java - 相同类型的代码，但有一个代码显示异常

Question

我编辑了问题。这是完整的代码。看看吧。

  public class AddStaff extends Application {

    private Connection connect = null;
    private Statement statement = null;
    private ResultSet rs = null;
    private PreparedStatement preparedStatement = null;
    TextField addSubName;
    String msg;
    private TableView<Staff> table = new TableView<Staff>();
    private final ObservableList<Staff> data
            = FXCollections.observableArrayList();
    final HBox hb = new HBox();

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

    @Override
    public void start(Stage stage) {
        Scene scene = new Scene(new Group());
        stage.setTitle("Add Staff");
        stage.setWidth(450);
        stage.setHeight(550);
        stage.setResizable(false);
        final Label label = new Label("Staff Details");
        label.setFont(Font.font("Calibri", FontWeight.BOLD, 22));
        label.setTextFill(Color.WHITE);
        TableColumn sub = new TableColumn("Staff Name");
        sub.setMinWidth(400);
        sub.setCellValueFactory(
                new PropertyValueFactory<Staff, String>("subName"));
        sub.setCellFactory(TextFieldTableCell.forTableColumn());
        sub.setOnEditCommit(
                new EventHandler<TableColumn.CellEditEvent<Staff, String>>() {
                    @Override
                    public void handle(TableColumn.CellEditEvent<Staff, String> t) {
                        ((Staff) t.getTableView().getItems().get(
                                t.getTablePosition().getRow())).setSubName(t.getNewValue());
                    }
                }
        );

        table.setItems(data);
        table.getColumns().addAll(sub);
        addSubName = new TextField();
        addSubName.setPromptText("Staff Name");
        addSubName.setPrefSize(200, 30);    
        final Button b2 = new Button("Add");
        b2.setFont(Font.font("Calibri", FontWeight.BOLD, 17));
        b2.setPrefSize(70, 30);
        b2.setStyle(" -fx-base: #0066ff;");
        b2.setTextFill(Color.BLACK);
        b2.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent e) {

                msg = addSubName.getText();
                try {
                    enterStaff();
                } catch (ClassNotFoundException | SQLException ex) {
                    Logger.getLogger(AddStaff.class.getName()).log(Level.SEVERE, null, ex);
                }
                data.add(new Staff(addSubName.getText()));
                addSubName.clear();
            }
        });

        hb.getChildren().addAll(addSubName, b2);
        hb.setSpacing(5);
        final VBox vbox = new VBox();
        vbox.setSpacing(10);
        vbox.setPadding(new Insets(20, 20, 20, 20));
        vbox.getChildren().addAll(label, table, hb);
        vbox.setStyle("-fx-background-color: #333333;");
        ((Group) scene.getRoot()).getChildren().addAll(vbox);
        stage.setScene(scene);
        stage.show();
    }

    public void enterStaff() throws ClassNotFoundException, SQLException {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            connect = DriverManager
                    .getConnection("jdbc:mysql://localhost:3306/project?"
                            + "user=root&password=virus");
            statement = connect.createStatement();

            preparedStatement = connect
                    .prepareStatement("SELECT count(*)FROM information_schema.tables\n"
                            + "WHERE table_schema = 'project' AND table_name = 'staff'");
            rs = preparedStatement.executeQuery();
            rs.next();
            int chk = rs.getInt(1);

            if (chk != 1) {
                preparedStatement = connect
                        .prepareStatement("create table staff (staffname varchar(30) primary key");
                preparedStatement.executeUpdate();
            }

            preparedStatement = connect
                    .prepareStatement("insert into staff values(?)");
            preparedStatement.setString(1, msg);
            preparedStatement.executeUpdate();
        } catch (ClassNotFoundException | SQLException e) {
            throw e;
        } finally {
            close2();
        }

    }

    private void close2() {
        try {

            if (statement != null) {
                statement.close();
            }

            if (connect != null) {
                connect.close();
            }
        } catch (SQLException e) {

        }
    }
}

此代码产生异常 -

com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '' for key 'PRIMARY'

不知道为什么会出现这个异常。我尝试了很长时间来纠正它。我怎样才能避免这种异常？

我之前问过这个。看那个-com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '' for key 'PRIMARY'

Answer 1

我不认为这是你的代码，但你正试图向数据库中插入一个具有相同主键的条目。

Duplicate entry '' for key 'PRIMARY'

这意味着您要插入的项目将 ''(空)作为主键，并且您已经使用该键插入了一个项目。

问题可能是您正在创建表格，但它在上一次运行时已经存在。当您创建新表时，请始终先尝试 DROP。