Java record pattern matching in JDK 19

Re:Java record pattern matching in JDK 19 by gizmo686 on Sunday May 15, 2022 @12:21AM (#31385236)

Consider the following pseudo-Java:

    Object foo = ...
    if (foo instanceof String) {
        System.out.println(foo.toLowerCase())
    }

The above is not valid Java, because foo has type Object, which does not have a toLowerCase() method, so the compiler refuses to compile it. What you need to do is something like:

    Object foo = ...
    if (foo instanceof String) {
        String sfoo = (String) foo
        System.out.println(sfoo.toLowerCase())
    }

Granted, you don't necessarily need to explicitly redefine in, as Java allows anonymous casting:

    Object foo = ...
    if (foo instanceof String) {
        System.out.println(((String) foo).toLowerCase())
    }

Regardless, the point still stands. Java (and most strongly typed languages with an inheritance based type system) requires an explicit cast, even if a straight forward static analysis would show that the object in question is already the desired type.

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Re:Java record pattern matching in JDK 19 by GavinBierman on Monday May 16, 2022 @05:20AM (#31394947)

This is all true, but with pattern matching for instanceof you don't need the explicit cast:

    Object foo = ...
    if (foo instanceof String s) {
        System.out.println(s.toLowerCase())
    }

In this case, the scope of the pattern variable `s` is just the then block, so are free to reuse it elsewhere in the block.

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