java - 为什么listView.getSelectionModel().getSelectedIndex()返回-1？

Question

在过去的几天里，这导致我在运行时出现大量错误- 并发修改异常和空指针异常

我不记得我添加了什么，但在这个段中添加了一段代码，它抛出了一个数组越界异常，导致我发现它在抛出的空指针异常之上抛出了一个负索引viewList 中没有任何内容。

为什么要这样做？这周我刚刚开始摆弄 javafx，这让我相信我一定在某个地方搞砸了？

ObservableList<BusinessCard> observableList = FXCollections.observableArrayList(); //initialized above

    // ListView Listener, changes text fields for the selected B.C in ViewList
    try {
        listView.getSelectionModel().selectedItemProperty().addListener(new ChangeListener<BusinessCard>() {
            @Override
            public void changed(ObservableValue<? extends BusinessCard> arg0, BusinessCard oldval,BusinessCard newVal) {
                int index = listView.getSelectionModel().getSelectedIndex();

                if(index == -1) return;  //Fix          

                BusinessCard card = observableList.get(index);                      
                setDataFields(card);         
            }
        });
    } catch (NullPointerException e1) {
        // Do nothing, NullPointer thrown when list is empty
    }

Answer 1

我认为documentation for the selectedIndex property这里说得很清楚了。

Refers to the selected index property, which is used to indicate the currently selected index value in the selection model. The selected index is either -1, to represent that there is no selection, or an integer value that is within the range of the underlying data model size.