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about equals

Sundar Murthi
Ranch Hand

Joined: Mar 05, 2004
Posts: 209
Hi..

What is the output of following code



the output given as

false
false
false

i understand first and last "false" result

but why the second one is false

tx in adv..
kapil munjal
Ranch Hand

Joined: May 11, 2004
Posts: 298
Even I am not sure why it returns false, but I have come across the API documentation of the equals method defined in the object class and it says that if and only if x and y refers to the same object, only then it returns true.

The following quote is the documentation of the equals method defined in the Object class.

equals
public boolean equals(Object obj)Indicates whether some other object is "equal to" this one.
The equals method implements an equivalence relation on non-null object references:

It is reflexive: for any non-null reference value x, x.equals(x) should return true.
It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
For any non-null reference value x, x.equals(null) should return false.
The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.


Parameters:
obj - the reference object with which to compare.
Returns:
true if this object is the same as the obj argument; false otherwise.
See Also:
hashCode(), Hashtable


Please if someone put a little more light on this.

Thanks
Kaps
[ June 09, 2004: Message edited by: kapil munjal ]

Kapil Munjal
SCJP 1.4, SCWCD 1.4
Shilpi M Ag
Ranch Hand

Joined: May 07, 2004
Posts: 40
In Dan's test - http://www.danchisholm.net/dec20/topic/section8/stringbuffer1.html

for ques 4 the explaination is :

The StringBuffer.equals method does not override Object.equals. The StringBuffer.equals method compares the reference values--not the contents of the StringBuffer instances. Since the reference values sb1 and sb2 are different, the method invocation expression sb1.equals(sb2) produces the value false.


So this explains the II false
Baps Vakkalagadda
Ranch Hand

Joined: May 25, 2004
Posts: 47
String class overrides the equals() method and overridding equals() method compares the contents of the String objects.

Unlike String class, StringBuffer class does not override the equals() method. So the statement sb1.equals(sb2) calls Object's equals() method, which works the way that Kapil mentioned.
Sundar Murthi
Ranch Hand

Joined: Mar 05, 2004
Posts: 209
Thanks Kapil & Shilpi now i understand about the equals methods in StringBuffer
 
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