When redeclaring Integer 'a' in line 33, why does jshell show the reference variable as an instance of Integer (refer to lines 38 & 39)? After the redeclaration, line 34 shows that 'a' is set to null. When 'a' is declared in line 6 but not given a value, or reset to null in line 22, 'a' is not considered an instance of Integer. I would expect that when the reference variable is redeclared, since its value is null, that it would not be an instance of a type; however, that is not the case.
01: java-lava:~ cafedude$ jshell
02: | Welcome to JShell -- Version 11
03: | For an introduction type: /help intro
04:
05: jshell> Integer a;
06: a ==> null
07: | created variable a : Integer
08:
09: jshell> a instanceof Integer;
10: $2 ==> false
11: | created scratch variable $2 : boolean
12:
13: jshell> a = 1;
14: a ==> 1
15: | assigned to a : Integer
16:
17: jshell> a instanceof Integer;
18: $4 ==> true
19: | created scratch variable $4 : boolean
20:
21: jshell> a = null;
22: a ==> null
23: | assigned to a : Integer
24:
25: jshell> a instanceof Integer;
26: $6 ==> false
27: | created scratch variable $6 : boolean
28:
29: jshell> a = 1;
30: a ==> 1
31: | assigned to a : Integer
32:
33: jshell> Integer a;
34: a ==> null
35: | modified variable a : Integer
36: | update overwrote variable a : Integer
37:
38: jshell> a instanceof Integer;
39: $9 ==> true
40: | created scratch variable $9 : boolean
The problem is that, though it says it is set to null, it actually isn't. See added comments in bug for more detail.
I've changed the bug title to: JShell: Redeclared variable should be reset
I'll attempt to fix in JDK 12.
The second issue is not a bug, Java does not allow instanceof operators that cannot be true -- behavior matches javac exactly.