As per my understanding, Java automatically takes care of Autoboxing and Unboxing i.e., conversion of Primitives to Object Wrappers and vice a versa. However, unboxing doesn't seem to be working in below code.
public class TestMath {
public static void main(String[] args) {
Long resultLong = (Long) Math.pow(10, 10);
System.out.println(resultLong);
}
}
The above code gives me compilation error until I manually do unboxing by replacing (Long) with (long). I would like to understand the reason behind this.
The compilation error is as shown below:
Exception in thread "main" java.lang.Error: Unresolved compilation problem: Cannot cast from double to Long
Math.pow(10, 10) returns a double, which can be auto-boxed into a Double, not into a Long.
If you explicitly cast the result to long, the compiler can auto-box the long to Long.
As to why (Long) Math.pow(10, 10) doesn't work - there is no conversion from double to Long defined in the JLS. The only supported boxing conversions are from a primitive to its corresponding reference type:
5.1.7. Boxing Conversion
Boxing conversion converts expressions of primitive type to corresponding expressions of reference type. Specifically, the following nine conversions are called the boxing conversions:
From type boolean to type Boolean
From type byte to type Byte
From type short to type Short
From type char to type Character
From type int to type Integer
From type long to type Long
From type float to type Float
From type double to type Double
From the null type to the null type