代碼不起作用，但沒有收到任何錯誤消息

Question

錯誤：奇怪，我沒有錯誤。

我想做的是：（我還添加了//消息，解釋了我的代碼的作用。）

我想做的很簡單。

1）在JTextField中填寫一個名稱，按Enter，該名稱應出現在JTextArea中。 名稱在JTextArea中之后，JTextField變為空，以便您可以填充另一個名稱，依此類推，應該在JTextArea中出現一個名稱列表。

2）按下按鈕kiesWin，使程序從列表中選擇一個隨機的人。 （這里出錯了）

3）按下按鈕resetL重置程序，這樣我可以列出一個新列表來從中選擇隨機獲勝者。

問題：當我按下按鈕Kies（翻譯：選擇）時，它應該從ArrayList中選擇一個隨機名稱，並在JTextField textvak2中顯示該隨機名稱。 但是，當我按Kies鍵時，程序什么也沒做。 它應該顯示從ArrayList中隨機選擇的名稱。

這是無法正常運行的課程：（我認為）

// This is the button that chooses a random name from the ArrayList.
// The random chosen name should appear in the JTextField textvak2. (but it doesn't)
// This is also the part where it goes wrong at the moment.
class Kies extends OnthoudNaam implements ActionListener {
    public void actionPerformed( ActionEvent e ) {
        Random r = new Random();
        if (lijst.size() > 0) {
            int n = r.nextInt(lijst.size());
            Naam kiesNaam = lijst.get(n);
            textvak2.setText(kiesNaam.getIngevoerdNaam());
            }
    }
}

如果您需要完整的代碼：

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

// Main method to make the frame.
public class Loterij3 extends JFrame {
public static void main( String args[] ) {
    JFrame frame = new Loterij3();
    frame.setExtendedState( frame.MAXIMIZED_BOTH );
    frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
    frame.setTitle( "My Lottery!" );
    frame.setContentPane( new Paneel() );
    frame.setVisible( true );
}
}

// This is the Panel that goes into the frame.
class Paneel extends JPanel {
private Boven boven;
JTextArea textvak1;
JTextField textvak2;
OnthoudNaam onthoudNaam = new OnthoudNaam();
JTextField invoervak1; // JTextField from class Boven.

public Paneel() {
setLayout( new BorderLayout() ); // using border Layout.
setBackground( Color.LIGHT_GRAY );

boven = new Boven(); 

textvak1 = new JTextArea();
add( new JScrollPane( textvak1 ) );
textvak1.setBackground( Color.WHITE );

textvak2 = new JTextField();
textvak2.setHorizontalAlignment(JTextField.CENTER); 

add( boven, BorderLayout.NORTH ); // Where the class Boven should be.
add( textvak1, BorderLayout.CENTER );
add( textvak2, BorderLayout.SOUTH );
}

// This is the class Boven (Translation up or upper).
// This is where the JButtons, JTextField and JLabels are.
public class Boven extends JPanel {
JButton kiesWin, resetL;
JLabel label1;

public Boven() {
    setBackground( Color.LIGHT_GRAY );
    setLayout( new GridLayout( 1, 4, 100, 5 ) ); // using GridLayout.
    Border border = 
        BorderFactory.createEmptyBorder( 10, 10, 10, 10 );
    setBorder( border );

    kiesWin = new JButton("Kies een Winnaar!");
    kiesWin.addActionListener( new Kies() );
    resetL = new JButton("Reset alles");
    resetL.addActionListener( new Reset() );
    label1 = new JLabel("Voer Persoon in en druk op enter: ", JLabel.RIGHT);
    invoervak1 = new JTextField( 20 );
    invoervak1.addActionListener( new InvoerVakHandler() );

    add( label1 );
    add( invoervak1 );
    add( kiesWin );
    add( resetL );
    }
}

// The class Naam (translation = name).
// This is what the ArrayList should remember
// In other words ArrayList should remember the Names I put in the JTextField.
class Naam {
    private String ingevoerdNaam;

    public Naam( String ingevoerdNaam) {
        this.ingevoerdNaam = ingevoerdNaam;
    }

    public String getIngevoerdNaam() {
        return ingevoerdNaam;
    }

    public String toString() {
        return ingevoerdNaam;
    }
}

// This is my ArrayList,
// This should remember the names I type in the JTextField.
class OnthoudNaam extends JPanel {
    protected ArrayList<Naam> lijst;

    public OnthoudNaam() {
        lijst = new ArrayList<Naam>();
        }

        public void voegNaamToe(Naam x ) {
        lijst.add(x);
        }

        public String toString() {
        StringBuffer buffer = new StringBuffer();
        for(Naam x : lijst ) {
        buffer.append( x );
        buffer.append( "\n" );
    }
    return buffer.toString();
}
}

// This is the JTextField where I enter the names.
// The Name I fill in the JTextField should be remembered by the ArrayList.
// The Name I fill in should be put in the JTextArea.
public class InvoerVakHandler implements ActionListener {
    public void actionPerformed( ActionEvent e ) {
        String invoer = invoervak1.getText();
        Naam Naam = new Naam( invoer );
        onthoudNaam.voegNaamToe( Naam );
        textvak1.setText( onthoudNaam.toString() );
        invoervak1.setText( "" );
    }
}
    // This is the button that chooses a random name from the ArrayList.
    // This is also the part where it goes wrong at the moment.
class Kies extends OnthoudNaam implements ActionListener {
    public void actionPerformed( ActionEvent e ) {
        Random r = new Random();
        if (lijst.size() > 0) {
            int n = r.nextInt(lijst.size());
            Naam kiesNaam = lijst.get(n);
            textvak2.setText(kiesNaam.getIngevoerdNaam());
            }
    }
}

// This should become the button that resets everything so you can start over.
class Reset implements ActionListener {
    public void actionPerformed( ActionEvent e ) {
    }
}
}

Answer 1

Random#nextInt需要一個正數，但List lijst的初始大小為0因此出現異常

文檔明確指出了這一點

拋出： IllegalArgumentException-如果n不為正

首先檢查List是否有條目。

if (lijst.size() > 0) {
   int n = r.nextInt(lijst.size());
   Naam kiesNaam = lijst.get(n);
   textvak2.setText(kiesNaam.getIngevoerdNaam());
}

另外：提取Naam對象並使用其getIngevoerdNaam方法，而不是從List轉換對象。

還要記住調試器是您的朋友