class A {}
class B extends A {}
class C extends A {}
A x = new B();
B y = new B();
x instanceof C
y instanceof C
Why does y instanceof C give a compilation error (incompatible types) when x instanceof C works fine?
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1what do you mean, an error? it should just return false.Stultuske– Stultuske2021-03-25 08:49:49 +00:00Commented Mar 25, 2021 at 8:49
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1 Answer
When the compiler can tell that y instanceof C can never return true, it produces a compilation error. The compile time type of y is B, and class B has no relation to class C. Therefore, an instance of class B can never be an instance of class C.
On the other hand, x instanceof C may return true, since the compile time type of x is A, and C is a sub-clsss of A.
JLS refrence:
15.20.2. Type Comparison Operator instanceof
If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.
