In the example below (from my coursepack), we want to give to the Square instance c1 the reference of some other object p1, but on开发者_StackOverflow中文版ly if those 2 are of compatible types.

if (p1 instanceof Square) {c1 = (Square) p1;}

What I don't understand here is that we first check that p1 is indeed a Square, and then we still cast it. If it's a Square, why cast?

I suspect the answer lies in the distinction between apparent and actual types, but I'm confused nonetheless...

Edit:

How would the compiler deal with:

if (p1 instanceof Square) {c1 = p1;}

Edit2:

Is the issue that instanceof checks for the actual type rather than the apparent type? And then that the cast changes the apparent type?

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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!