As traditional , the “impure” code of page 207 in the form of a pretend test.
(Doesn’t “impure” refer to performing uncontrolled side effects, like printing to Stdout in the middle of a function though? Here, the lambda of the of the map is really a function of two parameters, one of which is pulled from the context, but it being mutable ruins everything. The key is presence of mutability.
Suppose we have a lambda expression that depends on a field in a class.
That’s okay!
Suppose we have a lambda expression that depends on a mutable (non final) member in a surrounding context.
That’s not okay!
package chapter12;
import org.junit.jupiter.api.Test;
import java.util.List;
import java.util.stream.Collectors;
public class Impure {
private int factor = 2; // it's mutable, which is bad (it's not even volatile either)
@Test
void impureFunction() {
final var numbers = List.of(1, 2, 3);
final var stream = numbers.stream()
.map(number -> number * factor);
factor = 0;
System.out.println(stream.collect(Collectors.toList()));
}
}
The output here happens to be [0, 0, 0].