If you know that some code might fail, and you have an idea of what to do if it does, you can prevent an exception from crashing your program by catch-ing it.

To do this, you write try and put the code that might fail inside of { and }.

try {
    mightFail();
}

Then you write catch and in parentheses the kind of exception you want to handle as well as a variable name.¹

try {
    mightFail();
} catch (RuntimeException e) {

}

And inside the catch block you can write code to run when such an exception occurs.

void doThing(int x) {
    if (x == 0) {
        throw new RuntimeException("Cannot do something zero times");
    }
}

void main() {
    int x = 0;
    try {
        doThing(x);
    } catch (RuntimeException e) {
        IO.println("Something went wrong doing a thing.");
    }
}

Just as you cannot have an else without an if, you cannot have a catch without a try.

void main() {
    catch (RuntimeException e) {
        IO.println("Hello");
    }
}

Nor can you have a try without a catch.²

void main() {
    try {
        IO.println("Hello");
    }
}