我有A班和B班。 在A类中有一个构造函数：
public A() {
getSelectedRow();
}
此构造函数调用：
public int getSelectedRow() {
System.out.println("The row is : " + table.getSelectedRow());
return table.getSelectedRow();
}
到这里一切正常！
B类然后调用方法getSelectedRow()：
A results = new A();
System.out.println("YEAH! IT'S: " + results.getSelectedRow());
我只想找出A类中选定的表行。问题是我得到一个空指针异常，我不知道为什么。如果我不打电话给方法一切正常。
CLASS A：
public class AllResultsFromDB extends JFrame {
@SuppressWarnings("compatibility:9056676689615464658")
private static final long serialVersionUID = 188850508334531506L;
GUI ins = new GUI();
JTable table;
public AllResultsFromDB(GUI x) {
final Vector columnNames = new Vector();
final Vector data = new Vector();
this.ins = x;
try {
/** Initializing GUI class
* in order to call
* getSelectedTable() method. **/
Login sgui = new Login();
String dburl = "jdbc:oracle:thin:@localhost:1521:ORCL";
Connection connection = DriverManager.getConnection(dburl, sgui.getUsername(), sgui.getPassword());
// Fetch data from table specified by user
String query = "SELECT * FROM " + ins.getSelectedTable() + " ORDER BY id";
System.out.println(query);
Statement stmt = connection.createStatement();
ResultSet rset = stmt.executeQuery(query);
ResultSetMetaData metad = rset.getMetaData();
int columns = metad.getColumnCount();
// This loop gets the names of the columns
for (int i = 1; i <= columns; i++) {
columnNames.addElement(metad.getColumnName(i));
}
// This loop gets the data inside the rows
while (rset.next()) {
final Vector row = new Vector(columns);
for (int i = 1; i <= columns; i++) {
row.addElement(rset.getObject(i));
}
data.addElement(row);
}
rset.close();
stmt.close();
connection.close();
// Create table with results
table = new JTable(data, columnNames) {
public boolean isCellEditable(int row, int col) {
return false;
}
public Class getColumnClass(int column) {
for (int row = 0; row < getRowCount(); row++) {
Object obj = getValueAt(row, column);
if (obj != null) {
return obj.getClass();
}
}
return Object.class;
}
};
JScrollPane scroll = new JScrollPane(table);
getContentPane().add(scroll);
JPanel panel = new JPanel();
getContentPane().add(panel, BorderLayout.SOUTH);
table.addMouseListener(new MouseListener() {
public void mousePressed(MouseEvent e) {
//System.out.println(table.getSelectedRow());
}
public void mouseReleased(MouseEvent e) {
//System.out.println(table.getSelectedRow());
}
public void mouseEntered(MouseEvent e) {
//System.out.println(table.getSelectedRow());
}
public void mouseExited(MouseEvent e) {
//System.out.println(table.getSelectedRow());
}
public void mouseClicked(MouseEvent e) {
getSelectedRow();
if (e.getClickCount() == 2) {
//System.out.println(table.getSelectedRow());
Profile profile = new Profile();
try {
profile.getData();
//wait(500000);
profile.getImage();
} catch (Exception f) {
}
profile.setVisible(true);
}
}
});
} catch (SQLException e) {
}
}
public AllResultsFromDB(int x) {
x = getSelectedRow();
System.out.println(table.getSelectedRow());
}
public int getSelectedRow() {
System.out.println("The row is : " + table.getSelectedRow());
return table.getSelectedRow();
}
}
CLASS B：
public class Profile extends JFrame {
AllResultsFromDB results = new AllResultsFromDB();
public Profile(AllResultsFromDB x) {
this.results=x;
try {
getData();
getImage();
} catch (Exception e) {
e.printStackTrace();
}
try {
jbInit();
} catch (Exception e) {
e.printStackTrace();
}
}
public void getImage() throws Exception {
JLabel label;
Image img;
ImageIcon pic;
JPanel panel;
img = new ImageIcon("java.jpg").getImage();
pic = new ImageIcon(img);
label = new JLabel("", pic, JLabel.CENTER);
panel = new JPanel(new BorderLayout());
panel.setBounds(new Rectangle(0, 0, 340, 310));
panel.add(label, null);
panel.add(label, BorderLayout.CENTER);
this.getContentPane().setLayout(null);
this.setSize(new Dimension(1148, 336));
this.getContentPane().add(panel, null);
}
public void getData() throws Exception {
String url = "jdbc:oracle:thin:@localhost:1521:ORCL";
String username = "c##lambros";
String password = "16111111";
Connection conn = null;
try {
System.out.println("YEAH! IT'S: " + results.getSelectedRow());
Class.forName("oracle.jdbc.driver.OracleDriver");
conn = DriverManager.getConnection(url, username, password);
String sql = "SELECT foto FROM criminals WHERE id = 5";
PreparedStatement stmt = conn.prepareStatement(sql);
ResultSet resultSet = stmt.executeQuery();
while (resultSet.next()) {
//String name = resultSet.getString(1);
//System.out.println("Name = " + name);
File image = new File("java.jpg");
FileOutputStream fos = new FileOutputStream(image);
byte[] buffer = new byte[256];
//
// Get the binary stream of our BLOB data
//
InputStream is = resultSet.getBinaryStream(1);
while (is.read(buffer) > 0) {
fos.write(buffer);
}
fos.close();
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
if (conn != null && !conn.isClosed()) {
conn.close();
}
}
}
private void jbInit() throws Exception {
this.setSize(new Dimension(816, 380));
JLabel label;
Image img;
ImageIcon pic;
JPanel panel;
img = new ImageIcon("java.jpg").getImage();
pic = new ImageIcon(img);
label = new JLabel("", pic, JLabel.CENTER);
panel = new JPanel(new BorderLayout());
panel.setBounds(new Rectangle(0, 0, 340, 310));
panel.add(label, null);
panel.add(label, BorderLayout.CENTER);
this.getContentPane().setLayout(null);
this.setSize(new Dimension(1148, 336));
this.getContentPane().add(panel, null);
}
}
答案 0 :(得分：0)
在classB中，因为您正在创建一个新实例
A results = new A();
table.getSelectedRow（）中的值也会被新创建并指向null。
所以要确保你做了一些事情
结果=新A（selectedRow）;
并在A的构造函数中，将参数传递给函数
getSelectedRow(selectedRow);
请注意：确保维持“table.selectedRow”的值
答案 1 :(得分：0)
如果table是A类中的实例变量，那么当您尝试在A的构造函数中访问它时，它可能不会被初始化。 从构造函数中调用getSelectedRow也没有任何意义。
尝试在构造函数中初始化表变量而不是调用该方法，它应该在它之后工作。
答案 2 :(得分：0)
这是因为表对象没有被初始化 尝试在构造函数中初始化表对象....这是一个很好的做法