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What is a NullPointerException, and how do I fix it?
（12个答案）
3年前关闭。
我的例外是在我调用GUI的行中：
XmasGUI tableGUI =新的XmasGUI（xmasDataModel）;
我的另一个例外是在我的XmasGUI.java代码中将内容面板设置为（rootPanel）的那一行。我不明白什么可能是空的。有人可以指出我正确的方向吗？似乎我将被标记为我们的不赞成票。看来我的问题总是（太含糊），但对我来说，这很简单，而且很直截了当。
（12个答案）
3年前关闭。
我的例外是在我调用GUI的行中：
XmasGUI tableGUI =新的XmasGUI（xmasDataModel）;
我的另一个例外是在我的XmasGUI.java代码中将内容面板设置为（rootPanel）的那一行。我不明白什么可能是空的。有人可以指出我正确的方向吗？似乎我将被标记为我们的不赞成票。看来我的问题总是（太含糊），但对我来说，这很简单，而且很直截了当。
import java.sql.*;
public class XmasDB {
static final String DB_CONNECTION_URL = "jdbc:mysql://localhost:3306/";
static final String USER = "root"; //TODO replace with your username
static final String PASS = "****"; //TODO replace with your password
static private final String DB_NAME = "xmas";
public final static String WANT_TABLE_NAME = "want_list";
public final static String NEED_TABLE_NAME = "need_list";
// Each solver will have a unique ID
public final static String PK_COLUMN = "id";
// A primary key is needed to allow updates to the database on modifications to ResultSet
public final static String NAME_COLUMN = "name";
public final static String PRICE_COLUMN = "price";
public final static String PRIORITY_COLUMN = "riority";
public final static int MOVIE_MIN_RATING = 1;
public final static int MOVIE_MAX_RATING = 5;
// Name our database
static Statement statement = null;
static Connection conn = null;
static ResultSet rs = null;
// Create out data model
private static XmasDataModel xmasDataModel;
public static void main(String args[]) {
//setup creates database (if it doesn't exist), opens connection, and adds sample data
if (!setup()) {
System.exit(-1);
}
if (!loadAllMovies()) {
System.exit(-1);
}
//If no errors, then start GUI
XmasGUI tableGUI = new XmasGUI (xmasDataModel);
}
//Create or recreate a ResultSet containing the whole database, and give it to movieDataModel
public static boolean loadAllMovies(){
try{
if (rs!=null) {
rs.close();
}
String getSomeData = "SELECT * FROM " + WANT_TABLE_NAME;
String getRestData = "SELECT * FROM " + NEED_TABLE_NAME;
rs = statement.executeQuery (getSomeData);
rs = statement.executeQuery (getRestData);
if (xmasDataModel == null) {
//If no current movieDataModel, then make one
xmasDataModel = new XmasDataModel (rs);
} else {
//Or, if one already exists, update its ResultSet
xmasDataModel.updateResultSet(rs);
}
return true;
} catch (Exception e) {
System.out.println("Error loading or reloading lists");
System.out.println(e);
e.printStackTrace();
return false;
}
}
public static boolean setup(){
try {
//Load driver class
try {
String Driver = "com.mysql.jdbc.Driver";
Class.forName(Driver);
} catch (ClassNotFoundException cnfe) {
System.out.println("No database drivers found. Quitting");
return false;
}
conn = DriverManager.getConnection(DB_CONNECTION_URL + DB_NAME, USER, PASS);
statement = conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);
//Does the table exist? If not, create it.
if (! xmasTableOneExists ()) {
//Create a table
String createNeedTableSQL = "CREATE TABLE " + NEED_TABLE_NAME + " (" + PK_COLUMN + " int NOT NULL AUTO_INCREMENT, " + NAME_COLUMN + " varchar(50), " + PRICE_COLUMN + " int, " + PRIORITY_COLUMN + " int, PRIMARY KEY(" + PK_COLUMN + "))";
System.out.println ( createNeedTableSQL );
statement.executeUpdate ( createNeedTableSQL );
System.out.println ( "Created Need table..." );
String addDataSQL = "INSERT INTO " + NEED_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES ('New Winter Boots', 85, 3)";
statement.executeUpdate ( addDataSQL );
addDataSQL = "INSERT INTO " + NEED_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES('New Car', 3000, 2)";
statement.executeUpdate ( addDataSQL );
addDataSQL = "INSERT INTO " + NEED_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES ('Gift Card to Grocery Store', 300, 1)";
statement.executeUpdate ( addDataSQL );
}else if (! xmasTableTwoExists ()) {
String createWantTableSQL = "CREATE TABLE " + WANT_TABLE_NAME + " (" + PK_COLUMN + " int NOT NULL AUTO_INCREMENT, " + NAME_COLUMN + " varchar(50), " + PRICE_COLUMN + " int, " + PRIORITY_COLUMN + " int, PRIMARY KEY(" + PK_COLUMN + "))";
System.out.println (createWantTableSQL);
statement.executeUpdate(createWantTableSQL);
System.out.println("Created Want Table...");
String addMoreSQL = "INSERT INTO " + WANT_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES ('Gaming PC', 500, 3)";
statement.executeUpdate(addMoreSQL);
addMoreSQL = "INSERT INTO " + WANT_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES('Astrology Book', 150, 2)";
statement.executeUpdate(addMoreSQL);
addMoreSQL = "INSERT INTO " + WANT_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES ('Hover Board', 300, 1)";
statement.executeUpdate(addMoreSQL);
}
return true;
} catch (SQLException se) {
System.out.println(se);
se.printStackTrace();
return false;
}
}
private static boolean xmasTableOneExists () throws SQLException {
String checkNeedTablePresentQuery = "SHOW TABLES LIKE '" + NEED_TABLE_NAME + "'"; //Can query the database schema
ResultSet tablesRS = statement.executeQuery(checkNeedTablePresentQuery);
if (tablesRS.next()) { //If ResultSet has a next row, it has at least one row... that must be our table
return true;
}
return false;
}
private static boolean xmasTableTwoExists () throws SQLException {
String checkWantTablePresentQuery = "SHOW TABLES LIKE '" + WANT_TABLE_NAME + "'";
ResultSet tableTwoRS = statement.executeQuery(checkWantTablePresentQuery);
if (tableTwoRS.next()) { //If ResultSet has a next row, it has at least one row... that must be our table
return true;
}
return false;
}
//Close the ResultSet, statement and connection, in that order.
public static void shutdown(){
try {
if (rs != null) {
rs.close();
System.out.println("Result set closed");
}
} catch (SQLException se) {
se.printStackTrace();
}
try {
if (statement != null) {
statement.close();
System.out.println("Statement closed");
}
} catch (SQLException se){
//Closing the connection could throw an exception too
se.printStackTrace();
}
try {
if (conn != null) {
conn.close();
System.out.println("Database connection closed");
}
}
catch (SQLException se) {
se.printStackTrace();
}
}
}
//(This is my XmasGUI.java Class)
/
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;
import java.util.Calendar;
/**
* Created by Nnamdi on 11/29/2016.
*/
public class XmasGUI extends JFrame implements WindowListener{
private JTable needTable;
private JTextPane NEEDTABLETextPane1;
private JTextPane WANTTABLETextPane;
private JTable wantTable;
private JButton addButton;
private JButton exitButton;
private JTextPane welcomeToXmasListTextPane;
private JButton transferButton;
private JRadioButton radioNeed;
private JRadioButton radioWant;
private JTextField txtNameInput;
private JSpinner prioritySpinner;
private JTextField txtPriceInput;
private JPanel rootPanel;
private JButton deleteButton;
XmasGUI(final XmasDataModel xmasDataTableModel) {
setContentPane(rootPanel);
pack();
setTitle("Christmas List Database Application");
addWindowListener(this);
setVisible(true);
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
//Set up JTables
needTable.setGridColor(Color.BLACK);
needTable.setModel(xmasDataTableModel);
wantTable.setGridColor (Color.YELLOW);
wantTable.setModel (xmasDataTableModel );
//Set up the priority spinner.
//SpinnerNumberModel constructor arguments: spinner's initial value, min, max, step.
prioritySpinner.setModel(new SpinnerNumberModel(1, XmasDB.MOVIE_MIN_RATING, XmasDB.MOVIE_MAX_RATING, 1));
exitButton.addActionListener ( new ActionListener ( ) {
@Override
public void actionPerformed ( ActionEvent e ) {
XmasDB.shutdown();
System.exit (0);
}
} );
deleteButton.addActionListener ( new ActionListener ( ) {
@Override
public void actionPerformed ( ActionEvent e ) {
int currentRow = needTable.getSelectedRow();
int currentrow = wantTable.getSelectedRow();
if (currentRow == -1) { // -1 means no row is selected. Display error message.
JOptionPane.showMessageDialog(rootPane, "Please choose a item to delete");
}
boolean deleted = xmasDataTableModel.deleteRow(currentRow);
if (deleted) {
XmasDB.loadAllMovies();
} else {
JOptionPane.showMessageDialog(rootPane, "Error deleting item");
}
}
} );
}
@Override
public void windowOpened ( WindowEvent e ) {
}
@Override
public void windowClosing ( WindowEvent e ) {
System.out.println("closing");
XmasDB.shutdown();
}
@Override
public void windowClosed ( WindowEvent e ) {
}
@Override
public void windowIconified ( WindowEvent e ) {
}
@Override
public void windowDeiconified ( WindowEvent e ) {
}
@Override
public void windowActivated ( WindowEvent e ) {
}
@Override
public void windowDeactivated ( WindowEvent e ) {
}
private void createUIComponents () {
// TODO: place custom component creation code here
}
}
最佳答案
首先，请确保已导入XmasGUI类，然后再使用它：
例如：类import com.mypackage.XmasGUI;中的XmasDB。
其次，setContentPane(rootPanel);是拼写错误。应该是setContentPanel(rootPanel);
第三，您需要确保将rootPanel的值传递给setContentPanel();时不为空。
您可以在将其传递给setContentPanel();之前对其进行初始化。
10-04 11:26