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Comment by frant-hartm | Hacker News Reader

Two reasons - current convention and backwards compatibility. Java developers are simply used to the cast, it is commonly occurring pattern (even if it is from necessity).

The backwards compatibility part:

  class Foo {
    void foo(Object o) { }
  }
  class Bar extends Foo {
    void foo(Integer i) { }
  }
  ...
  Foo x = ...;
  Integer i = 42;
  if (x instanceof Bar) {
    x.foo(i);
  }

Currently this calls Foo.foo, if there was a smart cast it would call Bar.foo, possibly breaking existing code.

EDIT: Added method parameter.

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That's not correct. Java method dispatch is always dynamic, so x.foo() will always call the foo() method on whatever concrete type x actually is. The declared type of the variable that holds the reference to that object doesn't matter, nor does casting (with a couple exceptions, none of which apply here).

But don't trust me, try it!

    |  Welcome to JShell -- Version 11.0.5
    |  For an introduction type: /help intro
    
    jshell> class Foo { void foo() { System.out.println("Super"); } }
    |  created class Foo

    jshell> class Bar extends Foo { void foo() { System.out.println("Sub"); } }
    |  created class Bar

    jshell> Foo f = new Bar();
    f ==> Bar@58651fd0

    jshell> Bar b = new Bar();
    b ==> Bar@5419f379

    jshell> f.foo();
    Sub

    jshell> b.foo();
    Sub

    jshell> ((Foo)b).foo();
    Sub

That's right, I forgot a method parameter:

  jshell> class Foo { void foo(Object o) { System.out.println("Super"); } }
  |  created class Foo

  jshell> class Bar extends Foo { void foo(Integer i) { System.out.println("Sub"); } }
  |  created class Bar

  jshell> Foo f = new Bar();
  f ==> Bar@7f9a81e8

  jshell> Integer i = 1;
  i ==> 1

  jshell> f.foo(i)
  Super
  
  jshell> ((Bar)f).foo(i);
  Sub

Your statement is wrong, in the following sense: at runtime, the call to x.foo() would always start look at the runtime type (which in this case is Bar, or some subclass of it) - otherwise overriding methods would not work.

https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.htm...

Yes, I forgot a method parameter there.

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