short s = 'a';       // valid
Short ss = 'a';      // valid
int i = 'a';         // valid
Integer ii = 'a';    // invalid

byte b;
final short s = 1;
final Short ss = 1;
final int i =1;
final Integer ii = i;
final long L = 1;
final Long LL =1L;

b = s;     // valid
b = ss;    // invalid
b = i;     // valid
b = ii;    // invalid
b = L;     // invalid
b = LL;    // invalid

解决方法@H_301_16@

所以,行：

Short s = 'a'; // is valid ...

因为char是无符号的16位值(最大值是65,536)而short是有符号的16位值(最大值是32,767),所以有一个缩小的原始转换(char to short),然后是一个装箱转换(short to short) ).

short s = 'a'; // is valid - this is a narrowing primitive conversion (char -> short)

这些是special cases：

In addition,if the expression is a constant expression of
type byte,short,char,or int:

• A narrowing primitive conversion may be used if the type of the
  variable is byte,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.

我们来看下一个例子：

Integer ii = 'a'; // is invalid - not a special case according to Oracle docs
int i = 'a';      // is valid - widening primitive conversion (char -> int) is allowed

还有一个案例来自你的问题：

byte b;
final long L = 1;
b = L // error - incompatible types

为什么线b = L无效？因为它不是上面描述的特殊情况,我们可以在演员表中丢失信息,这就是你必须明确地执行它的原因：

b = (byte) L; // is valid - narrowing primitive conversion (long -> byte)

另外,请看一下非常有用的table.

在JLS文档中有很多关于所有这些规则的信息,您无需担心所有这些规则.关于你的上一个问题,我能说的是,如果没有隐式的缩小转换,任何整数文字都需要在接下来的情况下使用强制转换：

// Cast is permitted,but not required - profit!
byte  b = (byte)  100;
short s = (short) 100;

多亏了我们可以将它改为：

byte  b = 100;
short s = 100;