在下面的示例中，我创建了两个 texField。在第一个文本字段上书写时，如果用户输入空格或数字，则应在另一个文本字段_1 上显示消息。但是一旦用户输入数字/空格，它就会给出 java.lang.NullPointerException。
public class NewDemo extends JFrame {
private JPanel p1;
private JTextField textField,textField_1;
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
NewDemo frame = new NewDemo();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
public NewDemo() {
setTitle("New Demo");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 500, 500);
p1 = new JPanel();
p1.setBorder(new EmptyBorder(5, 5, 5, 5));
setContentPane(p1);
p1.setLayout(null);
textField = new JTextField();
textField.setBounds(70, 71, 86, 20);
p1.add(textField);
textField.setColumns(10);
JLabel lblNewLabel = new JLabel("");
lblNewLabel.setBounds(70, 96, 86, 14);
p1.add(lblNewLabel);
textField_1 = new JTextField();
textField_1.setBounds(204, 71, 86, 20);
p1.add(textField_1);
textField_1.setColumns(10);
System.out.println("before calling");
((AbstractDocument) textField.getDocument()).setDocumentFilter(new MyDocumentFilter());
System.out.println("AfterCalling");
}
public JTextField getTextField_1() {
return textField_1;
}}
这是第二个类MyDocumentFilter，其中replace方法的else block 中发生了java.lang.NullPointerException错误。
class MyDocumentFilter extends DocumentFilter {
private NewDemo n1;
@Override
public void replace(FilterBypass fb, int i, int i1, String string, AttributeSet as) throws BadLocationException {
System.out.println("Starting: replace Method");
for (int n = string.length(); n > 0; n--)
char c = string.charAt(n - 1);
System.out.println(c);
if (Character.isAlphabetic(c)) {
System.out.println("In if: replace method");
super.replace(fb, i, i1, String.valueOf(c), as);
} else {
System.out.println("Not allowed:BEFORE");
n1.getTextField_1().setText("not allowed");//***HERE IS THE ERROR
System.out.println("Not allowed:AFTER");
}
}
}
@Override
public void remove(FilterBypass fb, int i, int i1) throws BadLocationException {
System.out.println("In :remove method");
super.remove(fb, i, i1);
}
@Override
public void insertString(FilterBypass fb, int i, String string, AttributeSet as) throws BadLocationException {
System.out.println("In: insterString Method");
super.insertString(fb, i, string, as);
}}
最佳答案
您的 DocumentFilter 需要对显示的 NewDemo GUI 对象的引用。不要像另一个人建议的那样创建一个新的 NewDemo 实例，因为这将创建一个不显示的单独实例，而是传递适当的显示引用。
例如，
((AbstractDocument) textField.getDocument())
.setDocumentFilter(new MyDocumentFilter(this)); //!!
和
class MyDocumentFilter extends DocumentFilter {
private NewDemo n1;
public MyDocumentFilter(NewDemo n1) {
this.n1 = n1;
}
但是，是的，也请遵循 MadProgrammer 的任何建议，因为他对 Swing 和 Java 了如指掌。另外，您应该避免使用 null 布局和使用 setBounds(...) 进行组件放置，因为这会导致 GUI 非常不灵活，虽然它们在一个平台上看起来不错，但在大多数其他平台上看起来很糟糕，或者屏幕分辨率，并且很难更新和维护。
编辑使用 MadProgrammer 的建议，考虑允许 GUI 将 PropertyChangeListener 添加到 DocumentFilter。这样，任何类都可以监听 DocumentFilter 的“状态”变化。在这里，我创建了一个名为“valid”的 boolean 状态，每当 boolean 变量发生变化时都会通知监听器。
例如:
import java.awt.*;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import javax.swing.*;
import javax.swing.event.SwingPropertyChangeSupport;
import javax.swing.text.*;
public class NewDemo extends JPanel {
private static final long serialVersionUID = 1L;
private JTextField textField, textField_1;
private static void createAndShowGui() {
NewDemo mainPanel = new NewDemo();
JFrame frame = new JFrame("New Demo");
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.getContentPane().add(mainPanel);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
public NewDemo() {
setLayout(new GridLayout(1, 0, 5, 5));
setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
textField = new JTextField();
textField.setColumns(10);
add(textField);
textField_1 = new JTextField();
textField_1.setColumns(10);
add(textField_1);
MyDocumentFilter docFilter = new MyDocumentFilter(); // !!
((AbstractDocument) textField.getDocument()).setDocumentFilter(docFilter); // !!
docFilter.addPropertyChangeListener(MyDocumentFilter.VALID,
new PropertyChangeListener() {
@Override
public void propertyChange(PropertyChangeEvent evt) {
String text = ((Boolean) evt.getNewValue()) ? "Allowed"
: "Not Allowed";
textField_1.setText(text);
}
});
}
}
class MyDocumentFilter extends DocumentFilter {
public static final String VALID = "valid";
private SwingPropertyChangeSupport pcSupport = new SwingPropertyChangeSupport(
this);
private boolean valid = true;
@Override
public void replace(FilterBypass fb, int i, int i1, String string,
AttributeSet as) throws BadLocationException {
for (int n = string.length(); n > 0; n--) {
char c = string.charAt(n - 1);
System.out.println(c);
if (Character.isAlphabetic(c)) {
super.replace(fb, i, i1, String.valueOf(c), as);
setValid(true);
} else {
setValid(false);
}
}
}
@Override
public void remove(FilterBypass fb, int i, int i1)
throws BadLocationException {
super.remove(fb, i, i1);
}
@Override
public void insertString(FilterBypass fb, int i, String string,
AttributeSet as) throws BadLocationException {
super.insertString(fb, i, string, as);
}
public boolean isValid() {
return valid;
}
public void setValid(boolean valid) {
boolean oldValue = this.valid;
boolean newValue = valid;
this.valid = valid;
pcSupport.firePropertyChange(VALID, oldValue, newValue);
}
public void addPropertyChangeListener(PropertyChangeListener listener) {
pcSupport.addPropertyChangeListener(listener);
}
public void removePropertyChangeListener(PropertyChangeListener listener) {
pcSupport.removePropertyChangeListener(listener);
}
public void addPropertyChangeListener(String propertyName,
PropertyChangeListener l) {
pcSupport.addPropertyChangeListener(propertyName, l);
}
public void removePropertyChangeListener(String propertyName,
PropertyChangeListener l) {
pcSupport.removePropertyChangeListener(propertyName, l);
}
}
关于java - 如何使用 DocumentFIlter 从另一个类设置 JTextField 的文本？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/29054200/