现有键的哈希表为空

Question

I have a very strange error in this code from Vaadin's ContainerHierarchicalWrapper :

for (Object object : children.keySet()) {
    LinkedList<Object> object2 = children.get(object);
}

The debugger shows this state:

How is that even possible? How can object2 be null ?

This is my actual code which causes the NPE (class EstablishRelationWindow ):

childrenContainer = new BeanItemContainer<>(PlaylistDTO.class);
childrenContainerHierarchy = new ContainerHierarchicalWrapper(childrenContainer);
buildChildrenTree(somePlaylistDTO);

private void buildChildrenTree(PlaylistDTO current) {
    childrenContainer.addBean(current);
    if(current.getChildren().isEmpty()) {
        childrenContainerHierarchy.setChildrenAllowed(current, false);
    } else {
        current.getChildren().forEach(child -> {
            buildChildrenTree(child);
            childrenContainerHierarchy.setParent(child, current); //line 104
        });
    }
}

Relevant output:

java.lang.NullPointerException: null
    at java.util.Collections.sort(Collections.java:175) ~[na:1.8.0_60]
    at com.vaadin.data.util.ContainerHierarchicalWrapper.updateHierarchicalWrapper(ContainerHierarchicalWrapper.java:191) ~[vaadin-server-7.6.8.jar:7.6.8]
    at com.vaadin.data.util.ContainerHierarchicalWrapper$PiggybackListener.containerItemSetChange(ContainerHierarchicalWrapper.java:838) ~[vaadin-server-7.6.8.jar:7.6.8]
    at com.vaadin.data.util.AbstractContainer.fireItemSetChange(AbstractContainer.java:242) ~[vaadin-server-7.6.8.jar:7.6.8]
    at com.vaadin.data.util.AbstractContainer.fireItemSetChange(AbstractContainer.java:228) ~[vaadin-server-7.6.8.jar:7.6.8]
    at com.vaadin.data.util.ContainerHierarchicalWrapper.fireItemSetChangeIfAbstractContainer(ContainerHierarchicalWrapper.java:506) ~[vaadin-server-7.6.8.jar:7.6.8]
    at com.vaadin.data.util.ContainerHierarchicalWrapper.setParent(ContainerHierarchicalWrapper.java:495) ~[vaadin-server-7.6.8.jar:7.6.8]
    at com.xinra.listaide.frontend.EstablishRelationWindow.lambda$3(EstablishRelationWindow.java:104) ~[classes/:na]
    at java.lang.Iterable.forEach(Iterable.java:75) ~[na:1.8.0_60]
    at com.xinra.listaide.frontend.EstablishRelationWindow.buildChildrenTree(EstablishRelationWindow.java:102) ~[classes/:na]

Edit: Code that creates PlaylistDTO objects:

public class DynamicProxyDTOFactory implements DTOFactory {

    @SuppressWarnings("unchecked")
    @Override
    public <T extends DTO> T createDTO(Class<T> type) {
        return (T) Proxy.newProxyInstance(
                type.getClassLoader(),
                new Class<?>[] { type },
                new DynamicProxy());
    }

    private static class DynamicProxy implements InvocationHandler {

        private Map<String, Object> properties = new HashMap<>();

        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            if(method.getName().startsWith("set")) {
                properties.put(method.getName().substring(3), args[0]);
                return null;
            } else if(method.getName().startsWith("get")) {
                return properties.get(method.getName().substring(3));
            } else if(method.getName().startsWith("is")) {
                return properties.get(method.getName().substring(2));
            } else {
                return method.invoke(this, args); //for equals, hashCode etc.
            }
        }   
    }

}

Answer 1

I put together an SSCCE and investigated the issue further. Turns out, this actually doesn't work at all:

    } else {
        return method.invoke(this, args); //for equals, hashCode etc.
    }

Instead I extended the DynamicProxy as shown in this example :

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if(method.getName().startsWith("set")) {
            properties.put(method.getName().substring(3), args[0]);
            return null;
        } else if (method.getName().startsWith("get")) {
            return properties.get(method.getName().substring(3));
        } else if (method.getName().startsWith("is")) {
            return properties.get(method.getName().substring(2));
        } else if (method.getName().equals("equals")) {
            return proxy == args[0];
        } else if (method.getName().equals("hashCode")) {
            return System.identityHashCode(proxy);
        } else if (method.getName().equals("toString")) {
            return proxy.getClass().getName() + "@" + Integer.toHexString(System.identityHashCode(proxy))
                    + " with InvocationHandler " + this;
        } else {
            throw new IllegalStateException(String.valueOf(method));
        }
    }   

This way hashCode and especially equals work as intended and the NPE does not occur.