String object!

[juan Tsui]

Hi:
All,there is a question :
1.How many String objects are created when the following class is run?
Code ��
 public class TestClass {
public static void main(String args[])  {
String hello = "Hello"; //1
 String world = new String("World!");//2
   String helloworld = hello +" "+world;//3
   String alias = helloworld ; //4
   System.out.println(alias); //5
 }
}
I think there is only 5 object!
//1 has one in literal pool "Hello",
//2 has two ,one is in programm space,other is in literal pool,
//3 "" is one , the last is "Hello World!" as helloworld reference ! So only 5 ! But tha answer is 6 !
Who can tell me Why ? Thanks,very!!

[lu v thuan]

Hi Tsui:I think
//1 has one in literal pool "Hello",
//2 has two ,one is in programm space,other is in literal pool,
//3 "" is one , hello + "" is two, the object created by hello + "" plus world is three(three objects altogether)
Hope this help

[Jose Botella]

I think there are five:
//1 "Hello"
//2 world and "World!"
//3 helloworld and ""
please correct if wrong

[Manfred Leonhardt]

Hi Jose,
Lu has already explained how we get 6 strings out of the example. In case you missed it here it is again:
Literal Pool (compiler filled):
"Hello" -- from //1
"World!" -- from //2
" " -- from //3
Runtime creations:
"World!" -- from //2
"hello " -- from //3
"hello world" -- from //3
For a total of 6 string objects.
Regards,
Manfred.

[Jose Botella]

Hello Manfred thanks for aswering
I have run javap and I still see only 5 strings:

0 ldc #2 (String "Hello")
2 astore_1
3 new #3 (Class java.lang.String)
6 dup
7 ldc #4 (String "World!")
9 invokespecial #5 (Method java.lang.String(java.lang.String))
12 astore_2
13 new #6 (Class java.lang.StringBuffer)
16 dup
17 invokespecial #7 (Method java.lang.StringBuffer())
20 aload_1
21 invokevirtual #8 (Method java.lang.StringBuffer append(java.lang.String))
24 ldc #9 (String " ")
26 invokevirtual #8 (Method java.lang.StringBuffer append(java.lang.String))
29 aload_2
30 invokevirtual #8 (Method java.lang.StringBuffer append(java.lang.String))
33 invokevirtual #10 (Method java.lang.String toString())
36 astore_3
37 aload_3
38 astore 4
40 getstatic #11 (Field java.io.PrintStream out)
43 aload 4
45 invokevirtual #12 <Method void println(java.lang.String)>
48 return

Lines 0, 7 and 24 shows three String literals.
Lines 3 and 33 shows two strings more.
There is no "Hello " because all the additions to the StringBuffer are performed via the append method, and only after the last addition the content of the StringBuffer is returned as aString (line 33)
Please correct if wrong.

[Vikrama Sanjeeva]

I have run javap and I still see only 5 strings:

What is javap??
Bye
Viki.

------------------
Count the flowers of ur garden,NOT the leafs which falls away!

[Jose Botella]

javap is a disasambler converting from bytecodes to JVM instructions.
It is in the same directory as javac and the instructions are in the SDK Docs
Enjoy

[juan Tsui]

Thanks All ! Thanks for you!
I think the answer perhaps has some problem ! I am going to find it out !!
Thanks !
juantsui

[lu v thuan]

Hi Jose:
In the bytecode you got five String objects because of String
Optimization(An implementation may choose to perform conversion and concatenation in one step to avoid creating and concatenation in one step to avoid creating and then discarding an intermediate String objectsJLS 15.18.1.2)
you are correct
Thank Jose