javademos/recap26/src/FinalFieldMutation.java

import java.lang.reflect.Field;

/**
 * JEP 500 (Java 26): "Prepare to Make Final Mean Final". Deep reflection on a final field
 * (Field.setAccessible(true) followed by Field.setInt) used to work silently. On 26 it works but
 * warns once; a future release will deny it unless you opt in.
 *
 * The same file is run five ways by scripts/recap26.sh; the transcript is docs/output/70-final-field-mutation.txt.
 * Chapter: docs/11-recap-26.md
 */
public class FinalFieldMutation {
    static class Config {
        private final int port = 8080;
        int port() { return port; }
    }

    public static void main(String[] args) throws Exception {
        Config c = new Config();
        Field f = Config.class.getDeclaredField("port");
        f.setAccessible(true);
        try {
            f.setInt(c, 9090);
        } catch (IllegalAccessException e) {
            System.out.println("setInt refused : " + e.getMessage());
            return;
        }
        // The field reads 9090 but port() still says 8080: 'final int port = 8080' is a constant variable, so javac
        // copied the literal 8080 into port() at compile time. Nothing you write to the field can reach that copy.
        // That silent disagreement is the reason the platform wants this to stop, not a hypothetical.
        System.out.println("field reads    : " + f.getInt(c));
        System.out.println("port() returns : " + c.port());
    }
}