Why does Java require an explicit cast on a final variable if it was copied from an array?

Question

Starting with the following code...

byte foo = 1;
byte fooFoo = foo + foo;

When I try compiling this code I will get the following error...

Answer 1

It is indeed what compiler do in constant folding when used with final, as we can see from byte code:

    byte f = 1;
    // because compiler still use variable 'f', so `f + f` will 
    // be promoted to int, so we need cast
    byte ff = (byte) (f + f);
    final byte s = 3;
    // here compiler will directly compute the result and it know
    // 3 + 3 = 6 is a byte, so no need cast
    byte ss = s + s;
    //----------------------
    L0
    LINENUMBER 12 L0
    ICONST_1 // set variable to 1
    ISTORE 1 // store variable 'f'
    L1
    LINENUMBER 13 L1
    ILOAD 1 // use variable 'f'
    ILOAD 1
    IADD
    I2B        
    ISTORE 2 // store 'ff'
    L2

    LINENUMBER 14 L2
    ICONST_3 // set variable to 3
    ISTORE 3 // store 's'
    L3
    LINENUMBER 15 L3
    BIPUSH 6 // compiler just compute the result '6' and set directly
    ISTORE 4 // store 'ss'

And if you change your final byte to 127, it will also complain:

    final byte s = 127;
    byte ss = s + s;

in which cases, the compiler compute the result and know it out of limit, so it will still complain they are incompatible.

More:

And here is another question about constant folding with string: