Conversion from Integers

To convert from an int to a double, you don't need to do any special work. All ints are representable as doubles so it is a "widening conversion" and will be handled automatically by Java when performing an assignment.

void main() {
int x = 5;
double y = x;

System.out.println(x);
System.out.println(y);
}

This is not true in an expression. Even if the result of a computation between ints is being assigned to a double, the computation will still be performed using the same rules ints usually follow.

void main() {
int x = 7;
int y = 2;
// integer division of 7 and 2 gives 3.
double z = x / y;

System.out.println(z);
}

To perform math on an int and have that int behave as if it were a double, you need to convert said int into a double using a cast expression and the (double) cast operator.

void main() {
int x = 7;
int y = 2;
// This converts x into a double before performing the division
// so the result will be 3.5.
double z = (double) x / y;

System.out.println(z);
}