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There is no error during auto-boxing of constants with int and short types to Byte, but constant with long type do has error. Why?
final int i = 3;
Byte b = i; // no error
final short s = 3;
Byte b = s; // no error
final long l = 3;
Byte b = l; // error
270
Converting a primitive value (an int, for example) into an object of the corresponding wrapper class (Integer) is called autoboxing. The Java compiler applies autoboxing when a primitive value is: Passed as a parameter to a method that expects an object of the corresponding wrapper class.
Autoboxing in Java is a process of converting a primitive data type into an object of its corresponding wrapper class. For example, converting int to Integer class, long to Long class or double to Double class, etc.
Autoboxing and unboxing lets developers write cleaner code, making it easier to read. The technique lets us use primitive types and Wrapper class objects interchangeably and we do not need to perform any typecasting explicitly.
The automatic conversion of primitive data types into its equivalent Wrapper type is known as boxing and opposite operation is known as unboxing. This is the new feature of Java5. So java programmer doesn't need to write the conversion code.
From JLS Sec 5.2, "Assignment contexts" (emphasis mine):
In addition, if the expression is a constant expression (§15.28) of type byte, short, char, or int:
A narrowing primitive conversion may be used if the type of the variable is byte, short, or char, and the value of the constant expression is representable in the type of the variable.
A narrowing primitive conversion followed by a boxing conversion may be used if the type of the variable is:
- Byte and the value of the constant expression is representable in the type byte.
- ...
It's simply not allowed for longs by the spec.
Note that the second bullet point here says that this happens irrespective of the boxing: assigning a constant long expression to a byte variable would similarly fail:
// Both compiler errors.
byte primitive = 0L;
Byte wrapped = 0L;
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