javaif语句文本提示不起作用

Question

对编码来说非常陌生，一点也不是我的强项。。。最初，我的文本提示会说你需要"输入一个名字"，但由于某种原因，现在没有了。有其他方法写这个代码吗？

还有谁能告诉我如何用JTextArea创建JPanel吗？当按下按钮时，文本区域会显示"name"is饥饿？我看了看各种各样的东西，但还是想不通。感谢

    import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFormattedTextField;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
public class GuiEnvironment extends JFrame implements ActionListener {
private static final long serialVersionUID = 8573501273671847629L;
private JPanel firstPanel, labelPanel, wholeGUI, buttonPanel, textPanel, frog, fly;
private JButton newResetButton, newHungryButton, newPetButton;
private JTextField newName;
private JLabel state;
private Frog guiFrog;
private Fly guiFly;


public JPanel newEnvironment() {
    wholeGUI = new JPanel();
    wholeGUI.setLayout(null);

    // area for pet to chase upon 
    firstPanel = new JPanel();
    firstPanel.setLocation(20, 20);
    firstPanel.setBackground(Color.WHITE);
    firstPanel.setSize(550, 550);
    firstPanel.setLayout(null);
    wholeGUI.add(firstPanel);
    //panel for buttons
    buttonPanel = new JPanel();
    GridLayout newLayout = new GridLayout();
    buttonPanel.setLayout(newLayout);
    buttonPanel.setLocation(50, 575);
    buttonPanel.setSize(520, 50);
    wholeGUI.add(buttonPanel);
    //name button
    newName = new JFormattedTextField("");
    newName.setLocation(140,0);
    newName.setSize(50,30);
    buttonPanel.add(newName);
    //create pet button
    newPetButton = new JButton("Create Pet");
    newPetButton.setLocation (150, 0);
    newPetButton.setSize(50,30);
    newPetButton.addActionListener(this);
    buttonPanel.add(newPetButton);
    // clear pet area
    newResetButton = new JButton("Reset");
    newResetButton.setLocation(420, 0);
    newResetButton.setSize(50, 30);
    newResetButton.addActionListener(this);
    buttonPanel.add(newResetButton);
    // make pets hungry 
    newHungryButton = new JButton("Make Hungry");
    newHungryButton.setLocation(280, 0);
    newHungryButton.setSize(50, 30);
    newHungryButton.addActionListener(this);
    buttonPanel.add(newHungryButton);
    //input panel
    textPanel = new JPanel();
    textPanel.setLayout(null);
    textPanel.setLocation(0, 550);
    textPanel.setSize(120, 120);
    firstPanel.add(textPanel);

    wholeGUI.setOpaque(true);
    return wholeGUI;
    }


private static void createAndShowGUI() {
    JFrame.setDefaultLookAndFeelDecorated(true);
    JFrame frame = new JFrame("[=***] The Hungry Cyber Pet. [***=]");
    GuiEnvironment test = new GuiEnvironment();
    frame.setContentPane(test.newEnvironment());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setSize(800,800);
    frame.setVisible(true);
}


public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            createAndShowGUI();
        }
    });
}

public void actionPerformed(ActionEvent e){
    if(e.getSource() == newPetButton && newName.getText().length() != 0){
        String name = newName.getText();
        guiFrog = new Frog(name);
        guiFrog.getDisplayArea(frog);
        guiFrog.getTextArea();
        firstPanel.add(guiFrog);
        guiFrog.start();
        guiFly = new Fly();
        guiFly.getDisplayArea(fly);
        firstPanel.add(guiFly);
        guiFrog.insertFly(guiFly);
        guiFly.start();
        firstPanel.revalidate();
        firstPanel.repaint();
    }
    if (e.getSource() == newPetButton && newName.getText().length()==0)
     {
        state.setText("You must enter a name");      
     }
    else if(e.getSource() == newResetButton){
        firstPanel.removeAll();
        firstPanel.revalidate();
        firstPanel.repaint();
    }
    else if(e.getSource() == newHungryButton){
        guiFrog.setHungry();
    }
}
}

Answer 1

您是否尝试过使用JOptionPane作为提示，然后存储值？

if (e.getSource() == newPetButton && newName.getText().length()==0)
 {
    String name =  JOptionPane.showInputDialog(state, "You must enter a name");
    state.setText(name);      
 }

我对Swing做的不多，但我以前用过它来获取用户的输入。

至于您的另一个问题，当按下JButton时，下面的代码可能适用于在JTextArea中显示文本：

    // creates the JPanel that will hold JButton and JTextArea
    JPanel textAndButtonPanel = new JPanel();
    // the JButton the user will press to see if "name" is hungry
    JButton hungryButton = new JButton("Am I Hungry?");
    // the JTextArea that will initially be blank, but will display
    // "name" is hungry when the JButton is pressed
    JTextArea textArea = new JTextArea(10, 10);
    // sets up the event-handling for the JButton. When the button
    // is pressed, the JTextArea will display "name" is hungry.
    hungryButton.addActionListener(new ActionListener() {   
        @Override
        public void actionPerformed(ActionEvent e) {
            textArea.setText(String.format("%s is hungry", name));
        }
    });
    // adds the JButton and JTextArea to the JPanel
    textAndButtonPanel.add(hungryButton);
    textAndButtonPanel.add(textArea);

Answer 2

您的主要问题之一是代码在此处抛出NullPointerException：

state.setText("You must enter a name");

你的问题没有告诉我们。

这是由于您没有初始化状态JLabel变量造成的。我猜它曾经在某个地方初始化过（即state = new JLabel();），但出于某种原因，您删除了此代码。解决方案是初始化这个变量。。。为它分配一个新的JLabel，然后将它添加到GUI中。此外，在将来，当您有问题，并且是关于错误或异常时，请在您的问题中包含所有相关信息。

其他无关的问题，不要这样做：wholeGUI.setLayout(null);。虽然在Swing新手看来，空布局和setBounds()可能是创建复杂GUI的最简单、最好的方法，但创建的Swing GUI越多，在使用它们时就会遇到更严重的困难。当GUI调整大小时，它们不会调整组件的大小，它们是一个需要增强或维护的皇家女巫，当放在滚动窗格中时，它们会完全失败，当在所有平台上查看时，或者当屏幕分辨率与原始分辨率不同时，它们看起来非常糟糕。学习如何使用布局管理器，然后充分使用它们。