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Regarding StringBuffers

Manvir Singh
Greenhorn

Joined: Oct 19, 2003
Posts: 6
StringBuffer sb = new StringBuffer("Manvir");
StringBuffer sb1 = new StringBuffer("Manvir");
System.out.println( (sb.toString()).equals( (sb1.toString() )) //True
System.out.println(sb.equals(sb1));//False.
In the preceding code, the comments indicate the output.
Can someone please enlighten me as why SUn chose to over-ride the String' equals method and not StringBuffer's?
Panos Liaskos
Ranch Hand

Joined: Jul 28, 2003
Posts: 34

Can someone please enlighten me as why SUn chose to over-ride ...

1. Maybe, it has something to do with the imutability/ mutability of String/ StringBuffer.
2. Why SUN choose to override String's equals and not StringBuffer's? Why not Both?
Well, examine the available constructors for these classes:
String()
String(byte[] bytes)
String(byte[] bytes, int offset, int length)
String(byte[] bytes, int offset, int length, String charsetName)
String(byte[] bytes, String charsetName)
String(char[] value)
String(char[] value, int offset, int count)
String(String original)
String(StringBuffer buffer)
StringBuffer()
StringBuffer(int length)
StringBuffer(String str)
Which one you would choose?
Vivek Nidhi
Ranch Hand

Joined: Aug 10, 2003
Posts: 133
hi there,
My Humble opinion is that java.lang.StringBuffer class don't have a equals() method in it. Only in String, Wrappers and Object there are equals(). Here in you are invoking the equals() method in java.lang.Object which is looking for the reference, which is different. Hence returning false.
thanks
Vivek Nidhi
Jose Botella
Ranch Hand

Joined: Jul 03, 2001
Posts: 2120
Welcome to the Ranch Manvir and Vivek.
Maybe the reason is that they did not care about the fact that the class has no logical equality test. The StringBuffer is normally used to manipulate a string object. The string class represents the actual data. When a stringbuffer will be equal to another? are they equal if their string objects and their capacities are equal? This point seems not to be very useful for the programmer.


SCJP2. Please Indent your code using UBB Code
Arulkumar Gopalan
Ranch Hand

Joined: Oct 13, 2003
Posts: 104
Hi,
As per the Java Language Specification, the equals method returns true in two cases.
Case 1: If both the references are pointing to the same object
Case 2: If the hashCode value of the objects are same.
Well, now let us see what is the issue here.
Since the StringBuffer objects are mutable, they are not created as like String Objects. So each object has different hashCode value. that is the reason it returns false.
Please refer the below example.
StringBuffer strb1 = new StringBuffer("ArulkumarG");
StringBuffer strb2 = new StringBuffer("ArulkumarG");
System.out.println(strb1==strb2); //false
System.out.println(strb1.equals(strb2)); //false
System.out.println(strb1.toString().equals(strb2.toString())); //true
//strb1 = strb2;
//System.out.println(strb1.equals(strb2)); //true
System.out.println(strb1.hashCode()+" "+strb2.hashCode()); //7474923 3242435

------------------------------------------------
Can someone please enlighten me as why SUn chose to over-ride the String' equals method and not StringBuffer's?
------------------------------------------------
So, to answer to your question, I think it is not related to overridding.
It's all about how equals method works.
Please correct me if I am wrong.
Thanks,
~AK
-- No power in the world can withhold from anyone anything he really deserves.


Anbudan & Mahalo,<br />Arul<br /> <br />-Not a sun certified Java professional :-)
Wayne L Johnson
Ranch Hand

Joined: Sep 03, 2003
Posts: 399
java.lang.String does override the "equals()" method, while java.lang.StringBuffer does not. As has been mentioned by prior posts, String instances are immutable, so two Strings that are holding onto the same set of characters should be considered as equal. [Note that is different from "=="].
If you look at the JavaDoc for the ".equals()" method in Object it gives a good description of when two objects should be considered equal. One of the conditions stated is "that equal objects must have equal hash codes". Which means they can be used interchangeably in any collection that uses the HashCode as a key.
If SUN had chosen to implement ".equals()" for a StringBuffer based solely on the current contents of the underlying String value, then the following would cause problems.

The first key/value pair would go in fine, but the second one would over-ride the first since the Hashtable class would interpret the two keys (sb1 and sb2) to be the same object.
So SUN made the correct decision in this case in my opinion.
[ October 20, 2003: Message edited by: Wayne L Johnson ]
 
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