Determine if a lambda expression is stateless or stateful in Java

Question

Is there a function which accepts a reference to a lambda expression and returns a boolean saying whether the lambda expression is stateless or not? How can the statefulness of

Answer 1

Well, a lambda expression is just an instance of a special anonymous class that only has one method. Anonymous classes can "capture" variables that are in the surrounding scope. If your definition of a stateful class is one that carries mutable stuff in its fields (otherwise it's pretty much just a constant), then you're in luck, because that's how capture seems to be implemented. Here is a little experiment :

import java.lang.reflect.Field;
import java.util.function.Function;

public class Test {
    public static void main(String[] args) {
        final StringBuilder captured = new StringBuilder("foo");
        final String inlined = "bar";
        Function lambda = x -> {
            captured.append(x);
            captured.append(inlined);

            return captured.toString();
        };

        for (Field field : lambda.getClass().getDeclaredFields())
            System.out.println(field);
    }
}

The output looks something like this :

private final java.lang.StringBuilder Test$$Lambda$1/424058530.arg$1

The StringBuilder reference got turned into a field of the anonymous lambda class (and the final String inlined constant was inlined for efficiency, but that's beside the point). So this function should do in most cases :

public static boolean hasState(Function lambda) {
    return lambda.getClass().getDeclaredFields().length > 0;
}

EDIT : as pointed out by @Federico this is implementation-specific behavior and might not work on some exotic environments or future versions of the Oracle / OpenJDK JVM.