63. Introducing type pattern matching for switch

JDK 17 (JEP 406) added type pattern matching for switch as a preview feature. A second preview was available in JDK 18 (JEP 420). The final release is available in JDK 21 as JEP 441.

Type pattern matching for switch allows the selector expression (that is, o in switch(o)) to be of any type not just an enum constant, number, or string. By “any type,” I mean any type (any object type, enum type, array type, record type, or sealed type)! The type pattern matching is not limited to a single hierarchy as it happens in the case of inheritance polymorphism. The case labels can have type patterns (referred to as case pattern labels or, simply, pattern labels), so the selector expression (o) can be matched against a type pattern, not only against a constant.

In the next snippet of code, we rewrote the example from Problem 58 via a type pattern for switch:

public static String save(Object o) throws IOException {
  return switch(o) {
    case File file -> "Saving a file of size: " 
              + String.format("%,d bytes", file.length());
    case Path path -> "Saving a file of size: " 
              + String.format("%,d bytes", Files.size(path));
    case String str -> "Saving a string of size: " 
              + String.format("%,d bytes", str.length());
    case null -> "Why are you doing this?";
    default -> "I cannot save the given object";
  }; 
}

The following figure identifies the main players of a switch branch:

Figure 2.33: Type pattern matching for switch

The case for null is not mandatory. We have added it just for the sake of completeness. On the other hand, the default branch is a must, but this topic is covered later in this chapter.