April 4th, 2012 Uncategorized | No Comments »
There are many ways to accomplish this but this one does not use any external dependencies like Apache commons.
Two common pitfalls that I see are that people forget to flush the ByteArrayOutputStream and they call ‘baos.write(buffer)’ instead of ‘baos.write(buffer, 0, read)’ without actually clearing the buffer, which causes the last write to append previous bytes if the read returned less than what have been read from input stream.
private String extract(InputStream inputStream) throws IOException {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
byte[] buffer = new byte[1024];
int read = 0;
while ((read = inputStream.read(buffer, 0, buffer.length)) != -1) {
baos.write(buffer, 0, read);
}
baos.flush();
return new String(baos.toByteArray(), "UTF-8");
}
February 3rd, 2012 Uncategorized | No Comments »
Sometimes its nice to be able to transfer object between request or for flash messages without need for a whole session. Here we will use session as our store mechanism for our flash scope. This concept here can be extended into implementing a ‘Conversation Scope’ but that is a whole different animal.
Example Usage
@FlashAttribute(TABNAME_ATTRIB)
public String getTabName() {
return tabObject;
}
November 22nd, 2011 Uncategorized | No Comments »
Title might be little misleading since in java we pass always by value, but we can mimic passing by reference.
/**
* This class allows us to pass values by reference ranter than value.
* Not directly as java just passes reference of the object by value but indirecly
* @author Greg
*
* @param <E>
*/
public class IndirectReference<E> {
public E ref;
public IndirectReference(E ref) {
this.ref = ref;
}
public void set(E ref) {
this.ref = ref;
}
}Example usage
Just a snippet copied from my interpreter
IndirectReference<ControlSignal> signal = new IndirectReference<ControlSignal>(ControlSignal.NOOP);
if(signal.ref == ControlSignal.BREAK){
break;
}
November 19th, 2011 Uncategorized | No Comments »
While using HTML Tidy I needed to remove the DOCTYPE prolog to prevent
‘org.xml.sax.SAXParseException: Already seen doctype.’ exception.
Regex is quite simple, only catch is that we need to make sure we include the \n\r in our selecton and make it not greedy.
convertedData = convertedData.replaceAll("<!DOCTYPE((.|\n|\r)*?)\">", "");This will consume multiline as well as single declarations
/*
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
*/
October 28th, 2011 Uncategorized | No Comments »
This is snipper of my ANTLR grammar for parsing expressions with operator precedence,
By default the expressions are evaluated left to right which in some cases may produce undesired results, in order to fix that use of left and right parenthesis is required.
For example this two expressions will be evaluated differently
3 > 2 > 1
3 > (2 > 1)
Precedence
&&
||
< > <= >=
^
*/
+-
Grammar
According to ANTRL author currently we need a new rule for each precedence , which make is really messy but its not too bad after you get a hand of it. This grammar includes rewrite rules to generate our Abstract Syntax Tree(AST)
expression
: subExpr -> ^(EXPR subExpr)
;
subExpr : logicalAndExp (addSubtractOp^ logicalAndExp)*
;
logicalAndExp
: logicalOrExp (multiplyDivideOp^ logicalOrExp)*
;
logicalOrExp
: comparatorExp (CARET^ comparatorExp)*
;
comparatorExp
: powExp (comparatorOp^ powExp)*
;
powExp : multExp (BARBAR^ multExp)*
;
multExp
: expressionAtom (AMPAMP^ expressionAtom)*
;
expressionAtom
:
| NUMBER
| ( LPAREN! subExpr^ RPAREN! )
| VARNAME
| function
;
addSubtractOp
: PLUS
| MINUS
;
multiplyDivideOp
: STAR
| SLASH
;
comparatorOp
: GT
| LT
| GTE
| LTE
| NEQ
;
Abstract Syntax Trees
As stated before following two expression produce two different AST, ignore the semantics of the operators as they are only to show proper AST construction.
3 > 2 > 1
3 > (2 > 1)
Here is a expression parsed left to right
Here is a more complex expression that shows precedence and parenthesis operations
SET @me = 1*(2*3 + 3*2)/5+1
I think that for next post I will show on how to evaluate given expressions, also I am always looking for suggestions and comments
September 16th, 2011 Uncategorized | No Comments »
To work around this error
Custom port is not allowed or the host is not registered with this consumer key.
Register the app as Desktop/Client application instead of web app.
July 29th, 2011 Uncategorized | No Comments »
Simply adding @Transactional on the method will resolve this issue
July 21st, 2011 java | No Comments »
Other day I needed to convert excel spreadsheet column name to column index when having 200+ columns is easier to express as ‘FB’ instead of 157
A = 0
B = 1
AA = 26
AA = 27
FB = 157
This code will work for any number of column names.
/**
* Convert given excel column name to column Index, ex 'A=0', 'AA=26'
* @param columnName
* @return 0 based index of the column
*/
private static short convert2ColumnIndex(String columnName) {
columnName = columnName.toUpperCase();
short value = 0;
for (int i = 0, k = columnName.length() - 1; i < columnName.length(); i++, k--) {
int alpabetIndex = ((short) columnName.charAt(i)) - 64;
int delta = 0;
// last column simply add it
if (k == 0) {
delta = alpabetIndex - 1;
} else { // aggregate
if (alpabetIndex == 0)
delta = (26 * k);
else
delta = (alpabetIndex * 26 * k);
}
value += delta;
}
return value;
}
Converting from index to column name
This process is trivial we simply keep on taking mod 26 from index till we have nothing left, and converting that value to char.
July 8th, 2011 java | No Comments »
The other day I needed to read output stream directly into a string, but JDK does not provides us with a stream directly for that. There have been numerous discussions on why and why not we should have this build into the JDK, but this was a case where I did need it and knew the encoding.
My implementation
StringOutputStream backed by ByteArrayOutputStream that simplifies reading of OutputStream directly into string.
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.charset.Charset;
/**
* StringOutputStream backed by ByteArrayOutputStream that simplifies reading of {@link OutputStream} directly into string
* @author Grzegorz Bugaj
*
*/
public class StringOutputStream extends OutputStream{
private ByteArrayOutputStream buffer = new ByteArrayOutputStream();
private static final Charset DEFAULT_CHARACTER_SET = Charset.forName("UTF-8");
private Charset characterSet;
@Override
public void write(int b) throws IOException {
buffer.write(b);
}
@Override
public void write(byte[] b) throws IOException {
buffer.write(b);
}
@Override
public void write(byte[] b, int off, int len) throws IOException {
buffer.write(b, off, len);
}
/**
* Get currently set character set if none set default charset of UTF-8 will be used
* @return Charaset
*/
public Charset getCharacterSet() {
if(characterSet == null)
return DEFAULT_CHARACTER_SET;
return characterSet;
}
public void setCharacterSet(Charset characterSet) {
this.characterSet = characterSet;
}
@Override
public String toString() {
return new String(buffer.toByteArray(), getCharacterSet());
}
}Usage Example
/**
* Read in given stream into a string
* @param stream
* @return
*/
public static String readAsString(InputStream stream) throws IOException{
OutputStream sos = new StringOutputStream();
// Set chunk size to 64K chunk
byte[] buffer = new byte[0x10000];
int readLen;
while((readLen = stream.read(buffer, 0, buffer.length)) != -1){
sos.write(buffer, 0, readLen);
}
return sos.toString();
}
April 14th, 2011 Uncategorized | No Comments »
For cross compatability between sql and mysql servers when we do a division on a aggregated variable like count
we need to promote the INT to a DOUBLE type by multiplying it by 1.00 otherwise our results will loose precision
Examples
1/2=0 Incorrect Results
1.00/2=.5 or 1.00/2.00=.5 Good
For sake of consistency both variables have been promoted to DOUBLE
SELECT ROUND((COUNT(FieldA) * 1.00) / (COUNT(FieldB) * 1.00) * 100, 0)
FROM TESTTABLE