在JavaFX中创建行索引列 [英] Creating a row index column in JavaFX
问题描述
我有一个JavaFX TableView，我正在填充一个Observable Tasks of Tasks。我一直在尝试创建一个显示每行索引的列，它作为表中任务的ID，但我尝试了方法此处和其他来源的类似方法收效甚微。
I have a JavaFX TableView that I'm populating with an ObservableList of Tasks. I've been trying to create a column that displays the index of each row, which serves as the ID for the tasks in the table, but I've tried the method here and similar methods from other sources with little success.
我的代码供参考没有表面错误（就Eclipse而言）：
My code for reference, which has no superficial errors (as far as Eclipse can tell):
@FXML private TableColumn<Task, String> taskIndexCol;
Callback<TableColumn<Task, String>, TableCell<Task, String>> cb =
new Callback<TableColumn<Task, String>, TableCell<Task, String>>(){
@Override
public TableCell<Task, String> call(TableColumn<Task, String> col) {
TableCell<Task, String> cell = new TableCell<Task, String>() {
@Override
protected void updateItem(String item, boolean empty) {
super.updateItem(item, empty);
if (item == null) {
setText("");
} else {
setText(getIndex()+"");
}
}
};
return cell;
}
};
taskIndexCol.setCellFactory(cb);
目前，当我尝试设置列的CellFactory时，我的代码给了我一个NullPointerException。我已经尝试了一个填充的任务列表，但这没有帮助。我已经被困了很长时间了 - 理论上这应该很简单，因为它只是对行进行编号？感觉就像是在跳过一百万个箍来做一些令人沮丧的简单事情。
Currently, my code gives me a NullPointerException when I try to set the CellFactory of the column. I've tried it with a populated task list, but that didn't help. I've been stumped for a very long time -- and theoretically this should be pretty easy, since it's just numbering the rows? It feels like I'm jumping through a million hoops to do something frustratingly simple.
编辑：最后一行给了我NPE。
The last line gives me the NPE.
推荐答案
无法告诉Null指针异常的原因，因为你没有显示堆栈跟踪，识别出抛出异常的行，或者发布的是你的代码中没有一个代码（回调中的代码都没有抛出空指针异常，所以其他地方出了问题）。
It's impossible to tell the cause of the Null pointer exception, because you haven't shown the stack trace, identified the line which throws the exception, or posted enough of your code (none of the code in your callback can throw a null pointer exception, so something is wrong somewhere else).
对于你的实际单元格实现，你没有' t显示您是否在列上设置了 cellValueFactory 。如果你不这样做，那么项目将总是 null ，所以你永远不会在该列的单元格中看到任何文本。您可以检查 empty 属性（或方法参数），以检查单元格是空行还是具有实际数据。 （注意这意味着该列实际上根本不需要提供任何数据：它可以是 TableColumn< Task，Void> 。）
For your actual cell implementation, you didn't show if you had a cellValueFactory set on the column. If you don't, then the item will always be null, and so you will never see any text in the cells in that column. You can check the empty property (or method parameter) as a means to check if the cell is in an empty row or one with actual data. (Note this means the column really doesn't need to provide any data at all: it can be a TableColumn<Task, Void>.)
此外，依靠使用 updateIndex（...）代替 updateItem（...）可能更安全。保证在索引更改时调用 updateIndex ;如果您实现 updateItem ，则假设在项目之前设置了索引，这意味着您依赖于实现细节。
Additionally, it's probably safer to rely on using updateIndex(...) instead of updateItem(...). updateIndex is guaranteed to be called when the index changes; if you implement updateItem you are assuming the index is set before the item, which means you are relying on an implementation detail.
如果使用Java 8 lambda表达式，您的代码会更短，更容易阅读：
Your code is a lot shorter and easier to read if you use Java 8 lambda expressions:
taskIndexCol.setCellFactory(col -> new TableCell<Task, String>() {
@Override
protected void updateIndex(int index) {
super.updateIndex(index);
if (isEmpty() || index < 0) {
setText(null);
} else {
setText(Integer.toString(index));
}
}
});
或者
taskIndexCol.setCellFactory(col -> {
TableCell<Task, String> cell = new TableCell<>();
cell.textProperty().bind(Bindings.when(cell.emptyProperty())
.then("")
.otherwise(cell.indexProperty().asString()));
return cell ;
});
这是一个SSCCE：
import java.util.function.Function;
import static javafx.beans.binding.Bindings.when ;
import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.beans.value.ObservableValue;
import javafx.scene.Scene;
import javafx.scene.control.TableCell;
import javafx.scene.control.TableColumn;
import javafx.scene.control.TableView;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;
public class TableViewWithIndexColumn extends Application {
@Override
public void start(Stage primaryStage) {
TableView<Person> table = new TableView<>();
table.setEditable(true);
table.getItems().addAll(
new Person("Jacob", "Smith", "jacob.smith@example.com"),
new Person("Isabella", "Johnson",
"isabella.johnson@example.com"),
new Person("Ethan", "Williams", "ethan.williams@example.com"),
new Person("Emma", "Jones", "emma.jones@example.com"),
new Person("Michael", "Brown", "michael.brown@example.com"));
TableColumn<Person, String> firstNameCol = createColumn("First Name",
Person::firstNameProperty, 150);
TableColumn<Person, String> lastNameCol = createColumn("Last Name",
Person::lastNameProperty, 150);
TableColumn<Person, String> emailCol = createColumn("Email",
Person::emailProperty, 150);
// index column doesn't even need data...
TableColumn<Person, Void> indexCol = createColumn("Index", person -> null, 50);
// cell factory to display the index:
// indexCol.setCellFactory(col -> {
//
// // just a default table cell:
// TableCell<Person, Void> cell = new TableCell<>();
//
// cell.textProperty().bind(
// when(cell.emptyProperty())
// .then("")
// .otherwise(cell.indexProperty().asString()));
//
// return cell ;
// });
indexCol.setCellFactory(col -> new TableCell<Person, Void>() {
@Override
public void updateIndex(int index) {
super.updateIndex(index);
if (isEmpty() || index < 0) {
setText(null);
} else {
setText(Integer.toString(index));
}
}
});
table.getColumns().add(indexCol);
table.getColumns().add(firstNameCol);
table.getColumns().add(lastNameCol);
table.getColumns().add(emailCol);
primaryStage.setScene(new Scene(new BorderPane(table), 600, 400));
primaryStage.show();
}
private <S, T> TableColumn<S, T> createColumn(String title,
Function<S, ObservableValue<T>> property, double width) {
TableColumn<S, T> col = new TableColumn<>(title);
col.setCellValueFactory(cellData -> property.apply(cellData.getValue()));
col.setPrefWidth(width);
return col;
}
public static class Person {
private final StringProperty firstName = new SimpleStringProperty();
private final StringProperty lastName = new SimpleStringProperty();
private final StringProperty email = new SimpleStringProperty();
public Person() {
this("", "", "");
}
public Person(String firstName, String lastName, String email) {
setFirstName(firstName);
setLastName(lastName);
setEmail(email);
}
public final StringProperty firstNameProperty() {
return this.firstName;
}
public final java.lang.String getFirstName() {
return this.firstNameProperty().get();
}
public final void setFirstName(final java.lang.String firstName) {
this.firstNameProperty().set(firstName);
}
public final StringProperty lastNameProperty() {
return this.lastName;
}
public final java.lang.String getLastName() {
return this.lastNameProperty().get();
}
public final void setLastName(final java.lang.String lastName) {
this.lastNameProperty().set(lastName);
}
public final StringProperty emailProperty() {
return this.email;
}
public final java.lang.String getEmail() {
return this.emailProperty().get();
}
public final void setEmail(final java.lang.String email) {
this.emailProperty().set(email);
}
}
public static void main(String[] args) {
launch(args);
}
}
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