Question

当我尝试使用'j1.setText（“”）;'动态更新我的jlabel1时它不起作用并导致它发出大量错误，对此问题的解决方案的任何见解将不胜感激。出于测试目的，输入罪：130692544。

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


class SinChecker extends JFrame
{ //naming variables
JTextField t1;
static JLabel j, j1, j2, j3;
ButtonListener bl1;
ButtonListener2 bl2;

public SinChecker ()
{ //Get the container
    Container c = getContentPane ();

    //Set absolute layout
    c.setLayout (null);

    //Set Background Color
    c.setBackground (Color.WHITE);


    //Creating label Guess my number text
    JLabel j = new JLabel ("Social Insurance Calculator");
    j.setForeground (Color.BLUE);
    j.setFont (new Font ("tunga", Font.BOLD, 24));
    j.setSize (270, 20);
    j.setLocation (30, 35);

    //Creating label Enter a number.....
    JLabel j1 = new JLabel ("Enter your S.I.N. below.");
    j1.setFont (new Font ("tunga", Font.PLAIN, 17));
    j1.setSize (270, 20);
    j1.setLocation (66, 60);

    //Creating a label Instuctions
    JLabel j2 = new JLabel ("Enter a 9-digit Social Insurance Number");
    j2.setFont (new Font ("tunga", Font.PLAIN, 17));
    j2.setSize (270, 20);
    j2.setLocation (10, 165);

    //Creating a label Instuctions
    JLabel j3 = new JLabel ("with no spaces between the digits please.");
    j3.setFont (new Font ("tunga", Font.PLAIN, 17));
    j3.setSize (270, 20);
    j3.setLocation (10, 180);



    //Creating TextField for x input guess
    t1 = new JTextField (10);
    t1.setSize (70, 30);
    t1.setLocation (100, 80);

    //creating 2 buttons
    JButton b1 = new JButton ("Proceed");
    b1.setSize (120, 30);
    b1.setLocation (70, 200);
    bl1 = new ButtonListener ();
    b1.addActionListener (bl1);


    JButton b2 = new JButton ("Re-enter");
    b2.setSize (120, 30);
    b2.setLocation (70, 250);
    bl2 = new ButtonListener2 ();
    b2.addActionListener (bl2);

    //Place the components in the pane
    c.add (j);
    c.add (j1);
    c.add (j2);
    c.add (j3);

    c.add (t1);
    c.add (b1);
    c.add (b2);

    //Set the title of the window
    setTitle ("Social Insurance Number Checker");

    //Set the size of the window and display it
    setSize (300, 350);
    setVisible (true);
    setDefaultCloseOperation (EXIT_ON_CLOSE);
}


//implement  first action listener
private class ButtonListener implements ActionListener
{

    public void actionPerformed (ActionEvent e)
    {
        int a = Integer.parseInt (t1.getText ());
        boolean evenDigit = false;   //alternates between true and false
        int sum = 0; //accumulates the sum of the digits (as modified)

        while (a > 0)
        {
            int nextDigit = a % 10; //grab the last digit
            a = a / 10; //discard that digit
            if (evenDigit)
            {
                //double it, then add the two digits of the result
                nextDigit = 2 * nextDigit;
                nextDigit = (nextDigit / 10) + (nextDigit % 10);
            } // if(evenDigit)
            sum = sum + nextDigit;
            evenDigit = !evenDigit; //toggle the flag each time
        } // end while

        if (0 == sum % 10)
        {
            j1.setText ("That is a valid S.I.N.");
        }
        else
        {
            j1.setText ("That is not a valid S.I.N.");

        }
        t1.requestFocus ();
        t1.selectAll ();
    }
}


private class ButtonListener2 implements ActionListener
{
    public void actionPerformed (ActionEvent e)
    {
        //reset the label messages
        t1.setText ("");


        t1.requestFocus ();
        t1.selectAll ();



    }
}


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

}

Answer 1

有两个问题......

static的用户应该是什么实例字段
可变阴影

例如......

class SinChecker extends JFrame
{ //naming variables
    // This is a BAD idea
    static JLabel j, j1, j2, j3;

    public SinChecker ()
    { //Get the container
        //...
        // Now what does SinCheck.j1 actually equal?
        //Creating label Enter a number.....
        JLabel j1 = new JLabel ("Enter your S.I.N. below.");

j1在SinChecker的构造函数中被重新声明为局部变量，使SinCheker.j1仍为null

即使你解决了这个问题，你也应该问自己一个问题，如果你创建SinCheck的第二个副本会怎么样？您现在引用哪个标签......

首先删除构造函数中的static引用和标签声明

class SinChecker extends JFrame
{ //naming variables
    private JLabel j, j1, j2, j3;

    public SinChecker ()
    { //Get the container
        //...
        // Now what does SinCheck.j1 actually equal?
        //Creating label Enter a number.....
        j1 = new JLabel ("Enter your S.I.N. below.");

避免使用null布局，像素完美布局是现代ui设计中的一种幻觉。影响组件个体大小的因素太多，您无法控制。 Swing旨在与布局管理器一起工作，放弃这些将导致问题和问题的终结，您将花费越来越多的时间来纠正

仔细查看Laying Out Components Within a Container了解更多详情

Answer 2

您正在重新声明方法中的JLabel。您已将它们声明为字段。它们被视为局部变量。替换：

 JLabel  j1 = new JLabel ("Enter your S.I.N. below.");

与

  j1 = new JLabel ("Enter your S.I.N. below.");

出于某种原因，在我的程序中动态更改Jlabel会导致它发送一堆错误，但不会崩溃

2 个答案: