ListSurprise.java

gistfile1.txt

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;

import sun.misc.Unsafe;

public class ListSurprise {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        Collections.addAll(numbers, 3, 1, 4, 1, 5, 5, 9);
        Iterator<Integer> it = numbers.iterator();
        System.out.print(it.next());
        System.out.print('.');
        System.out.print(it.next());
        System.out.print(it.next());
        System.out.print(it.next());
        System.out.print(it.next());
        doSomething(numbers);
        System.out.print(it.next());

        if (!numbers.equals(List.of(3,1,4,1,5,9)))
            throw new AssertionError();
    }

    private static void doSomething(List<Integer> numbers) {
        try {
            // Don't log anything fancy https://stackoverflow.com/a/46458447/521799
            System.setErr(new PrintStream(new ByteArrayOutputStream()));
            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
            theUnsafe.setAccessible(true);
            Unsafe u = (Unsafe) theUnsafe.get(null);

            Class<?> cls = Class.forName("jdk.internal.module.IllegalAccessLogger");
            Field logger = cls.getDeclaredField("logger");
            u.putObjectVolatile(cls, u.staticFieldOffset(logger), null);

            // From now on, 5 is 9
            Class<?> integerCacheClass = Class.forName("java.lang.Integer$IntegerCache");
            Field integerCacheField = integerCacheClass.getDeclaredField("cache");
            Field integerValue = Integer.class.getDeclaredField("value");
            integerCacheField.setAccessible(true);
            integerValue.setAccessible(true);

            Object[] integerCache = (Object[]) integerCacheField.get(Integer.class);
            int modifiers = integerValue.getModifiers();
            Field modifiersField = integerValue.getClass().getDeclaredField("modifiers");
            modifiersField.setAccessible(true);
            modifiers = modifiers & ~Modifier.FINAL;
            modifiersField.setInt(integerValue, modifiers);
            integerValue.set(integerCache[133], 9);
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
}