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実行をすると
Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1044)
at java.awt.Container.add(Container.java:363)
at JComboBoxTest1.<init>(comb.java:31)
at JComboBoxTest1.main(comb.java:15)
のようなエラーメッセージが出てきます。
全くjavaがわからないので、
どのように改善すれば良いのか、教えてください。
よろしくお願いします。。。
以下がコードです。
import javax.swing.*;
import java.awt.BorderLayout;
import java.util.Vector;
import java.awt.event.*;
import java.awt.Color;
import java.awt.Component;
class JComboBoxTest1 extends JFrame implements ActionListener{
JComboBox colorCombo;
JComboBox lineCombo;
JLabel label;
public static void main(String[] args){
JComboBoxTest1 frame = new JComboBoxTest1();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setBounds(10, 10, 300, 200);
frame.setTitle("write your favorite");
frame.setVisible(true);
}
JComboBoxTest1(){
String[] colordata = {"red", "blue", "yellow", "black", "pink",
"green"};
DefaultComboBoxModel model = new
DefaultComboBoxModel(colordata);
colorCombo = new JComboBox(model);
colorCombo.setMaximumRowCount(3);
MyCellRenderer renderer = new MyCellRenderer();
colorCombo.setRenderer(renderer);
JPanel p = new JPanel();
p.add(lineCombo);
getContentPane().add(p, BorderLayout.CENTER);
colorCombo.addActionListener(this);
String[] linedata = {"5", "10", "15", "20"};
DefaultComboBoxModel model2 = new
DefaultComboBoxModel(linedata);
lineCombo = new JComboBox(model2);
lineCombo.setMaximumRowCount(3);
MyCellRenderer renderer2 = new MyCellRenderer();
lineCombo.setRenderer(renderer2);
JPanel p2 = new JPanel();
p2.add(colorCombo);
getContentPane().add(p, BorderLayout.CENTER);
lineCombo.addActionListener(this);
JPanel p3 = new JPanel();
p3.add(new JLabel("color:"));
p3.add(new JLabel(" line:"));
label = new JLabel();
JPanel labelPanel = new JPanel();
labelPanel.add(label);
getContentPane().add(labelPanel, BorderLayout.PAGE_END);
}
public void actionPerformed(ActionEvent e){
String color;
String line;
if(colorCombo.getSelectedIndex() == -1){
color = "(not select)";
}
else{
color = (String)colorCombo.getSelectedItem();
}
if(lineCombo.getSelectedIndex() == -1){
line = "(not select)";
}
else{
line = (String)lineCombo.getSelectedItem();
}
label.setText("COLOR:" + color + " , LINE:"+ line);
}
class MyCellRenderer extends JLabel implements
ListCellRenderer{
MyCellRenderer(){
setOpaque(true);
}
public Component getListCellRendererComponent(
JList list,
Object value,
int index,
boolean isSelected,
boolean cellHasFocus){
String data = value.toString();
setText(data.toUpperCase());
if(isSelected){
setForeground(Color.white);
setBackground(Color.black);
}
else{
setForeground(Color.black);
setBackground(Color.white);
}
return this;
}
}
}
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