Why cast after an instanceOf?

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Old code will not work correctly

The implied cast feature is justified after all but we have trouble to implement this FR to java because of backward-compatibility.

See this:

public class A {
    public static void draw(Square s){...} // with implied cast
    public static void draw(Object o){...} // without implied cast
    public static void main(String[] args) {
        final Object foo = new Square();
        if (foo instanceof Square) {
            draw(foo);
        }
    }
}

The current JDK would compile the usage of the second declared method.
If we implement this FR in java, it would compile to use the first method!

🔴 JDK 14

We finally implemented this feature in JDK 14. As you might have noticed you can declare a new variable within the instanceof-linkage. This new variable has been defined by the value of a automatically downcast to the specified type.

if (any instanceof String s) {
  System.out.println(s);
}

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