far ran straight down the page, every line, every time. That is fine for a banner. It is useless for advice, because advice depends on the situation.

if is how a program asks a question before it acts. You give it something to check, and inside a pair of curly braces a block of lines that run only when the answer is yes. Follow that with else and a second braced block, and you have covered the no as well. Exactly one of the two blocks runs. Never both, never neither.

if (rainChance >= umbrellaMark) {
    System.out.println("Rain is likely today.");
} else {
    System.out.println("It should stay dry.");
}

The braces are what mark off each block, the same way they marked off the body of main itself. Indentation helps you read it; the braces are what Java actually reads.

The question has to be a boolean

Java is strict about one thing here that trips almost everybody once: whatever sits inside the round brackets of an if has to be a boolean — a value that is either true or false, nothing else. A comparison produces one of those automatically:

• == asks whether two values are the same, and hands back true or false
• >= asks whether the left one is at least as big as the right one, and does the same

A single equals sign does something completely different. rainChance = umbrellaMark is not a question at all — it stores umbrellaMark's value into rainChance and, as an expression, that whole thing works out to an int. Put that inside an if, and javac refuses it outright: an int is not a boolean, and no amount of running the program will make it one. This is not a warning either. incompatible types: int cannot be converted to boolean stops compilation before a single line executes, which means the report above the if never gets the chance to print. Once you know to look for it, the fix is always the same: a second equals sign, or whichever comparison you actually meant.

A worked example

A cinema till, not a weather report, so the answer to this exercise stays yours to write:

public class Main {
    public static void main(String[] args) {
        int age = 14;
        int adultFrom = 18;

        System.out.println("Age ...... " + age);
        if (age >= adultFrom) {
            System.out.println("Ticket type: adult");
        } else {
            System.out.println("Ticket type: child");
        }
        System.out.println("Enjoy the film.");
    }
}

Age ...... 14
Ticket type: child
Enjoy the film.

14 is not at least 18, so the first block is skipped over entirely and the second one runs. Only one ticket line ever appears. The last line sits outside both braces, so it runs either way. Change age to 30 and the middle line changes while the other two stay put.

Your turn

The editor holds a rain report. Two numbers sit at the top: rainChance is today's forecast, and umbrellaMark is the point at which carrying an umbrella is worth the bother.

Run it before changing anything. The card prints, and then tells you to leave the umbrella at home on a day with a seventy percent chance of rain. That is because the test currently reads:

if (rainChance == 0) {

which asks whether the chance is exactly zero — a question with nothing to do with umbrellas, and one whose answer today is no.

Replace that test with one asking whether rainChance has reached umbrellaMark: at the mark or above it counts as take it. Everything else stays exactly as it is.

Then change rainChance to 20 and run it again. The same program should now tell you to leave the umbrella at home. That second run is the real proof — it shows the program is reading the numbers rather than printing a fixed answer.

If something goes wrong

The likeliest slip is the equals sign. Written as if (rainChance = umbrellaMark), javac stops on the spot with incompatible types: int cannot be converted to boolean, and nothing runs, not even the card above it. That message is Java refusing to treat a stored value as a yes-or-no answer. Add the second equals sign, or the comparison you actually meant.

If the message instead mentions a missing brace or bracket, count them against the worked example above — every if and else here needs both an opening and a closing { } pair. Nothing here can break.