如何制作数字猜谜游戏

Question

I need to make a guessing game program. I can't figure out any way to successfully log 2 guesses for comparison. I am having the most problems with the "actionPerformed" method. It's not linking to the constructor - like with txtFld and Lbl3, it says it's a null pointer. Here's what I have so far:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.Random;


public class GuessTheNumber extends JFrame
{
    Random randNum = new Random();
    int numToGuess = randNum.nextInt(1000);
    int guess1;
    int guess2;

    public static void main(String [] args)
    {
        new GuessTheNumber();

    } // end main

    public GuessTheNumber()
    {
        setTitle("Guess The Number");
        setLayout(new FlowLayout());

        JLabel promptLbl1 = new JLabel("I have a number between 1 and 1000.  Can you guess my number?");
        add(promptLbl1);
        JLabel promptLbl2 = new JLabel("Please enter your guess.");
        add(promptLbl2);
        JTextField txtFld = new JTextField(4);
        add(txtFld);
        JLabel Lbl3 = new JLabel();
        add(Lbl3);
        MyHandler handler = new MyHandler();
        txtFld.addActionListener(handler);
        setSize(300,300);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    }  //end constructor

    private class MyHandler implements ActionListener
    {
        public void actionPerformed(ActionEvent ev)
        {
            String gs1;
            guess1 = txtFld.getText();  
            guess1 = Integer.parseInt(gs1);
            String gs2;
            guess2 = txtFld.getText();  
            guess2 = Integer.parseInt(gs2);

            if (gs1 > gs2)
                txtFld.setBackground(color.blue);

            else if (gs1 < gs2)
                txtFld.setBackground(color.red);

            if (gs2 == numToGuess)  
            {
                Lbl3("Correct!"); 
                txtFld.setBackground(color.green);
            }

            else if (gs2 > numToGuess)
                Lbl3("Too High");

            else if (gs2 < numToGuess)
                Lbl3("Too Low");

        } // end actionPerformed  

    } // end MyHandler

} // end GuessTheNumber

Answer 1

The question you were asking about the null fields is because you declared your fields in your constructor instead of the class body. Just add them below the other declared fields and it should work:

public class GuessTheNumber extends JFrame
{
    Random randNum = new Random();
    int numToGuess = randNum.nextInt(1000);
    int guess1;
    int guess2;
    JLabel promptLbl1;
    JLabel promptLbl2;
    JTextField txtFld;
    JLabel Lbl3;

I found another problem as well:

You cannot compare String with == you need to use .equals instead like so:

if (gs2.equals(numToGuess))

Then it should work.

EDIT

Whoops, I just noticed that numToGuess is an int . In that case you should write

if (Integer.parseInt(gs2) == numToGuess)

or equivalently:

if (gs2.equals(String.valueOf(numToGuess))

Since you want to compare gs2 with numToGuess they need to be of the same type. So either you change gs2 to an Integer using Integer.parseInt() or you change numToGuess to a String using String.valueOf()

Answer 2

You have to move the declarations of the labels and text-fields from the constructor to GuessTheNumber.

public class GuessTheNumber extends JFrame
{
Random randNum = new Random();
int numToGuess = randNum.nextInt(1000);
int guess1;
int guess2;

JLabel promptLbl1;
JLabel promptLbl2;
JTextField txtFld;
JLabel Lbl3;
//...


public GuessTheNumber()
{
    setTitle("Guess The Number");
    setLayout(new FlowLayout());

    promptLbl1 = new JLabel("I have a number between 1 and 1000.  Can you guess my number?");
    add(promptLbl1);
    promptLbl2 = new JLabel("Please enter your guess.");
    add(promptLbl2);
    txtFld = new JTextField(4);
    add(txtFld);
    Lbl3 = new JLabel();
    add(Lbl3);
    //...

Otherwise you only have access to them in the constructor.