javafxjava为什么我的静态变量被设置为null？

Question

编辑1：我想我可能已经弄明白了

这是我的initialize（）方法

public void initialize(URL location, ResourceBundle resources) {
    groupList.setItems(createGroupFilter());
    groupList.getSelectionModel().selectedItemProperty().addListener(obs -> {
        Group.setSelectedGroup(groupList.getSelectionModel().getSelectedItem());
        handleGroupSelected();
    });

我认为，当我重新加载groupList ListView时，将调用此SelectEditeProperty侦听器并将其设置为null

我已通过在侦听器中添加一个if条件修复了此问题：

public void initialize(URL location, ResourceBundle resources) {
    groupList.setItems(createGroupFilter());
    groupList.getSelectionModel().selectedItemProperty().addListener(obs -> {
        if(groupList.getSelectionModel().getSelectedItem() != null) {
            Group.setSelectedGroup(groupList.getSelectionModel().getSelectedItem());
            handleGroupSelected();
        }
    });

---

出于某种原因，我的静态“selectedGroup”变量被设置为null，我无法找出原因

我已执行了以下故障排除

我有一个名为“refresh（）”的方法，它调用一个名为“createGroupFilter（）”的方法。我已经包括了以下系统。出来println语句，以查看变量何时为空

private void refresh() {
    System.out.println("before groupList.setItems(createGroupFilter()); "  + Group.getSelectedGroup()); //TROUBLESHOOTING
    groupList.setItems(createGroupFilter());
    System.out.println("after groupList.setItems(createGroupFilter()); "  + Group.getSelectedGroup()); //TROUBLESHOOTING
    if(Group.getSelectedGroup() != null) {
        groupList.getSelectionModel().select(Group.getSelectedGroup());
        int n = groupList.getSelectionModel().getSelectedIndex();
        groupList.getFocusModel().focus(n);
        groupList.scrollTo(n);
        userList.setItems(createUserFilter());
        if(User.getSelectedUser() != null) {
            userList.getSelectionModel().select(User.getSelectedUser());
        }
    }
}

private FilteredList<Group> createGroupFilter() {
    System.out.println("start createGroupFilter(); " + Group.getSelectedGroup()); //TROUBLESHOOTING
    groups = MainScreenModel.getGroups();
    FilteredList<Group> filteredGroups = new FilteredList<Group>(groups, s -> true);
    groupSearchTxt.textProperty().addListener(obs -> {
        String filter = groupSearchTxt.getText().toLowerCase();
        if(filter == null || filter.trim().length() == 0) {
            filteredGroups.setPredicate(s -> true);
        } else {
            filteredGroups.setPredicate(s -> s.toString().toLowerCase().contains(filter));
        }
    });
    System.out.println("end createGroupFilter(); " + Group.getSelectedGroup()); //TROUBLESHOOTING
    return filteredGroups;
}

这将提供以下输出：

before groupList.setItems(createGroupFilter()); Group 1
start createGroupFilter(); Group 1
end createGroupFilter(); Group 1
after groupList.setItems(createGroupFilter()); null

所以我觉得这个组列表。setItems（）。。。是什么导致了这个问题

以下是主屏幕模型，供参考。getGroups（）方法：

public static ObservableList<Group> getGroups() {
    ObservableList<Group> groups = FXCollections.observableArrayList();
    try {
        PreparedStatement stmt = DatabaseConnection.getConnection()
                .prepareStatement("SELECT * FROM groups ORDER BY group_name ASC");
        ResultSet rs = stmt.executeQuery();
        while(rs.next()) {
            String gn = rs.getString("GROUP_NAME");
            String desc = rs.getString("DESCRIPTION");
            groups.add(new Group(gn, desc));
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return groups;
}

还有我的小组课：

public class Group {

    private static Group selectedGroup;

    private final StringProperty groupName   = new SimpleStringProperty();
    private final StringProperty description = new SimpleStringProperty();

    public Group(String hn, String desc) {
        setGroupName(hn);
        setDescription(desc);
    }

    public StringProperty hostNameProperty() {
        return this.groupName;
    }

    public String getGroupName() {
        return this.groupName.get();
    }

    public void setGroupName(String hn) {
        this.groupName.set(hn);
    }

    public StringProperty descriptionProperty() {
        return this.description;
    }

    public String getDescription() {
        return this.description.get();
    }

    public void setDescription(String desc) {
        this.description.set(desc);
    }

    public static Group getSelectedGroup() {
        return selectedGroup;
    }

    public static void setSelectedGroup(Group group) {
        selectedGroup = group;
    }

    @Override
    public String toString() {
        return this.groupName.get();
    }

}

Answer 1

我看不到你在代码中调用setSelectedGroup的任何地方。由于selectedGroup默认初始化为null，如果不调用setSelectedGroup，它将保持null

另一方面，如果（看起来）selectedGroup在某些点上是非空的，那么您没有向我们展示的一些代码必须调用该方法。Java变量不会自动更改其值

所以

---

Why is my static variable being set to null?

一定有什么东西在用null参数调用setSelectedGroup某个地方

So it appears to me that the groupList.setItems()... is what is causing the issue.

这是有道理的。也有可能是其他原因造成的；e、 g.另一条线

---

^{1-这就是问题所在}