Question

还出现了一些其他问题，但程序运行时出现了很多错误，没有输出。一个是空指针。现在它还说“类型Talk必须实现继承的抽象方法ActionListener.actionPerformed（ActionEvent）”。它也说“void是变量actionPerformed的无效类型。”

import javax.swing.*;

import java.awt.*;
import java.awt.event.*;

public class Talk extends JFrame implements ActionListener{
    JTextField first;
    JTextField second;
    JButton b = new JButton("ok");
    public Talk(){
        JPanel panel = new JPanel();
        add(panel);
        panel.setLayout(new FlowLayout(FlowLayout.LEFT,10,20));
        panel.add(new JLabel("First Name"));
        JTextField first = new JTextField(8);
        panel.add(first);
        panel.add(new JLabel("Last Name"));
        JTextField last = new JTextField(8);
        panel.add(last);
        panel.add(b);
        b.addActionListener(this);





public void actionPerformed(ActionEvent e) {
    if (e.getSource()==b){
        String data = first.getText();
        String data2 = second.getText();
        System.out.println("Welcome " + data + data2);
    }

}}

public static void main(String[] args){
    Talk frame = new Talk();
    frame.setTitle("Your Name");
    frame.setSize(500,500);
    frame.setLocationRelativeTo(null);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
}}

Answer 1

您已尝试在方法中声明方法
你试图在课堂外声明一种方法......

这两者是相关的......

    public Talk() { // Start of constructor
        //...still in method
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == b) {
            String data = first.getText();
            String data2 = second.getText();
            System.out.println("Welcome " + data + data2);
        }

    }
} // end of constructor

采用actionPerformed方法并将其移出构造函数...

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Talk extends JFrame implements ActionListener {

    JTextField first;
    JTextField second;
    JButton b = new JButton("ok");

    public Talk() {
        JPanel panel = new JPanel();
        add(panel);
        panel.setLayout(new FlowLayout(FlowLayout.LEFT, 10, 20));
        panel.add(new JLabel("First Name"));
        JTextField first = new JTextField(8);
        panel.add(first);
        panel.add(new JLabel("Last Name"));
        JTextField last = new JTextField(8);
        panel.add(last);
        panel.add(b);
        b.addActionListener(this);
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == b) {
            String data = first.getText();
            String data2 = second.getText();
            System.out.println("Welcome " + data + data2);
        }
    }

    public static void main(String[] args) {
        Talk frame = new Talk();
        frame.setTitle("Your Name");
        frame.setSize(500, 500);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

<强>更新

你也会得到一个NullPointerException，因为你正在影响你的变量......

// You declare these instance variables here...
JTextField first;
JTextField second;
JButton b = new JButton("ok");

public Talk() {
    But re-declare them here (as local variables)...
    //...
    JTextField first = new JTextField(8);
    //...
    JTextField last = new JTextField(8);

这意味着当actionPerformed方法被调用时first（second}仍然是null。

删除本地重新声明......

public Talk() {
    //...
    first = new JTextField(8);
    //...
    last = new JTextField(8);

Answer 2

试试这个，

你得到null pointer exception，因为你没有使用你声明的类变量（改为使用局部变量）。

import javax.swing.*;

import java.awt.*;
import java.awt.event.*;

public class Talk extends JFrame implements ActionListener{
    JTextField first;
    JTextField second;
    JTextField result;
    JLabel res=new JLabel("result");
    JButton b = new JButton("ok");
    public Talk(){
        JPanel panel = new JPanel();
        add(panel);
        panel.setLayout(new FlowLayout(FlowLayout.LEFT,10,20));
        panel.add(new JLabel("First Name"));
        first = new JTextField(8);
        panel.add(first);
        panel.add(new JLabel("Last Name"));
        second = new JTextField(8);
        panel.add(second);
        panel.add(b);
        b.addActionListener(this);

        panel.add(res);
        result = new JTextField(8);
        panel.add(result);


    }


public void actionPerformed(ActionEvent e) {
    if (e.getSource()==b){
        String data = first.getText();
        String data2 = second.getText();
        result.setText("Welcome " + data +" "+data2);
        //System.out.println("Welcome " + data + data2);
    }

}

public static void main(String[] args){
    Talk frame = new Talk();
    frame.setTitle("Your Name");
    frame.setSize(700,700);
    frame.setLocationRelativeTo(null);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
}

}

Answer 3

这是你的解决方案..你已经在类级别声明变量第一和第二，但没有使用它们

 import javax.swing.*;

    import java.awt.*;
    import java.awt.event.*;

    public class Talk extends JFrame implements ActionListener{
        JTextField first;
        JTextField second;
        JButton b = new JButton("ok");
        public Talk(){
            JPanel panel = new JPanel();
            add(panel);
            panel.setLayout(new FlowLayout(FlowLayout.LEFT,10,20));
            panel.add(new JLabel("First Name"));
            first = new JTextField(8);
            panel.add(first);
            panel.add(new JLabel("Last Name"));
             second = new JTextField(8);
            panel.add(second);
            panel.add(b);
            b.addActionListener(this);
        }

    public static void main(String[] args){
        Talk frame = new Talk();
        frame.setTitle("Your Name");
        frame.setSize(500,500);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        System.out.println("in actionPerformed1");
        if (arg0.getSource()==b){
            String data = first.getText();
            String data2 = second.getText();
            System.out.println("Welcome " + data + data2);
        }

    }}

为什么这个程序不会显示文本？

3 个答案: