从Java原语转换为包装类 [英] Converting from Java primitive to wrapper classes

问题描述

在将基元分配给包装类引用时，我对Java编译器的行为感到困惑。请参阅下面的代码。带注释的行不会编译。

I am mystified by the behavior of the Java compiler when assigning primitives to wrapper class references. Please see the code below. The lines with comments don't compile.

我不明白原因的逻辑：

1. 一个字节可以分配给字节或短，但不是整数或长参考


2. a 短可以分配到字节或短，但是不是整数或长参考


3. 一个 int 可以分配到字节，短或整数，但不是长参考


4. a long 可以分配到长，但不能字节，短或整数参考

1. a byte can be assigned to a Byte or Short, but not Integer or Long reference
2. a short can be assigned to a Byte or Short, but not Integer or Long reference
3. an int can be assigned to a Byte, Short, or Integer, but not Long reference
4. a long can be assigned to a Long, but not Byte, Short or Integer reference

我看不到模式。对此的任何见解都将非常有用。
谢谢。

I cannot see the pattern. Any insight into this will be really helpful. Thanks.

Byte s5 = (byte)7;
Short s6 = (byte)7;
Integer s7 = (byte)7;   // Does not compile
Long s8 = (byte)7;      // Does not compile

Byte s9 = (short)7;
Short s10 = (short)7;
Integer s11 = (short)7; // Does not compile
Long s12 = (short)7;    // Does not compile

Byte s1 = (int)7;
Short s2 = (int)7;
Integer s3 = (int)7;
Long s4 = (int)7;       // Does not compile

Byte s13 = (long)7;     // Does not compile
Short s14 = (long)7;    // Does not compile
Integer s15 = (long)7;  // Does not compile
Long s16 = (long)7;

推荐答案

让我们看一下<允许的转换类型a href =http://docs.oracle.com/javase/specs/jls/se8/html/jls-5.html#jls-5.2\"rel =nofollow>分配上下文。

主要：

分配上下文允许使用一个以下内容：

• 身份转换

• an identity conversion

扩大原始转换

扩大参考转化

一个拳击转换，可选择后跟扩展引用转换

a boxing conversion optionally followed by a widening reference conversion

取消装箱转换，可选择后跟扩展原始转换。

an unboxing conversion optionally followed by a widening primitive conversion.

（注意我强调一个。）

大多数您编译的示例，例如

Most of your examples that do not compile, for example

Integer s11 = (short)7;

要求扩展原始转换，然后进行装箱转换。这不是允许的转换。

require a widening primitive conversion followed by a boxing conversion. This is not a permitted conversion.

但是你可能想知道为什么下面的例子会编译：

But then you might wonder why the following example does compile:

Byte s9 = (short)7;

这是缩小的原始转换，然后是装箱转换。

这是一个特例：

此外，如果表达式是常量表达式 byte ， short ， char 或 int [...]如果变量的类型为：

In addition, if the expression is a constant expression of type byte, short, char, or int [...] a narrowing primitive conversion followed by a boxing conversion may be used if the type of the variable is:

• 字节，常量表达式的值可在字节。

• Byte and the value of the constant expression is representable in the type byte.

短，常量表达式的值可在<$类型中表示c $ c>短。

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

字符及其值常量表达式可在 char 类型中表示。

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

这种特殊情况是必要的，因为无法表达整数li比 int 更窄的类型的teral。

This special case is necessary because there is no way to express an integer literal of a type narrower than int.

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