Test your JVM's string interning behaviour under string mutation!

Evil.java

import java.lang.reflect.Field;

public class Evil {
    private static final Field valueField;

    static {
        try {
            valueField = String.class.getDeclaredField("value");
            valueField.setAccessible(true);
        } catch (NoSuchFieldException exc) {
            throw new AssertionError(exc);
        }
    }

    private static char[] getValue(String str) {
        try {
            return (char[]) valueField.get(str);
        } catch (IllegalAccessException exc) {
            throw new AssertionError(exc);
        }
    }

    private static String string(char... chars) {
        return new String(chars);
    }

    private static void dump(String legend, String str) {
        String intern = str.intern();
        System.out.printf("%15s: %s[%#x]; intern: %s[%#x]%n", legend,
                str, System.identityHashCode(str),
                intern, System.identityHashCode(intern));
    }

    public static void main(String[] args) {
        dump("'foo' initial", "foo");
        dump("'goo' initial", "goo");
        ++getValue("foo")[0];
        dump("'foo' changed", "foo");
        dump("'goo' unchanged", "goo");
        dump("'f', 'o', 'o'", string('f', 'o', 'o'));
        --getValue("goo")[0];
        dump("'foo' changed", "foo");
        dump("'goo' changed", "goo");
        dump("'g', 'o', 'o'", string('g', 'o', 'o'));
    }
}