Buffer Mania

trytryagain · 2009-10-06 15:18

Peter,

This one is VERY STRANGE.

I am trying to create a list of transactions

static List scanLog = new List(100);

static StringBuffer myBuffer = new StringBuffer();··

1. First I use a string buffer to create a transaction string
···then add this string to a list

myBuffer.clear();
myBuffer.append("@");
myBuffer.append("123");
scanLog.add(0,myBuffer.toString());·

2. Now I clear myBuffer, Create a second string and store
··· the next string in the list as a new index

myBuffer.clear();
myBuffer.append("@");
myBuffer.append("456");
scanLog.add(1,myBuffer.toString());·

3. Now I look at whats in the list by index

System.out.println((String)scanLog.get(0));
System.out.println((String)scanLog.get(1));

This reports:
@456
@456

It some how holds a reference to last myBuffer string
and doesn't hold the correct strings in the List. Bizzare.

The Strange part is when I do not use a stringbuffer
it seems to work:

String str = "@123"
scanLog.add(0,str);

str = "@456"
scanLog.add(1,str);

System.out.println((String)scanLog.get(0));
System.out.println((String)scanLog.get(1));

Now I get:
@123
@456

My problem is I need to use a stringbuffer to
assemble my strings for memory efficiency.

What don't I get about these stupid stringbuffers
I am clearing it and it still won't work properly.

PLEASE HELP!

PS: Even when I create a string array -vs- string buffer
I get the same result when I set it equal to a string buffer
I am reusing with myBuffer.clear(); (I've also tried myBuffer.delete(0,4))

Jeff(trytryagain)

·

Peter Verkaik · 2009-10-06 16:00

Remember that StringBuffer only uses one character array.
Applying toString merely returns the reference to this one array, but as type String.

So
1. First I use a string buffer to create a transaction string
···then add this string to a list

myBuffer.clear();
myBuffer.append("@");
myBuffer.append("123");
scanLog.add(0,myBuffer.toString());·

2. Now I clear myBuffer, Create a second string and store
··· the next string in the list as a new index

myBuffer.clear();
myBuffer.append("@");
myBuffer.append("456");
scanLog.add(1,myBuffer.toString());·

3. Now I look at whats in the list by index

System.out.println((String)scanLog.get(0));
System.out.println((String)scanLog.get(1));

This reports:
@456
@456

makes perfect sense as you store the references, not the content of the array.

When using Strings,

String str = "@123"
scanLog.add(0,str);

str = "@456"
scanLog.add(1,str);

System.out.println((String)scanLog.get(0));
System.out.println((String)scanLog.get(1));

Now I get:
@123
@456

str gets the reference of the constant strings and those references are different,
and thus you store different references. and thus reading back shows different content.

What you need to do is to store the content of the StringBuffer in a new StringBuffer,
and store the reference of this new StringBuffer.

regards peter

trytryagain · 2009-10-06 16:13

Pete,

Can I create an array of 100 string buffers

each with a max length of 30

up front so I know my memory allocation.

If so, could you do me big favor and show me

the proper declaration for the string buffer array

and change my supplied code as an example.

I have·been working on this for over 6 hours.

Thanks as always Pete,

Jeff

Peter Verkaik · 2009-10-06 16:26

Here you go

import stamp.core.*;

public class StringBufferList_test {

  static StringBuffer sArray[noparse][[/noparse]] = new StringBuffer[noparse][[/noparse]100];
  static {
    for (int i=0; i<100; i++) {
      sArray[noparse][[/noparse]i] = new StringBuffer(30);
    }
  }

  static void main() {
  }

}

regards peter

trytryagain · 2009-10-06 17:25

·Pete,

I initialized the buffer per your example:

static StringBuffer sArray[noparse]/noparse = new StringBuffer[noparse][[/noparse]100];
· static {
··· for (int i=0; i<100; i++) {
····· sArray[noparse][[/noparse]i] = new StringBuffer(30);
··· }
· }

myBuffer.clear();
myBuffer.append("@");
myBuffer.append("123");

int scanIndex = 0;

scanIndex++;
sArray[noparse][[/noparse]scanIndex] = myBuffer;

myBuffer.clear();
myBuffer.append("@");
myBuffer.append("456");

scanIndex++;
sArray[noparse][[/noparse]scanIndex] = myBuffer;

I am still getting the same values for
sArray[noparse][[/noparse]1] = @456
sArray[noparse][[/noparse]2] = @456

I am using a different index of the sArray buffer
each time and Its still happening!

Jeff

·

Peter Verkaik · 2009-10-06 18:55

Do it like this:

static StringBuffer sArray[noparse]/noparse = new StringBuffer[noparse][[/noparse]100];
· static {
··· for (int i=0; i<100; i++) {
····· sArray[noparse][[/noparse]i] = new StringBuffer(30);
··· }
· }

int scanIndex = 0;

sArray[noparse][[/noparse]scanIndex].clear();
sArray[noparse][[/noparse]scanIndex].append("@");
sArray[noparse][[/noparse]scanIndex].append("123");

scanIndex++;

sArray[noparse][[/noparse]scanIndex].clear();
sArray[noparse][[/noparse]scanIndex].append("@");
sArray[noparse][[/noparse]scanIndex].append("456");

scanIndex++;
etc.

Better to create a method clear and append

static void clear(int i) {
· sArray[noparse][[/noparse]i].clear();
}

static void append(int i, String s) {
· sArray[noparse][[/noparse]i].append(s);
}

and call these methods

regards peter

trytryagain · 2009-10-06 19:08

Pete

Does that work because the reference is coming from a function?

Jeff

Peter Verkaik · 2009-10-06 19:15

It works because you use another StringBuffer each time you
change the index.
The nice thing about a OOP language like java is
that you can call a method via the variable.
sArray[noparse][[/noparse]i].append(s);
sArray[noparse][[/noparse]i] is the variable (of type StringBuffer in this case), append(s) is the method.
And that also allows it to be embedded in another method.

regards peter

trytryagain · 2009-10-06 19:24

·Pete,

After all this I have really grown to dislike strings

These references backword certainly save on space

but make it frustrating to use strings in logs.

Luckily all the values in my string are integers

so I made arrays out of each piece:

index++

Month[noparse][[/noparse]index] = 9

Day[noparse][[/noparse]index] = 14

Year[noparse][[/noparse]index] = 2009

index++

Month[noparse][[/noparse]index] = 10

Day[noparse][[/noparse]index] = 25

Year[noparse][[/noparse]index] = 2009

With integers the values stick.

I assemble the same buffer each time I send a different index of data

buff.clear();

int indextosend = 0;

buff.append(Month[noparse][[/noparse]indextosend]);

buff.append(Day[noparse][[/noparse]indextosend]);

buff.append(Year[noparse][[/noparse]indextosend]);

I will try your suggestion as well Pete

Thanks so much for the explanation

I hope in future versions of Javelin they

give us a little more memory and a better

method to work with strings.

Jeff(trytryagain)

Peter Verkaik · 2009-10-06 20:08

Since you are dealing with dates and times,
check out my Calendar class. It provides methods
to convert between binary stored dates and times
and human readable strings.

regards peter

trytryagain · 2009-10-06 21:31

·Peter,

I appreciate the·calendar code.

Thanks so much for all your help today.

Jeff

Buffer Mania

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