import javax.swing.*;
import javax.swing.table.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;public class Test7
extends JFrame {
String longTitle = "Last Name / Maiden Name (if divorced)";
MultilineHeaderRenderer multilineRenderer =
new MultilineHeaderRenderer(longTitle); JTable table = new JTable(
new Object[][] { {"Lynn", "M.", "Seckinger"}, {"Carol", "R.", "Seckinger"},
{"Roy", "D.", "Martin"}, {"Richard", "A.", "Tattersall"}, {"Philip", "B.",
"Edwards"}, {"Moore", "T.", "Moore"},
// shorten scrollbar grip with these ... {"Lynn", "M.", "Seckinger"}, {"Carol", "R.", "Seckinger"}, {"Roy", "D.",
"Martin"}, {"Richard", "A.", "Tattersall"}, {"Philip", "B.", "Edwards"},
{"Moore", "T.", "Moore"},
},
new Object[] {
"First Name", "MI", longTitle}); public Test7() {
TableColumn middleColumn = table.getColumn("MI"),
lastColumn = table.getColumn(longTitle); lastColumn.setHeaderRenderer(multilineRenderer); TableCellRenderer hdrRenderer =
middleColumn.getHeaderRenderer(); Component hdrComponent =
hdrRenderer.getTableCellRendererComponent(table,
"MI", false, false, 0, 0); if (hdrComponent instanceof JComponent) {
JComponent c = (JComponent) hdrComponent;
c.setToolTipText("Middle Initial");
} table.getTableHeader().setToolTipText("Table Header!"); getContentPane().add(
new JScrollPane(table), BorderLayout.CENTER);
} public static void main(String args[]) {
GJApp7.launch(new Test7(),
"Multi-Line Column Headers", 300, 300, 300, 250);
}
}class MultilineHeaderRenderer
implements TableCellRenderer {
MultilineHeader mll;
JScrollPane scrollPane; public MultilineHeaderRenderer(String title) {
mll = new MultilineHeader(title);
scrollPane = new JScrollPane(mll); scrollPane.setHorizontalScrollBarPolicy(
JScrollPane.HORIZONTAL_SCROLLBAR_NEVER); scrollPane.setVerticalScrollBarPolicy(
JScrollPane.VERTICAL_SCROLLBAR_NEVER); scrollPane.setBorder(null);
} public Component getTableCellRendererComponent(JTable table,
Object value,
boolean isSelected,
boolean hasFocus,
int row, int col) {
mll.setText( (String) value);
return scrollPane;
}
}class MultilineHeader
extends JTextArea {
public MultilineHeader(String s) {
super(s);
} public void updateUI() {
super.updateUI();// turn on wrapping and disable editing and highlighting setLineWrap(true);
setWrapStyleWord(true);
setHighlighter(null);
setEditable(false);// make the text area look like a table header LookAndFeel.installColorsAndFont(this,
"TableHeader.background",
"TableHeader.foreground",
"TableHeader.font"); LookAndFeel.installBorder(this, "TableHeader.cellBorder");
}
}class GJApp7
extends WindowAdapter {
static private JPanel statusArea = new JPanel();
static private JLabel status = new JLabel(" ");
static private ResourceBundle resources; public static void launch(final JFrame f, String title,
final int x, final int y,
final int w, int h) {
launch(f, title, x, y, w, h, null);
} public static void launch(final JFrame f, String title,
final int x, final int y,
final int w, int h,
String propertiesFilename) {
f.setTitle(title);
f.setBounds(x, y, w, h);
f.setVisible(true); statusArea.setBorder(BorderFactory.createEtchedBorder());
statusArea.setLayout(new FlowLayout(FlowLayout.LEFT, 0, 0));
statusArea.add(status);
status.setHorizontalAlignment(JLabel.LEFT); f.setDefaultCloseOperation(
WindowConstants.DISPOSE_ON_CLOSE); if (propertiesFilename != null) {
resources = ResourceBundle.getBundle(
propertiesFilename, Locale.getDefault());
} f.addWindowListener(new WindowAdapter() {
public void windowClosed(WindowEvent e) {
System.exit(0);
}
});
} static public JPanel getStatusArea() {
return statusArea;
} static public void showStatus(String s) {
status.setText(s);
} static Object getResource(String key) {
if (resources != null) {
return resources.getString(key);
}
return null;
}
}想学习table,随便找了段代码,运行了一下报空指针。拿过来请教一下,想知道为什么。
还有,我碰到过一些空指针的情况,不知道怎么解决,什么情况下会出现NullPointerException呢?
分都拿去押德国队夺冠了,就剩这么点了,呵呵。
楼主【helanpiaoxue】截止到2008-06-25 09:41:11的历史汇总数据(不包括此帖):
发帖数:9 发帖分:420
结贴数:8 结贴分:320
未结数:1 未结分:100
结贴率:88.89 % 结分率:76.19 %
楼主加油
当一个对象是null时,却要用它去操作
NullPointerException的出现一般是操作了一个空对象的属性或方法
Exception in thread "main" java.lang.NullPointerException
at Test7.<init>(Test7.java:35)
at Test7.main(Test7.java:51)这一行出的错
Component hdrComponent =
hdrRenderer.getTableCellRendererComponent(table,
"MI", false, false, 0, 0);
static JTable table 试下
跟踪一下这个变量看看。是不是
TableCellRenderer hdrRenderer =
middleColumn.getHeaderRenderer();
得到的hdrRenderer 为null
2. NullPointerException 空引用(也叫空指针)异常,String s = null; s.trim(); 时会抛出,也就是通过对象变量找不到它引用的对象时。
import javax.swing.table.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;public class Test7
extends JFrame {
String longTitle = "Last Name / Maiden Name (if divorced)";
MultilineHeaderRenderer multilineRenderer =
new MultilineHeaderRenderer(longTitle);
MultilineHeaderRenderer multilineRenderer2 =
new MultilineHeaderRenderer("MI"); JTable table = new JTable(
new Object[][] { {"Lynn", "M.", "Seckinger"}, {"Carol", "R.", "Seckinger"},
{"Roy", "D.", "Martin"}, {"Richard", "A.", "Tattersall"}, {"Philip", "B.",
"Edwards"}, {"Moore", "T.", "Moore"},
// shorten scrollbar grip with these ... {"Lynn", "M.", "Seckinger"}, {"Carol", "R.", "Seckinger"}, {"Roy", "D.",
"Martin"}, {"Richard", "A.", "Tattersall"}, {"Philip", "B.", "Edwards"},
{"Moore", "T.", "Moore"},
},
new Object[] {
"First Name", "MI", longTitle}); public Test7() {
TableColumn middleColumn = table.getColumn("MI"),
lastColumn = table.getColumn(longTitle); lastColumn.setHeaderRenderer(multilineRenderer);
middleColumn.setHeaderRenderer(multilineRenderer2);
TableCellRenderer hdrRenderer =
middleColumn.getHeaderRenderer(); Component hdrComponent =
hdrRenderer.getTableCellRendererComponent(table,
"MI", false, false, 0, 0); if (hdrComponent instanceof JComponent) {
JComponent c = (JComponent) hdrComponent;
c.setToolTipText("Middle Initial");
} table.getTableHeader().setToolTipText("Table Header!"); getContentPane().add(
new JScrollPane(table), BorderLayout.CENTER);
} public static void main(String args[]) {
GJApp7.launch(new Test7(),
"Multi-Line Column Headers", 300, 300, 300, 250);
}
}class MultilineHeaderRenderer
implements TableCellRenderer {
MultilineHeader mll;
JScrollPane scrollPane; public MultilineHeaderRenderer(String title) {
mll = new MultilineHeader(title);
scrollPane = new JScrollPane(mll); scrollPane.setHorizontalScrollBarPolicy(
JScrollPane.HORIZONTAL_SCROLLBAR_NEVER); scrollPane.setVerticalScrollBarPolicy(
JScrollPane.VERTICAL_SCROLLBAR_NEVER); scrollPane.setBorder(null);
} public Component getTableCellRendererComponent(JTable table,
Object value,
boolean isSelected,
boolean hasFocus,
int row, int col) {
mll.setText( (String) value);
return scrollPane;
}
}class MultilineHeader
extends JTextArea {
public MultilineHeader(String s) {
super(s);
} public void updateUI() {
super.updateUI();// turn on wrapping and disable editing and highlighting setLineWrap(true);
setWrapStyleWord(true);
setHighlighter(null);
setEditable(false);// make the text area look like a table header LookAndFeel.installColorsAndFont(this,
"TableHeader.background",
"TableHeader.foreground",
"TableHeader.font"); LookAndFeel.installBorder(this, "TableHeader.cellBorder");
}
}class GJApp7
extends WindowAdapter {
static private JPanel statusArea = new JPanel();
static private JLabel status = new JLabel(" ");
static private ResourceBundle resources; public static void launch(final JFrame f, String title,
final int x, final int y,
final int w, int h) {
launch(f, title, x, y, w, h, null);
} public static void launch(final JFrame f, String title,
final int x, final int y,
final int w, int h,
String propertiesFilename) {
f.setTitle(title);
f.setBounds(x, y, w, h);
f.setVisible(true); statusArea.setBorder(BorderFactory.createEtchedBorder());
statusArea.setLayout(new FlowLayout(FlowLayout.LEFT, 0, 0));
statusArea.add(status);
status.setHorizontalAlignment(JLabel.LEFT); f.setDefaultCloseOperation(
WindowConstants.DISPOSE_ON_CLOSE); if (propertiesFilename != null) {
resources = ResourceBundle.getBundle(
propertiesFilename, Locale.getDefault());
} f.addWindowListener(new WindowAdapter() {
public void windowClosed(WindowEvent e) {
System.exit(0);
}
});
} static public JPanel getStatusArea() {
return statusArea;
} static public void showStatus(String s) {
status.setText(s);
} static Object getResource(String key) {
if (resources != null) {
return resources.getString(key);
}
return null;
}
}
TableCellRenderer hdrRenderer =
middleColumn.getHeaderRenderer();
在这里hdrRenderer就是null了
middleColumn.setHeaderRenderer(multilineRenderer);
lastColumn.setHeaderRenderer(multilineRenderer); TableCellRenderer hdrRenderer =
middleColumn.getHeaderRenderer();
middleColumn.setHeaderRenderer(multilineRenderer);
其实还是回到set和get方法的运用上了,在没有设置HeaderRenderer时,默认的是null。
谢谢大家,学习了。