Java boxing is violable

Foo.java

import java.lang.reflect.*;
import java.util.*;

public class Foo {
    public static void main(String[] argv) throws Exception {
        Class<?> integerCacheClass = Arrays.stream(Integer.class.getDeclaredClasses())
            .filter(c -> c.getSimpleName().equals("IntegerCache"))
            .findFirst()
            .get();
        Field cacheField = Arrays.stream(integerCacheClass.getDeclaredFields())
            .filter(c -> c.getName().equals("cache"))
            .findFirst()
            .get();
        cacheField.setAccessible(true);
        Integer[] cache = (Integer[]) cacheField.get(null);
        Arrays.fill(cache, new Integer(1337));

        Integer boxed = 3;
        System.out.println("" + boxed);
    }
}

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by Foo (file:/private/tmp/) to field java.lang.Integer$IntegerCache.cache
WARNING: Please consider reporting this to the maintainers of Foo
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
1337