Tuesday, November 30, 2010

Java final String optimisation

Some times I need a sub string of a string and I noticed that if you add final keyword, Java compiler makes a small optimization, isn't it?

For example :

String name = "this is a simple string";
String simple = name.substring(0,4);

Here Java creates two strings
but if you write this code in different way you get rid of one extra allocation

final String name = "this is a simple string";
final String simple = name.substring(0,4);

Both objects points to the same memory block and String object itself has a constant context
Am I right?

I am not sure about this conclusions, so, I will check it again when I have time.

If you know something how to use final for Strings and what exactly java compiler does, 

I'll be pleased if someone share his knowledges in this area

Bugs aligned to CString

Today I noticed a funny thing. Class CString C++ sometimes works like class String in Java
For example :
class Example
  CString mName;

  void setName(const CString & name)
     mName = name; 
  void modify()
     memset((VPVOID)(LPTSTR) mName,65,mName.GetLength()*sizeof(TCHAR));

class Other
// we set up this name somewere in code 
  CString getName() const
     return mName; 

void main()
   std::autoptr o1 =
std::autoptr(new  Other(_T("simple string")));
   Example ex1;
   ex.setName( o1->getName() );
   // here you will have empty string for o1 and for ex classes 
   // because it refers to the same string data in memory 

   // so both classes will contain modified data

Conclusion : don't mix API because you never know about class implementation.
Fix is simple, just use mText.Empty() or mText[n] to modify string and never (LPTSTR)(mText)[n]