Null pointer exception while executing query

Question

Following is the code to display the tuples which has the username and password matching the strings

String usr = tf1.getText();
String pwd = tf2.getText();

System.out.println("Username is " + usr + " Password is " + pwd);

String sql = "select * from student where first = '"+usr+"' and last = '"+pwd+"';";
System.out.println("SQL is " + sql);

try {
    rset = stmt.executeQuery(sql);
    if(rset.next()){
          JOptionPane.showMessageDialog(null, " Login Successfull...");
    }
}
catch(SQLException e){
    JOptionPane.showMessageDialog(null, e);
}
// TODO add your handling code here:

}

Is there any mistake in framing the query here.pPlease check the string 'sql'. The null pointer exception is at the following statement.

rset = stmt.executeQuery(sql);

This sql string is not null because, when I print it on console it shows me

select * from student where first = 'rohit' and last = 'rocks';

So, its not null at the runtime. If sql is not null then why am I getting the NullPointer Exception?