具有三个按钮的Java程序，每个按钮显示一个不同的文本，按下该按钮将在文本框中显示按钮上的文本

Question

I have an assignment to write a program that has three buttons, each displaying a different text that when pressed will display the text on the button in a text box. I think I have the basics of the program down but I can't get it to run. I've tried watching tutorials and reading up on the error I've been getting but I can't seem to figure it out. I am new to programming in java and have been pretty confused throughout the whole course. Any help would be greatly appreciated!

right now my errors are:
non-static variable this cannot be referenced from a static context
objButton1.addActionListener(this);

non-static variable this cannot be referenced from a static context
objButton2.addActionListener(this);

non-static variable this cannot be referenced from a static context
objButton3.addActionListener(this);

cannot find symbol
if (e.getSource()==objButton1)

cannot find symbol
else if (e.getSource()==objButton2)

import java.applet.Applet.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Option3 extends Frame implements ActionListener
{
    Option3()
    {
        setTitle("Option 3");
        setSize (300,300);
        show();
    }
    public static void main (String args[])
    {
        Frame objFrame;
        Button objButton1;
        Button objButton2;
        Button objButton3;
        TextField objTextField;

        objFrame = new Option3();
        objButton1 = new Button("A");
        objButton2 = new Button("B");
        objButton3 = new Button("C");
        objTextField = new TextField(100);

        objButton1.addActionListener(this);
        objButton2.addActionListener(this);
        objButton3.addActionListener(this);
    }

    public void actionPerformed(ActionEvent e)
    {
        if (e.getSource() == objButton1 )

            System.out.println("A");
        else if (e.getSource() == objButton2 )
            System.out.println("B");
        else
            System.out.println("C");    
    }
}

Answer 1

There are a couple of things you need to change in order to make this work.

1) If you want to access the buttons in the actionPerformed(...) method, you need to increase their scope. As it is now, those variables can be accessed only in the main(...) method.

2) You need to pass an instance of ActionListener to the addActionListener(...) method and because main(...) is a static method, you can't use the this keyword, what you can do is to use the Option3 instance that you've just created or a better solution will be to create those components inside the Option3 constructor where you can use this .

3) If you want to display the components you've created, you need to add them to the frame.

import java.applet.Applet.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Option3 extends Frame implements ActionListener {

    Button objButton1;
    Button objButton2;
    Button objButton3;
    TextField objTextField;

    Option3() {
        setTitle("Option 3");
        setSize (300,300); // is better to control the frame's size by using panels with appropriate layout managers.

        objButton1 = new Button("A");
        objButton2 = new Button("B");
        objButton3 = new Button("C");
        objTextField = new TextField(100);

        objButton1.addActionListener(this);
        objButton2.addActionListener(this);
        objButton3.addActionListener(this);

        Panel panel = new Panel(); // set a layout to this panel based on how you want the components to be displayed.
        panel.add(objButton1);
        panel.add(objButton2);
        panel.add(objButton3);
        panel.add(objTextField);

        add(panel);

        show();
    }
    public static void main (String args[]) {
        Frame objFrame = new Option3();

    }

    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == objButton1 ) { //delimit this kind of statements using curly braces to avoid confusion and bugs.
            System.out.println("A");
        } else if (e.getSource() == objButton2 ) {
            System.out.println("B");
        } else {
            System.out.println("C");    
        }
    }
}