import java.util.*;
import java.lang.reflect.*;
 
class Ideone {
	public static void main(String[] args) throws Exception {
		ClassA a = new ClassA();
		new ClassB().entry(a);
		a.print();
	}
}
class ClassA {
	private List<Object> list = new ArrayList<>();
	void print() {
		System.out.println(this.list);
	}
}
class ClassB {
	@SuppressWarnings("unchecked")
	public void entry(ClassA instance) throws Exception {
		Field list = instance.getClass().getDeclaredField("list");
		list.setAccessible(true);
		List<Object> actualList = (List<Object>) list.get(instance);
		actualList.add(new Object());
//		list.set(instance, actualList);
	}
}

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