为什么我的JOptionPane错误消息不会弹出？

Question

我制作了一个GUI程序，它计算在主文本字段中输入的内容的某些元素。如果文本字段为空，则应弹出一条消息，提示用户应在文本字段中输入文本。如果tfMain == null，我做了一个if语句，应该弹出一个JOptionPane消息，但是由于某种原因它不会。为什么没有弹出提示？

这是我的代码：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.lang.*;

public class LabExcer9 extends WindowAdapter implements ActionListener
{
    //Container
    private Frame f;
    private Panel p1,p2,p3;
    //Component
    private Button bReadAndComp, bClear;
    private TextField tfMain,tf1,tf2,tf3,tf4,tf5,tf6;
    private Label l1,l2,l3,l4,l5,l6;

    public LabExcer9()
    {
        f = new Frame("Character Counter of Miguel Martin");

        p1 = new Panel();
        p2 = new Panel();
        p3 = new Panel();

        bReadAndComp = new Button("Read and Compute");
        bClear = new Button("Clear ALL Values");

        tfMain = new TextField(null);
        tf1 = new TextField("0");
        tf2 = new TextField("0");
        tf3 = new TextField("0");
        tf4 = new TextField("0");
        tf5 = new TextField("0");
        tf6 = new TextField("0");

        l1 = new Label("Number of Words ");
        l2 = new Label("Number of Characters ");
        l3 = new Label("Number of Vowels ");
        l4 = new Label("Number of Consonants ");
        l5 = new Label("Number of Digits ");
        l6 = new Label("Number of Symbols and Spaces ");
    }

    public void assembleGUI()
    {
        p1.setLayout(new GridLayout(1,1));
        p1.setPreferredSize(new Dimension(200, 200));
        p1.add(tfMain);
        p2.setLayout(new GridLayout(6,2));
        p2.add(l1);
        p2.add(tf1);
        p2.add(l2);
        p2.add(tf2);
        p2.add(l3);
        p2.add(tf3);
        p2.add(l4);
        p2.add(tf4);
        p2.add(l5);
        p2.add(tf5);
        p2.add(l6);
        p2.add(tf6);
        p3.setLayout(new GridLayout(1,2));
        p3.add(bReadAndComp);
        p3.add(bClear);
        f.add(p1,BorderLayout.NORTH);
        f.add(p2,BorderLayout.CENTER);
        f.add(p3,BorderLayout.SOUTH);
        f.pack();
        f.addWindowListener(this);
        f.setVisible(true);

        //registers
        bReadAndComp.addActionListener(this);
        bClear.addActionListener(this);

    }

    public void windowClosing(WindowEvent we)
    {
        System.exit(0);
    }
    public void actionPerformed(ActionEvent ae)
    {
        Object source = ae.getSource();
        //gets main text
        String textString = tfMain.getText();
        //puts words into array

        int vowels = 0, consonants = 0, digits = 0, symbolsAndSpaces = 0;
        int characters = textString.length();
        if(tfMain.getText() == "" || tfMain.getText() == null )
        {
            if(source == bReadAndComp)
                JOptionPane.showMessageDialog(null, "Please Enter Text!");
        }
        else if(tfMain.getText() != null && tfMain.getText() != "")
        {
            if(source == bReadAndComp)
            {
                for(int i = 0;i<textString.length();i++)
                {
                    if(Character.isDigit(textString.charAt(i)))
                        digits++;
                    if(Character.isLetterOrDigit(textString.charAt(i)) == false)
                        symbolsAndSpaces++;
                    if(isVowel(textString.charAt(i)) == true)
                        vowels++;
                    else if(Character.isDigit(textString.charAt(i)) == false && isVowel(textString.charAt(i)) == false && textString.charAt(i) != ' ' && Character.isLetter(textString.charAt(i)) == true)
                        consonants++;
                }
                tf1.setText(""+textString.split(" ").length);
                tf2.setText(""+characters);
                tf3.setText(""+vowels);
                tf4.setText(""+consonants);
                tf5.setText(""+digits);
                tf6.setText(""+symbolsAndSpaces);
                //System.out.println(textStringArray[0]+"wat");
                //JOptionPane.showMessageDialog(null, "Please Enter Text!");
            }
            else if (source == bClear)
            {
                tf1.setText("0");
                tf2.setText("0");
                tf3.setText("0");
                tf4.setText("0");
                tf5.setText("0");
                tf6.setText("0");
                tfMain.setText(null);
            }
        }

    }

    public boolean isVowel(char c)
    {
        if (c == 'a' || c == 'A' ||c== 'e' ||c == 'E' ||c == 'i' ||c == 'I' ||c == 'o' ||c == 'O' ||c == 'u' ||c == 'U')
            return true;
        else
            return false;
    }

    public static void main(String args [])
    {
        LabExcer9 GUI = new LabExcer9();
        GUI.assembleGUI();
    }
}

Answer 1

要比较字符串，请使用equals或equalsIgnoreCase。

替换您的

if(tfMain.getText() == "" || tfMain.getText() == null ) 

带有以下内容的陈述：

if("".equals(tfMain.getText())){
}