getting null pointer exception when retrieving value

[Mohammad Ashari Rahman]

table rows: Pin#, Account#, Balance

I have added jdbc driver to class path and in JRE system libraries and in JRE lib folder also

public class InterfaceDatabase {
private Connection connection;
private PreparedStatement stmt;
private ResultSet res_set;
public InterfaceDatabase() throws InstantiationException, IllegalAccessException
{
try {
Class.forName("com.mysql.jdbc.Driver");
} catch (ClassNotFoundException e) {
e.getMessage();
}
connection=null;
}
public void Connect(){
try {
connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/user");
} catch (SQLException e) {
System.out.println("catch");//this line always executes
e.getMessage();
}
}
public float getCurrentBalance(int id) throws SQLException{
float currentBalance=0;
String sql= "select Balance from user where Pin#=?";
stmt=connection.prepareStatement(sql);
stmt.setInt(1, id);
res_set=stmt.executeQuery();
System.out.println(res_set==null);
System.out.println(res_set.getFloat(3));
if(!res_set.isBeforeFirst())
return 0;
else
currentBalance=res_set.getFloat(3);
return currentBalance;
}
}
public class User {
private int pin;
private int account;
private float balance;
public void deposit() throws SQLException, InstantiationException, IllegalAccessException{
InterfaceDatabase userAcc=new InterfaceDatabase();
userAcc.Connect();
float amountToAdd=Float.parseFloat(JOptionPane.showInputDialog(null, "Amount to deposit? ", "Add Money", JOptionPane.PLAIN_MESSAGE));
balance=userAcc.getCurrentBalance(859069);
System.out.println(balance);
System.out.println(balance);
}
public static void main(String[] args) throws SQLException, InstantiationException, IllegalAccessException {
User u=new User();
u.deposit();
}}
OUTPUT:
catch
Exception in thread "main" java.lang.NullPointerException
at labtask5.InterfaceDatabase.getCurrentBalance(InterfaceDatabase.java:78)
at labtask5.User.deposit(User.java:20)
at labtask5.User.main(User.java:37)

[Bear Bibeault]

 

at labtask5.InterfaceDatabase.getCurrentBalance(InterfaceDatabase.java:78)

What's at line 78?

[Paul Clapham]

 try {
connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/user");
} catch (SQLException e) {
System.out.println("catch");//this line always executes
e.getMessage();
}

So you already know that the DriverManager.getConnection() method is throwing an exception. It would be useful to know why that is the case. So instead of just printing the word "catch", which provides no information except the fact that an exception was thrown, put this line of code in the catch block:

e.printStackTrace();

That will tell you more about the exception and you can proceed with fixing whatever problem it tells you about.

[Dave Tolls]

public InterfaceDatabase() throws InstantiationException, IllegalAccessException
{
try {
Class.forName("com.mysql.jdbc.Driver");
} catch (ClassNotFoundException e) {
e.getMessage();
}
connection=null;
}
public void Connect(){
try {
connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/user");
} catch (SQLException e) {
System.out.println("catch");//this line always executes
e.getMessage();
}
}

Both catch blocks here should at the minimum be logging the exception (see Paul's post for how to print the stack trace) and should, to be honest, simply crash out entirely as you can't continue anyway. Possibly throw some form of RuntimeException, wrapping the original exception:

throw new MyRuntimeException("Arrrggghh", e);

[Roel De Nijs]

Besides the valuable comments of other ranchers I have a few as well.

Let's start by looking at your Connect method

public void Connect(){
try {
connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/user");
} catch (SQLException e) {
System.out.println("catch");//this line always executes
e.getMessage();
}
}

First of all, you should not print just "catch" in the catch handler, because that's pretty useless information. You should print (at least) the whole stack trace (like already suggested by Paul and Dave). Secondly, the statement e.getMessage(); is completely useless as well. It will retrieve the message of the exception and then it's immediately "lost", because it's not stored into another variable or printed or ... So you should remove that line as it's useless.

Now we'll take a look at the InterfaceDatabase constructor

public InterfaceDatabase() throws InstantiationException, IllegalAccessException
{
try {
Class.forName("com.mysql.jdbc.Driver");
} catch (ClassNotFoundException e) {
e.getMessage();
}
connection=null;
}

Here we'll see again the useless e.getMessage(); statement. But you'll be probably using the latest (most recent) MySQL driver. This driver is a JDBC 4.0 driver and one of the enhancements/improvements in JDBC 4.0 is auto-loading of JDBC drivers. You no longer need to explicitly load JDBC drivers using code like Class.forName(), so you can remove the complete constructor from your code. Less is More!

Hope it helps!
Kind regards,
Roel