Java strange allocation rules

short s = 'a';       // valid

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

Why is integer II = 'a' invalid, but int i = 'a' valid? Why is short SS = 'a' valid, but integer II = 'a' invalid?

Another question:

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

Why B = L; Invalid, and B = s; Does it work?

Please don't say it's all because JLS says so I want to know why JLS has these inconsistent and non intuitive rules What did I miss?

Solution

So, line:

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

Because char is an unsigned 16 bit value (the maximum value is 65536) and short is a signed 16 bit value (the maximum value is 32767), there is a reduced original conversion (char to short), followed by a boxing conversion (short to short))

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

These are special cases:

Let's look at the next example:

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

Another case comes from your question:

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

Why is line B = l invalid? Because it is not a special case described above, we can lose information in the cast, which is why you must explicitly execute it:

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

In addition, please take a look at the very useful table

There is a lot of information about all these rules in JLS documents. You don't need to worry about all these rules On your last question, what I can say is that if there is no implicit reduction conversion, any integer text needs to be cast in the following cases:

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

Thanks to the fact that we can change it to:

byte  b = 100;
short s = 100;