In general, apex requires explicit conversion from one datatype to another however a few datatype can be implicitly converted like variables of lower numeric type to higher types.

Hierarchy of numbers: integer<long<double<decimal

Note: Once a value has been passed from a number of lower type to a number of higher type, the value is converted to the higher type of number.

• Ids can always be assigned to strings.
• String can be assigned to ids however at runtime the value is checked to ensure that it is a legitimate id, if it is not then a runtime error is thrown.
• If the numeric value of the right hand side exceeds the maximum value for an integer you get a compilation error. In this case, the solution is to append L or l to the numeric value so that it represents a long value which has a wider range.
• Arithmetic computation that produce values larger than the max values of the current type are set to overflow.

Ex: Integer i= 2147483647 + 1; // overflow