java - 为什么可以对 TableColumn<T, S> 中的两种泛型类型使用相同的泛型类型？

Question

在这个问题的答案中:Automatic row numbering in javafx table

我找到了原始问题的以下解决方案:

import javafx.application.Application;
import javafx.beans.property.ReadOnlyObjectWrapper;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.value.ObservableValue;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.geometry.Insets;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.TableCell;
import javafx.scene.control.TableColumn;
import javafx.scene.control.TableColumn.CellDataFeatures;
import javafx.scene.control.TableView;
import javafx.scene.control.cell.PropertyValueFactory;
import javafx.scene.layout.VBox;
import javafx.scene.text.Font;
import javafx.stage.Stage;
import javafx.util.Callback;

public class NumberedTableViewSample extends Application {

private TableView<Person> table = new TableView<Person>();
private final ObservableList<Person> data =
    FXCollections.observableArrayList(
        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")
    );

public static void main(String[] args) {
    launch(args);
}

@Override
public void start(Stage stage) {
    Scene scene = new Scene(new Group());
    stage.setTitle("Table View Sample");
    stage.setWidth(470);
    stage.setHeight(500);

    final Label label = new Label("Address Book");
    label.setFont(new Font("Arial", 20));

    table.setEditable(true);

    TableColumn numberCol = new TableColumn("#");
    numberCol.setMinWidth(20);
    numberCol.setCellValueFactory(new Callback<CellDataFeatures<Person, Person>, ObservableValue<Person>>() {
        @Override public ObservableValue<Person> call(CellDataFeatures<Person, Person> p) {
            return new ReadOnlyObjectWrapper(p.getValue());
        }
    });

    numberCol.setCellFactory(new Callback<TableColumn<Person, Person>, TableCell<Person, Person>>() {
        @Override public TableCell<Person, Person> call(TableColumn<Person, Person> param) {
            return new TableCell<Person, Person>() {
                @Override protected void updateItem(Person item, boolean empty) {
                    super.updateItem(item, empty);

                    if (this.getTableRow() != null && item != null) {
                        setText(this.getTableRow().getIndex()+"");
                    } else {
                        setText("");
                    }
                }
            };
        }
    });
    numberCol.setSortable(false);

    TableColumn firstNameCol = new TableColumn("First Name");
    firstNameCol.setMinWidth(100);
    firstNameCol.setCellValueFactory(
            new PropertyValueFactory<Person, String>("firstName"));

    TableColumn lastNameCol = new TableColumn("Last Name");
    lastNameCol.setMinWidth(100);
    lastNameCol.setCellValueFactory(
            new PropertyValueFactory<Person, String>("lastName"));

    TableColumn emailCol = new TableColumn("Email");
    emailCol.setMinWidth(200);
    emailCol.setCellValueFactory(
            new PropertyValueFactory<Person, String>("email"));

    table.setItems(data);
    table.getColumns().addAll(numberCol, firstNameCol, lastNameCol, emailCol);

    final VBox vbox = new VBox();
    vbox.setSpacing(5);
    vbox.setPadding(new Insets(10, 0, 0, 10));
    vbox.getChildren().addAll(label, table);

    ((Group) scene.getRoot()).getChildren().addAll(vbox);
    stage.setScene(scene);
    stage.show();
}

public static class Person {

    private final SimpleStringProperty firstName;
    private final SimpleStringProperty lastName;
    private final SimpleStringProperty email;

    private Person(String fName, String lName, String email) {
        this.firstName = new SimpleStringProperty(fName);
        this.lastName = new SimpleStringProperty(lName);
        this.email = new SimpleStringProperty(email);
    }

    public String getFirstName() {
        return firstName.get();
    }

    public void setFirstName(String fName) {
        firstName.set(fName);
    }

    public String getLastName() {
        return lastName.get();
    }

    public void setLastName(String fName) {
        lastName.set(fName);
    }

    public String getEmail() {
        return email.get();
    }

    public void setEmail(String fName) {
        email.set(fName);
    }
}

}

但是，我的问题是: 为什么可以使用

TableColumn<Person, Person> and TableCell<Person, Person>

当单元格的实际类型是 TableColumn 的类型而不是 Person 时？ JavaDoc 说第二个类型应该是:

S - The TableView type T - The TableColumn type

代码以某种方式工作，但是你能解释一下为什么两种类型都是 Person，尽管编号列只包含数字？这可能有助于理解代码。

据我了解，TableView 的类型应该是 Person，TableColumn 的类型应该是 String 或 int

Answer 1

第二个参数是相应列中包含的对象的类型。

正如您已经提到的，这些列通常包含从 Person 获取的单个属性，例如 String 或 Integer。

但是，在本例中，第一列仅包含所有 Person 对象。它们不显示。您可以通过更改行来显示它们

setText(this.getTableRow().getIndex()+"");

至

setText(this.getTableRow().getIndex()+": "+item);

但是，您也可以使用 SomethingElse 作为第二种类型:

class SomethingElse {}

TableColumn<Person, SomethingElse> numberCol = new TableColumn<Person, SomethingElse>("#");
numberCol.setMinWidth(20);
numberCol.setCellValueFactory(new Callback<CellDataFeatures<Person, SomethingElse>, ObservableValue<SomethingElse>>() {
    @Override public ObservableValue<SomethingElse> call(CellDataFeatures<Person, SomethingElse> p) {
        return new ReadOnlyObjectWrapper<SomethingElse>(new SomethingElse());
    }
});

numberCol.setCellFactory(new Callback<TableColumn<Person, SomethingElse>, TableCell<Person, SomethingElse>>() {
    @Override public TableCell<Person, SomethingElse> call(TableColumn<Person, SomethingElse> param) {
        return new TableCell<Person, SomethingElse>() {
            @Override protected void updateItem(SomethingElse item, boolean empty) {
                super.updateItem(item, empty);

                if (this.getTableRow() != null && item != null) {
                    setText(this.getTableRow().getIndex()+"");
                } else {
                    setText("");
                }
            }
        };
    }
});

它仅用于枚举行(顺便说一下，这是一种令人困惑的复杂方法)