Null Pointer on creating an Element Wrapper

Question

I am trying to create an editable TreeTableView from a XML Document. For this, i am wrapping the Elements from the Document within a class. The Data of the Elements is stored within the attributes key and val. My Wrapper consists of

private Element node;

private final StringProperty key = new SimpleStringProperty(this,  node.getAttribute("key"), "temp");
private final StringProperty val = new SimpleStringProperty(this, node.getAttribute("val"), "temp");

public ElementWrapper(Element n){
    System.out.println("creating element "+n.getNodeName());
    node = n;
}
public String getKey(){
    return key.get();
}
@Override
public String getVal(){
    return key.get();
}
public void setKey(String key){
    ((Element)node).setAttribute("key", key);
}
@Override
public void setVal(String value){
    ((Element)node).setAttribute("val", value);
}
@Override
public Element getElement(){
    return node;
}
@Override
public StringProperty keyProperty(){
    return key;
}
@Override
public StringProperty valProperty(){
    return val;
}

I wrote a recursive algorithm which creates the tree items and sets them with

TreeItem<NodeWrapper> newsub = new TreeItem<>(new ElementWrapper(current));

where current is the XML Element. At this Point i get a NullPointerException for lib.ElementWrapper.<init>(ElementWrapper.java:21) which is the second line of the Wrapper class posted above. How do i set this correctly?

Community · Accepted Answer · 2017-05-23 12:07:17Z

3

Change it to:

private Element node;

private final StringProperty key;
private final StringProperty val;

public ElementWrapper(Element n){
    System.out.println("creating element " + n.getNodeName());
    node = n;
    key = new SimpleStringProperty(this, node.getAttribute("key"));
    val = new SimpleStringProperty(this, node.getAttribute("val"));
}

Because these initializations are done before executing costructor:

private final StringProperty key = new SimpleStringProperty(this,  node.getAttribute("key"), "temp");
private final StringProperty val = new SimpleStringProperty(this, node.getAttribute("val"), "temp");

Here is nice question about initialization: Java order of Initialization and Instantiation.
Official documentation: Initializing Fields.

edited May 23, 2017 at 12:07

CommunityBot

11 silver badge

answered Nov 23, 2016 at 14:42

Andrii Abramov

10.9k12 gold badges81 silver badges107 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

chenino Over a year ago

Well this seems to be the reason with the Null Pointer, only Problem i got now is the elements wont be set with the desired value from the Attribute. maybe i got to figure out why they only see the initial value

Andrii Abramov Over a year ago

@chenino Sorry, could you provide more details what is wrong ?

chenino Over a year ago

Found the solution already, did an edit to your answer and marked it. If i set the last value on the constructur to null, values wouldnt be updated on the tree table. fixed this, now everythign is working for me. Thank you very much Andrii!

Collectives™ on Stack Overflow

Null Pointer on creating an Element Wrapper

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related