Thursday, October 13, 2011

why a nested java class can only access final local variables

The explanation is tedious because it has to cover primitive/reference type local variables.

1) Primitive is easier. Say there's local_boolean isGood variable. If it's final, then the nested class can simply get an implicit isGood final_private_boolean_field when the nested instance (I'm not saying "class") is allocated. Probably 1 byte added. All final primitive variables are immutable, which makes things simple.

2) Suppose this local variable is not final, and it's accessed by a static or non-static NestedClass.m1(). At run time, when m1() runs, isGood is quite possibly out of scope if the stack frame is wiped out. The "last" value of isGood is lost.

3) Contrast 2) with a final_local_reference variable acct, with a mutable Boolean field acct.isGood. Final, so NestedClass instantiation can allocate a 32-bit immutable pointer to the object behind acct. This will put off the garbage collector. When NestedClass.m1() runs, the real time value of acct.isGood will be used, even though it's mutable.

4) Now consider non-final_local_reference variable acct. acct can point at one object then another object. These objects are on heap, but the 32-bit local pointer is on stack. Stack frame could be gone when m1() runs. The address of the "last" object referenced by acct would be gone.

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