空指针异常错误

Question

当我运行程序时，出现此错误nullPointerException：null。

import model.*;

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

public class ButtonPanel extends JPanel implements View
{
private Prison prison;
private LeftInputPanel leftInput;
private DaysPanel days;
private MonthsPanel months;
private YearsPanel years;
private CrimePanel crime;
private AllocateListener aListener;

public ButtonPanel()
{
    setup();
    build();
}

public void setup()
{
}

public void build()
{
    JButton button = new JButton("Allocate Cell");
    Dimension size = new Dimension(240, 70);

    button.setPreferredSize(size);
    button.setMinimumSize(size);
    button.setMaximumSize(size);
    button.addActionListener(aListener);
    add(button);
    update();
}

public void update()
{
    leftInput.update(); //ERROR ON THIS LINE
}

private class AllocateListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
    Criminal criminal = new Criminal(leftInput.name());
    Period period = new Period(days.days(), months.months(), years.years());
    criminal.set(new Crime(crime.getCrime()));
    prison.add(criminal);
}
}
}

我的LeftInputPanel-从那里调用update方法。

import model.*;
import java.awt.*;
import java.text.*;
import javax.swing.*;

public class LeftInputPanel extends JPanel
{    
public JTextField field = new JTextField();

public LeftInputPanel()
{
    setup();
    build();
}

public void setup()
{
    //setLayout(new GridLayout());
    Dimension size = new Dimension(100, 190);

    setPreferredSize(size);
    setMinimumSize(size);
    setMaximumSize(size);
}

public void build()
{
    setLayout(new FlowLayout(FlowLayout.LEFT));
    JLabel label = new JLabel("          Name");
    field.setPreferredSize(new Dimension(90, 20));
    add(label);
    add(field);

    add(new DaysPanel());
    add(new MonthsPanel());
    add(new YearsPanel());
}

public String name()
{
    return field.getText();
}

public void update()
{
    field.setText("");
}
}

失误

java.lang.NullPointerException
at ButtonPanel.update(ButtonPanel.java:43)
at ButtonPanel.build(ButtonPanel.java:38)
at ButtonPanel.<init>(ButtonPanel.java:21)
at RightInputPanel.build(RightInputPanel.java:30)
at RightInputPanel.<init>(RightInputPanel.java:14)
at InputPanel.build(InputPanel.java:24)
at InputPanel.<init>(InputPanel.java:13)
at Panel.build(Panel.java:30)
at Panel.<init>(Panel.java:13)
at Window.build(Window.java:31)
at Window.<init>(Window.java:11)
at Root.main(Root.java:9)

Answer 1

aListener尚未初始化（即，它仍然为null ），并且您试图将其添加为动作侦听器。 我怀疑所需要的是：

private AllocateListener aListener = new AllocateListener();

Answer 2

当您遇到异常时，通常会得到堆栈跟踪，包括完整的调用顺序-这对于跟踪问题非常有用。

如果您无法从该堆栈跟踪中找出问题，则需要将其与相关代码以及行号一起提供给我们。

然后，我们将能够更好地帮助您解决问题， 更重要的是 ，向您展示问题的解决方法。