java - Passing char into a method with an int parameter

Question

The output from the following code is 123 because substring takes from beginIndex to EndIndex - 1. However, I am surprised how char here is understood as 3 (int) because substring take two ints. What is the concept behind this?

String x = "12345";
char a = 3;
x = x.substring(0, a);
System.out.println(x);

Answer 1

This goes all the way back to C, where char is in essence a narrow integer type and gets implicitly converted to int whenever necessary.

In Java, this is technically known as a "widening primitive conversion", and is covered in section 5.1.2 of the JLS.