Читать книгу OCP Oracle Certified Professional Java SE 17 Developer Study Guide - Jeanne Boyarsky - Страница 181

The compiler applies flow scoping when working with pattern matching. Flow scoping means the variable is only in scope when the compiler can definitively determine its type. Flow scoping is unlike any other type of scoping in that it is not strictly hierarchical like instance, class, or local scoping. It is determined by the compiler based on the branching and flow of the program.

Given this information, can you see why the following does not compile?

void printIntegersOrNumbersGreaterThan5(Number number) { if(number instanceof Integer data || data.compareTo(5)>0) System.out.print(data); }

If the input does not inherit Integer, the data variable is undefined. Since the compiler cannot guarantee that data is an instance of Integer, data is not in scope, and the code does not compile.

What about this example?

void printIntegerTwice(Number number) { if (number instanceof Integer data) System.out.print(data.intValue()); System.out.print(data.intValue()); // DOES NOT COMPILE }

Since the input might not have inherited Integer, data is no longer in scope after the if statement. Oh, so you might be thinking that the pattern variable is then only in scope inside the if statement, right? Well, not exactly! Consider the following example that does compile:

void printOnlyIntegers(Number number) { if (!(number instanceof Integer data)) return; System.out.print(data.intValue()); }

It might surprise you to learn this code does compile. Eek! What is going on here? The method returns if the input does not inherit Integer. This means that when the last line of the method is reached, the input must inherit Integer, and therefore data stays in scope even after the if statement ends.