问题描述
在以下示例中
int i = -128;
Integer i2 = (Integer) i; // compiles
Integer i3 = (Integer) -128; /*** Doesn't compile ***/
Integer i4 = (Integer) (int) -128; // compiles
Integer i4 = -128; // compiles
Integer i5 = (int) -128; // compiles
Integer i6 = (Integer) (-128); // compiles
Integer i7 = (Integer) 0-128; // compiles
我不能演示 -128 （Integer）但我可以转换（int）-128 。
I can't cast -128 with (Integer) but I can cast (int) -128.
我一直认为 -128 是 int 类型并使用（int）应该是多余的。
I always thought -128 was of int type and casting it with (int) should be redundant.
i3 是
cannot find symbol variable Integer
我尝试使用Java 6 update 29和Java 7 update 1.
I tried this with Java 6 update 29 and Java 7 update 1.
编辑： $ c> +128 而不是 -128 。
You get the same behavior with +128 instead of -128. It does appear to be confusion between unary and binary operators.
推荐答案
编译器尝试减去 128 到（Integer）而不是将 -128 / code>。添加（）修复它
The compiler tries to subtract 128 from (Integer) instead of casting -128 to Integer. Add () to fix it
Integer i3 = (Integer) -128; // doesn't compile
Integer i3 = (Integer) (-128); // compiles
根据BoltClock的意见，转换为 int 按预期工作，因为它是一个保留字，因此不能解释为一个标识符，这对我有意义。
According to BoltClock in the comments the cast to int works as intended, because it is a reserved word and therefore can't be interpreted as an identifier, which makes sense to me.
和Bringer128找到JLS参考。
And Bringer128 found the JLS Reference 15.16.
CastExpression:
( PrimitiveType Dims ) UnaryExpression
( ReferenceType ) UnaryExpressionNotPlusMinus
如你所见，转换为原始类型需要任何 UnaryExpression 转换为引用类型需要 UnaryExpressionNotPlusMinus 。这些是在之前的CastExpression之前定义的。
As you can see, casting to a primitive type requires any UnaryExpression, whereas casting to a reference type requires a UnaryExpressionNotPlusMinus. These are defined just before the CastExpression at JLS 15.15.
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