long

If an int is not big enough for your needs, a long is twice as big as an int and can represent numbers from -2^63 to 2^63 - 1.

You can make a long from an integer literal, but integer literals do not normally allow for numbers that an int cannot store.

void main() {
// Smaller numbers work without issue
long smallNumber = 5;
IO.println(smallNumber);
// This is too big for an int
long bigNumber = 55555555555;
IO.println(bigNumber);
}

For those cases you need to add an L to the end of the literal.¹

void main() {
long smallNumber = 5;
IO.println(smallNumber);
// But with an L at the end, its not too big for a long
long bigNumber = 55555555555L;
IO.println(bigNumber);
}

All operations with a long will result in a long. Conversions to int and other "smaller" integer types will be narrowing and require a cast.

void main() {
long a = 5;
int b = 3;
// a long times an int will result in a long
long c = a * b;
IO.println(c);
}

And if you have need of a potentially nullable long, Long with a capital L is the boxed version.

void main() {
// Can't have a null "long"
// long l = null; 

// But you can have a null "Long"
Long l = null;
IO.println(l);
}

The reason you will likely end up using int more than long is that int works more with other parts of Java. Like array indexing - you can't get an item in an array with a long, you need an int for that.

void main() {
String[] sadRobots = { "2B", "9S", "A2" };
long index = 2;
// Longs can't be used as indexes
String sadRobot = sadRobots[index];
}

But there is nothing wrong with a long. If you need to represent a number that is potentially bigger than an int then it is useful.