Agregar un oyente de acción a un JCombobox

Question

Solo quiero imprimir la opción seleccionada en el cuadro Combo, en un campo de texto. Explique lo que está mal porque tengo que completarlo y explicarlo en clase. Cualquier ayuda sería muy apreciada. Gracias por adelantado.

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

public class App3 extends JFrame implements ActionListener
{
    private JPanel boxPanel,textPanel;
    private JLabel selectName,selectedName;
    private JComboBox nameCombo;
    private JTextField valueOfSelectedName;
    private Container c;

    public App3()
    {
        super("Combo example");
        setup();
        setSize(200,200);
        setLocation(50,50);
        show();
    }

    public void setup()
    {
        c = getContentPane();

        boxPanel = new JPanel();
        c.add(boxPanel,BorderLayout.NORTH);

        selectName = new JLabel("Select Name : ");
        selectedName = new JLabel("The selected Name : ");

        String[] names = {"Ramila","Hashan","Shaad","Gus","Mahasen","Hasaru","Shabba"};
        nameCombo = new JComboBox(names);
        nameCombo.addActionListener(this);

        valueOfSelectedName = new JTextField(10);

        boxPanel.add(selectName);
        boxPanel.add(nameCombo);

        c.add(textPanel,BorderLayout.CENTER);

        textPanel.add(selectedName);
        textPanel.add(valueOfSelectedName);

    }

    public void actionPerformed(ActionEvent e) 
    {
        JComboBox nameCombo = (JComboBox)e.getSource();
        String newSelection = (String)nameCombo.getSelectedItem();
        valueOfSelectedName.setText(newSelection);
    }

    public static void main(String args[])
    {
        App3 a = new App3();
    }
}

No recibo ningún error de tiempo de compilación, recibo estos errores cuando lo ejecuto.

Exception in thread "main" java.lang.NullPointerException
    at java.awt.Container.addImpl(Container.java:1041)
    at java.awt.Container.add(Container.java:927)
    at App3.setup(App3.java:42)
    at App3.(App3.java:16)
    at App3.main(App3.java:58)

Process completed.

Answer 1

private JPanel boxPanel,textPanel;
...
textPanel = new JPanel();

No has creado el objeto jpanel, por lo tanto, TextPanel apunta a NULL, por lo que se lanza la excepción. Crear el objeto y todo debería funcionar bien

Answer 2

TextPanel es nulo cuando intenta agregarlo al panel de contenido:

c.add(textPanel, BorderLayout.CENTER);

Answer 3

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;


public class App3 extends JFrame implements ActionListener
{
    private JPanel boxPanel,textPanel;

    private JLabel selectName,selectedName;

    private JComboBox nameCombo;

    private JTextField valueOfSelectedName;

    private Container c;


    public App3()
    {
        super("Combo example");
        setup();
        setSize(200,200);
        setLocation(50,50);
        show();
    }

    public void setup()
    {
        c = getContentPane();

        boxPanel = new JPanel();
        c.add(boxPanel,BorderLayout.NORTH);

        selectName = new JLabel("Select Name : ");
        selectedName = new JLabel("The selected Name : ");

        String[] names = {"Ramila","Hashan","Shaad","Gus","Mahasen","Hasaru","Shabba"};
        nameCombo = new JComboBox(names);
        nameCombo.addActionListener(this);

        valueOfSelectedName = new JTextField(10);

        boxPanel.add(selectName);
        boxPanel.add(nameCombo);

        textPanel = new JPanel(); // panel is not init that why its give error

        c.add(textPanel,BorderLayout.CENTER);

        textPanel.add(selectedName);
        textPanel.add(valueOfSelectedName);

    }

    public void actionPerformed(ActionEvent e) 
    {
        JComboBox nameCombo = (JComboBox)e.getSource();
        String newSelection = (String)nameCombo.getSelectedItem();
        valueOfSelectedName.setText(newSelection);
    }

    public static void main(String args[])
    {
        App3 a = new App3();
    }

}