Java widening conversions

Question

I\'m preparing for Java 7 certification and have the following question.

Byte b = 10 compiles ok. Looks like the compiler is narrowing int 10 to byte 10

Answer 1

Section 5.2 of the JLS covers the types of conversions that are allowed in assignment contexts.

Assignment contexts allow the use of one of the following:

• an identity conversion (§5.1.1)

• a widening primitive conversion (§5.1.2)

• a widening reference conversion (§5.1.5)

• a boxing conversion (§5.1.7) optionally followed by a widening reference conversion

Additionally,

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.

  • Short and the value of the constant expression is representable in the type short.

  • Character and the value of the constant expression is representable in the type char.

Byte b = 10 compiles ok because 10 is a constant expression and is representable as a byte.

Byte b = new Byte(10) won't compile because 10 is an int literal, and method invocation conversion won't perform primitive narrowing conversions. To get this call to a Byte constructor to compile, you can explicitly cast 10 to byte:

Byte b = new Byte( (byte) 10);

Long l = new Long(10) compiles because method invocation conversion will perform primitive widening conversions, including from int to long.

Long l = 10 won't compile, because Java will not specifically allow a widening conversion followed by a boxing conversion, as I discussed in a recent answer. To get this to compile, you can use a long literal, so only boxing is necessary.

Long l = 10L;