i need to create a client-server connection (in JAVA), where the server is connected to a database and the client can send queries and get answers.
I use the OCSF (by Dr. Timothy C. Lethbridge) , and JDBC driver for JAVA.
at the server's side, i have the following code
ResultSetMetaData rsmd=rs.getMetaData();
int numOfCols=rsmd.getColumnCount();
String[] tuple=new String[numOfCols];
Vector result=new Vector();
while (rs.next()){
for (int i=1;i<=numOfCols;i++)
tuple[i-1]=rs.getString(i);
result.addElement(tuple);
isResultEmpty=0;
}
if (isResultEmpty==0)
this.sendToAllClients(result);
else
this.sendToAllClients("No appropriate results!");
so far, it seems to work fine, i made some test-prints if the for loop and it works fine.
for the client side, i have the following code:
public void handleMessageFromServer(Object msg)
{
//clientUI.display(msg.toString());
if (msg instanceof Vector){
Vector result=(Vector)msg;
System.out.println("The result of the query is:\n\n");
//System.out.println((String)result.firstElement());
System.out.println("result size is "+result.size());
for (int i=1;i<=result.size();i++){
System.out.println("here");
System.out.println((String)result.get(i));
System.out.println("here2");
}
}
else
clientUI.display(msg.toString());
}
the problem starts when i try to print the results:
the first "here" is printed, and nothing else happens,
it never gets to the "here2", and seems to just terminate the printing and waits for new input.
is there any problem with the casting I make?
i just can't figure out where is the exact problem....
thank you very much in advance for helping
You don't have a vector of String objects. You have a vector of string arrays (String[])
Edit in response to comments:
System.out.println("result size is "+result.size());
for (int i = 0;i < result.size();i++){
System.out.println("here");
String[] array = (String[])result.get(i));
for (int j = 0; j < array.length; j++)
System.out.println(array[j]);
System.out.println("here2");
}
This is of course assuming the server side works. I didn't look at it closely.
Edit for server side:
This is all kinds of broke:
while (rs.next()){
for (int i=1;i<=numOfCols;i++)
tuple[i-1]=rs.getString(i);
result.addElement(tuple);
isResultEmpty=0;
}
On each loop iteration, you're simply changing what your string array contains, and adding the same reference over and over. You need:
String[] tuple;
while (rs.next()){
tuple = new String[numOfCols];
for (int i=1;i<=numOfCols;i++)
tuple[i-1]=rs.getString(i);
result.addElement(tuple);
isResultEmpty=0;
}
First thing that I noticed is your iteration starts at one and goes through the size (inclusive). You must start at zero and go through the size (exclusive). i=0,i
In the client code, I don't understand the while loop. When you call rs.next() (I am assuming here rs is a ResultSet), you move your pointer one more, unless there are no more entries. Then it seems that you iterate through all the items again. I may be incorrect, as I am unfamiliar with the usage of ResultSet, though from browsing the JavaDoc, it seems problematic.
Try printing the raw msg in your client code, see what spits out. Also go ahead an do the same after the cast, see if the output makes sense.
You should not be passing a ResultSet around at all. It's an interface that does not extend Serializable. It's an abstraction for a database cursor. When you pass it from one machine to another, without any information about the connection, what do you expect to happen? I don't see how it can work.
A ResultSet should never be passed out of the method in which it was created. The right thing to is the load it into an object or data structure, close it in a finally block, and return the object or data structure.
Your code is riddled with problems, but I think this is the chief one.
Related
I wrote a for loop that is supposed to determine if there is user input. If there is, it sets the 6 elements of int[] valueArr to the input, a vararg int[] statValue. If there is no input, it sets all elements equal to -1.
if (statValue.length == 6) {
for (int i = 0; i < 6; i++) {
valueArr[i] = statValue[i];
}
} else {
for (int i : valueArr) {
i = -1;
}
}
I am using Visual Studio Code, and it is giving me a message in for (int i : valueArr) :
"The value of the local variable i is not used."
That particular for loop syntax is still new to me, so I may be very well blind, but it was working in another file:
for(int i : rollResults) {
sum = sum + i;
}
I feel that I should also mention that the for loop giving me trouble is in a private void method. I'm still fairly new and just recently started using private methods. I noticed the method would give the same message when not used elsewhere, but I do not see why it would appear here.
I tried closing and reopening Visual Studio Code, deleting and retyping the code, and other forms of that. In my short experience, I've had times where I received errors and messages that should not be there and fixed them with what I mentioned, but none of that worked here.
for (int i : valueArr) {
.... CODE HERE ...
}
This sets up a loop which will run CODE HERE a certain number of times. Inside this loop, at the start of every loop, an entirely new variable is created named i, containing one of the values in valueArr. Once the loop ends this variable is destroyed. Notably, i is not directly the value in valueArr - modifying it does nothing - other than affect this one loop if you use i later in within the block. It does not modify the contents of valueArr.
Hence why you get the warning: i = -1 does nothing - you change what i is, and then the loop ends, which means i goes away and your code hasn't changed anything or done anything, which surely you didn't intend. Hence, warning.
It's not entirely clear what you want to do here. If you intend to set all values in valueArr to -1, you want:
for (int i = 0; i < valueArr.length; i++) valueArr[i] = -1;
Or, actually, you can do that more simply:
Arrays.fill(valueArr, -1);
valueArr[i] = -1 changes the value of the i-th value in the valueArr array to -1. for (int i : valueArr) i = -1; does nothing.
Here is my MySql table:
I want to show the output of the query in commandline as below:
I have written the code below to loop but I am getting only the first row, What i have to modify ??
ResultSet rs2 = stmt.executeQuery(table_retrive);
String[] cols = new String[itemList.size()];
int[] rec =new int[itemList.size()];
for (int i = 0; i < itemList.size(); i++) {
while (rs2.next()) {
cols[i] =(String) itemList.get(i);
rec[i] = rs2.getInt(cols[i]);
System.out.println(rec[i]+" ");
}
}
Your two loops are wrong. Start at i=0 and then iterate once over the whole ResultSet, filling yor first array position. When this is done, i is incremented and you try to iterate the ResultSet a second time but the cursor is at the end of the ResultSet, so rs2.next() returns false and the code will not be executed.
So you have two Solutions:
Handle the loops correctly. Unfortunately I do not know, what you are trying to do anyways because this is some C-like code without OOP, which doesn't show semantics and then you have this itemList which seems to hold preset values and you read out of this list, which column to take for the i-th position. This seems odd. Maybe switching the loops does the desired: Start with the while and nest the for.
Reset the cursor of the ResultSet after the while with rs2.beforeFirst(). WARNING: This could throw a SQLFeatureNotSupportedException. Not all Databases can move the cursor backwards. This is of course a very ugly solution, since you should first parse the whole row a once.
Try to use printf() Or format() method. It is same as printf method in c lang. you can pass parameters and difference. Look at link1
And link 2
Example : System.out.printf("%d%5s%10d", 5,"|",10);
output : 5 | 10
Using this the I got all the values but in one row :
while (rs2.next()) {
for (int i = 0; i < itemList.size(); i++) {
cols[i] =(String) itemList.get(i);
rec[i] = rs2.getInt(cols[i]);
System.out.print(rec[i]+" ");
}
}
But I need to divide like the rows.
Usage of the inner loop is your problem.
You can enhance your code to remove the usage of the second loop in your code, it basically does nothing. You can loop over your result set and in the same loop using the incremented variable to persist the values accordingly.
The code shown half implemented in your question, hence it will be difficult to give you exactly what need to be done. Nevertheless, here's an attempt to resolve the problem for you:
while (rs2.next()) {
System.out.println(rs2.getInt(1) + "\t |" + rs2.getString(2) + "\t |" + rs2.getString(3));
}
Based on the column names from the table in the question, assuming that column2 and column3 are String's.
You can add the necessary details to this code to complete it according to your usecase, but I've just taken the example of showing a record in one line.
EDIT:
OP has his own way of programming, but to satisfy his question in the comment - this is how you can do it.
while (rs2.next()) {
for (int i = 0; i < itemList.size(); i++)
{
cols[i] =(String) itemList.get(i);
rec[i] = rs2.getInt(cols[i]);
System.out.print(rec[i]+"\t |");
}
System.out.println();
}
I am building a Sudoku solver that use the Try and Fail technique to solve any problem. My algorithm is:
1)Update (method that remove any possible value that already given as a final value to element in the same Row, column or squar)
2)Get the minimum element that has minimum number of possible values
3)start solve assuming the first possible value is the final value
4)save the current sate into a stack
5)Try to solve
5-a)If solved, return
5-b)if not solved and with invalid Sudoku, then Pop previous state
6)Repeat step 3) for all possible vaues (9)
7)Repeat step 2) until the puzzel is solved
This is my code
Stack<Element[][]> myStack= new Stack<>();
private Element[][] mySudoku;
public void solve(){
update();//remove all final values from all possible values for each element
if(isSudokuSolved(mySudoku)){
return;
}
//find a cell that is not confirmed and has the minimal candidates
int celli=-1,cellj=-1, p=10;
for(int i=0;i<9;i++){
for(int j=0;j<9;j++){
if(mySudoku[i][j].getValue()==0){
if(mySudoku[i][j].getPossibleValues().size()<p){
celli=i;
cellj=j;
p=mySudoku[i][j].getPossibleValues().size();
}
}
}
}
try {
for (int c = 0; c < mySudoku[celli][cellj].getPossibleValues().size() - 1; c++) {
//save state
Element[][] copy=deepCopy(mySudoku);//copy the current state
myStack.push(copy);
//apply candidate to cell
mySudoku[celli][cellj].setValue(mySudoku[celli][cellj].getPossibleValues().get(c));
update();//check is solved
if(checkValidInputSudoku(mySudoku)){
solve();
}else{
try {
mySudoku = myStack.pop();
} catch (EmptyStackException est) {
//do nothing
}
}
}
} catch (Exception e) {
}
//if we have reached here then we are at the last possible value for the candidates so confirm candidate in cell
if(celli!=-1 && cellj!=-1 && p!=10) {//Some problems happen here "out of Boundry -1 Error"
mySudoku[celli][cellj].setValue(mySudoku[celli][cellj].getPossibleValues().get(mySudoku[celli][cellj].getPossibleValues().size()-1));
}
}//end of solve method
I have spent more than 6 hours trying to find out the problem. I have checked for the Update() method, deepCopy() method and checkValidInputSudoku() method. They all works fine. Thank you in Advance
I can see one problem in your code. You have a loop that is sawing off the branch it sits on:
for(int c = 0; c < mySudoku[celli][cellj].getPossibleValues().size() - 1; c++) {
...
mySudoku[celli][cellj].setValue(mySudoku[celli]cellj].getPossibleValues().get(c));
...
}
Apart from that, you are missing one of the values, it should be for(c=0; c!=size; ++c), i.e. not size - 1. Also, calling getPossibleValues() just once would make this code much more readable. Lastly, catching and ignoring a stack underflow is just stupid, because it hides errors in your algorithm, as far as I can tell. If you don't know how to handle an error, don't just silence it. Since java requires you to catch it, put it in the outermost place possible or at least abort or do something, but don't ignore it!
One more thing: You are recursing and passing the context data via mySodoku and myStack. This is completely missing the point of recursion (or at least the way it's typically used), because the function call stack is the only stack you need. Using these to pass parameters only makes things more complicated than necessary. Instead, the function should return a partial sodoku puzzle and return either the fully solved puzzle or null. Using is easier to distinguish than the exception you're using now, and it's a regular and expected thing, not really exceptional. Then, when trying different choices, you set the cell to the values in turn and recurse, until the call doesn't return null. If none of the choices returns a solution, you clear the cell and return null yourself.
solve(sodoku):
if sodoku is solved:
return true
if sodoku is invalid:
return false
c = some empty cell
for v in 1...9:
// set to a value and recurse
c = v
if solve(sodoku):
// found a solution
return true
// no solution found, clear cell and return failure
c = null
return false
BTW: This strategy is called "backtracking". Using a cell with the least amount of possible values is called "pruning", which allows you to cut off whole branches from the search tree. Actually determining the possible values also helps avoiding a few futile attempts.
This is the code I am working on:
if(connection.doDatabaseRead(findSQL))
{
ResultSet retRES = connection.getResultSet();
int i = 0;
// did we find anything
while( retRES.next() )
{
//read result from query
suiteNum.add(retRES.getString(i)); // this is the problem
i++;
//let other threads breathe
Thread.yield();
}
}
suiteNum is a string vector
When I try to add the database results to the vector the code crashes with this error.
java.sql.SQLException: Column Index out of range, 0 > 1.
I have the same piece of code working elsewhere in the program but I use real numbers like 0, 1 and 2 instead of i and it works fine.
As I do not know how many results the database request will have I need it to be dynamic but it will only work hard coded.
How can I make it work with i ?
The argument to getString is the column index, not the row index as you seem to think. The function returns the value of the given column in the current row, while next advances the cursor to the next row.
You probably mean:
suiteNum.add(retRES.getString(1));
in which case you can lose i altogether.
Java ResultSet objects are 1-indexed in this regard. The first element is at 1, not 0. See the javadoc.
EDIT: That's true too, but indeed the problem is this appears to be used as a row index! it's certainly the column.
This is your problem:
i = 0;
...
retRES.getString(i);
ResultSet.getString(i) gets a String from column number i
You want something like
while(retRes.next()) {
add(retRes.getString(1);
}
column index starts from 1
As I do not know how many results the database request will have I need it to be dynamic but it will only work hard coded. How can I make it work with i
ResultSetMetaData rsMetaData = rs.getMetaData();
int numberOfColumns = rsMetaData.getColumnCount();
See Also
ResultSetMetaData
Let your i start with 1 as specified in the API docs
if(connection.doDatabaseRead(findSQL))
{
ResultSet retRES = connection.getResultSet();
int i = 1;
// did we find anything
while( retRES.next() )
{
//read result from query
suiteNum.add(retRES.getString(i)); // this is the problem
i++;
//let other threads breathe
Thread.yield();
}
}
public boolean catDog(String str)
{
int count = 0;
for (int i = 0; i < str.length(); i++)
{
String sub = str.substring(i, i+1);
if (sub.equals("cat") && sub.equals("dog"))
count++;
}
return count == 0;
}
There's my code for catDog, have been working on it for a while and just cannot find out what's wrong. Help would be much appreciated!*/
EDIT- I want to Return true if the string "cat" and "dog" appear the same number of times in the given string.
One problem is that this will never be true:
if (sub.equals("cat") && sub.equals("dog"))
&& means and. || means or.
However, another problem is that your code looks like your are flailing around randomly trying to get it to work. Everyone does this to some extent in their first programming class, but it's a bad habit. Try to come up with a clear mental picture of how to solve the problem before you write any code, then write the code, then verify that the code actually does what you think it should do and that your initial solution was correct.
EDIT: What I said goes double now that you've clarified what your function is supposed to do. Your approach to solving the problem is not correct, so you need to rethink how to solve the problem, not futz with the implementation.
Here's a critique since I don't believe in giving code for homework. But you have at least tried which is better than most of the clowns posting homework here.
you need two variables, one for storing cat occurrences, one for dog, or a way of telling the difference.
your substring isn't getting enough characters.
a string can never be both cat and dog, you need to check them independently and update the right count.
your return statement should return true if catcount is equal to dogcount, although your version would work if you stored the differences between cats and dogs.
Other than those, I'd be using string searches rather than checking every position but that may be your next assignment. The method you've chosen is perfectly adequate for CS101-type homework.
It should be reasonably easy to get yours working if you address the points I gave above. One thing you may want to try is inserting debugging statements at important places in your code such as:
System.out.println(
"i = " + Integer.toString (i) +
", sub = ["+sub+"]" +
", count = " + Integer.toString(count));
immediately before the closing brace of the for loop. This is invaluable in figuring out what your code is doing wrong.
Here's my ROT13 version if you run into too much trouble and want something to compare it to, but please don't use it without getting yours working first. That doesn't help you in the long run. And, it's almost certain that your educators are tracking StackOverflow to detect plagiarism anyway, so it wouldn't even help you in the short term.
Not that I really care, the more dumb coders in the employment pool, the better it is for me :-)
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Fgevat fho = fge.fhofgevat(v, v+3);
vs (fho.rdhnyf("png")) {
qvssrerapr++;
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}
Another thing to note here is that substring in Java's built-in String class is exclusive on the upper bound.
That is, for String str = "abcdefg", str.substring( 0, 2 ) retrieves "ab" rather than "abc." To match 3 characters, you need to get the substring from i to i+3.
My code for do this:
public boolean catDog(String str) {
if ((new StringTokenizer(str, "cat")).countTokens() ==
(new StringTokenizer(str, "dog")).countTokens()) {
return true;
}
return false;
}
Hope this will help you
EDIT: Sorry this code will not work since you can have 2 tokens side by side in your string. Best if you use countMatches from StringUtils Apache commons library.
String sub = str.substring(i, i+1);
The above line is only getting a 2-character substring so instead of getting "cat" you'll get "ca" and it will never match. Fix this by changing 'i+1' to 'i+2'.
Edit: Now that you've clarified your question in the comments: You should have two counter variables, one to count the 'dog's and one to count the 'cat's. Then at the end return true if count_cats == count_dogs.