Java: Long result = -1: cannot convert from int to long

Question

I'm using eclipse java ee to perform java programming.

I had the following line of code in one of my functions:

Long result = -1;

I got the following error:

Type mismatch: cannot convert from int to Long

I can't quite understand why when i add a number to the variable it provides this error.

How can this issue be resolved and why did it happen in the first place?

Answer 1

There is no conversion between the object Long and int so you need to make it from long. Adding a L makes the integer -1 into a long (-1L):

Long result = -1L;

However there is a conversion from int a long so this works:

long result = -1;

Therefore you can write like this aswell:

Long result = (long) -1;

Converting from a primitive (int, long etc) to a Wrapper object (Integer, Long etc) is called autoboxing, read more here.

Answer 2

-1 can be auto-boxed to an Integer.

Thus:

Integer result = -1;

works and is equivalent to:

Integer result = Integer.valueOf(-1);

However, an Integer can't be assigned to a Long, so the overall conversion in the question fails.

Long result = Integer.valueOf(-1);

won't work either.

If you do:

Long result = -1L;

it's equivalent (again because of auto-boxing) to:

Long result = Long.valueOf(-1L);

Note that the L makes it a long literal.