用户界面-为什么我得到这个异常?尝试用Java做一个简单的GUI

Question

import java.awt.*;

import javax.swing.*;


public class userInput extends JFrame {

private JButton newEntry;
private JButton deleteEntry;
private JButton editEntry;
private JButton saveEntry;
private JButton cancelEntry;
private FlowLayout layout;


public userInput() {
    super("My Address Book"); //sets the title!
    JTextField field = new JTextField(20);
    Container content = getContentPane();
    JPanel buttonPanel = new JPanel();
    buttonPanel.setLayout(new FlowLayout());
    buttonPanel.add(newEntry);
    buttonPanel.add(deleteEntry);
    buttonPanel.add(editEntry);
    buttonPanel.add(saveEntry);
    buttonPanel.add(cancelEntry);
    add(buttonPanel, BorderLayout.SOUTH);
    content.setLayout(new BorderLayout());
    content.add(buttonPanel, "South");
    setVisible(true);


}
}

下面是我的驱动程序:

import javax.swing.*;

public class AddressBookGui {
   public static void main (String[] args)
   {
        userInput addressBook = new userInput();
        addressBook.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //whenever you hit x you will exit the program
        addressBook.setSize(750, 600);
        addressBook.setVisible(true);
   }
}

Answer 1

您必须在执行

操作之前初始化newEntry

newEntry = new JButton("foo");
buttonPanel.add(newEntry);

以及其他按钮

Answer 2

你忘了分配你的按钮:

newEntry = new JButton();
deleteEntry = new JButton();
...