Java actionPerformed方法中存在NullPointerException，但不在main中？
我理解NullPointerException是什么，但我的代码不理解当我移动Java actionPerformed方法中存在NullPointerException，但不在main中？,java,Java,我理解NullPointerException是什么，但我的代码不理解当我移动 public class DictionaryClient implements ActionListener { JFrame frame = new JFrame("Amazing CW"); JPanel panel = new JPanel(); JButton button = new JButton("Send"); JTextField tex
public class DictionaryClient implements ActionListener {
JFrame frame = new JFrame("Amazing CW");
JPanel panel = new JPanel();
JButton button = new JButton("Send");
JTextField text = new JTextField("Field");
Book book;
DictionaryService port;
public DictionaryClient() {
panel.add(button);
panel.add(text);
frame.add(panel);
button.addActionListener(this);
frame.setSize(300,300);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
public static void main(String[] args){
DictionaryClient client = new DictionaryClient();
DictionaryServiceService service = new DictionaryServiceService();
DictionaryService port = service.getDictionaryServicePort();
Book book = port.sendBook();
}
@Override
public void actionPerformed(ActionEvent e) {
if(e.getSource() == button){
System.out.print(book.getAuthor() + " " + book.getTitle());
}
}
}
System.out.print(book.getAuthor() + " " + book.getTitle());
bookDictionaryClientbooknullBook book = port.sendBook();
DictionaryClientprintactionPerformedbookbookDictionaryClientbooknullBook book = port.sendBook();
DictionaryClient现在，只要主方法结束，局部变量超出范围，该值就会丢失。当您将
printactionPerformedbookclient.book = port.sendBook();
这将正确设置类级别book变量。然后，actionPerformed方法将相应地引用该实例变量。您的Book对象在main方法中只有局部作用域。您需要将其范围更改为类的本地范围（您已经引用了该类） 只要换一行就行了
client.book = port.sendBook();
client.book = port.sendBook();
actionPerformedclient.book = port.sendBook();
actionPerformed但is从未初始化，因此是NPE。Perfect:）谢谢！完美：）谢谢你！由于
mainDictionaryClientbookmainDictionaryClientbook