A horrible hack to change a String.

gistfile1.java

import java.lang.reflect.Field;

class MainClass {
    // requires Java 7 update 5+
    static {
        try {
            Field value = String.class.getDeclaredField("value");
            value.setAccessible(true);
            value.set("B", value.get("ABC"));
        } catch (Throwable e) {
            throw new AssertionError(e);
        }
    }
    public static void main(String[] args) {
        System.out.println("B"); // prints ABC
    }
}