Problem with contains() method in ArrayList

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Chanchal - 04 Jan 2006 06:04 GMT

hi,
In an application i'm developing, i have a class say X and
an arraylist of X ie,

list = new ArrayList<X>() ;

and i have added an object of X into list

list.add(x); // x ib object of X

and i'm calling the contains() function of the arraylist to check if an
object of X, say 'y' is in there.

I have overridden the equals() method in X and it returns true for
x.equals(y).

But the contains() method returns false. what could be the reason.
please advice

Chanchal

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Torkel Franzen - 04 Jan 2006 06:11 GMT

> I have overridden the equals() method in X and it returns true for
> x.equals(y).
>
> But the contains() method returns false. what could be the reason.

That you haven't in fact overridden equals.

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Chanchal - 04 Jan 2006 06:16 GMT

Hi Franzen,
i haven't fully understood your comment that i haven't overridden
equals

this is my equals() function

public boolean equals(SaleItem saleItem){
return (saleItem instanceof Service &&
this.name.equals(saleItem.getName()));
}

am i missing anything here??

Chanchal

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Jean-Francois Briere - 04 Jan 2006 06:20 GMT

> this is my equals() function
>
> public boolean equals(SaleItem saleItem){
> return (saleItem instanceof Service &&
>this.name.equals(saleItem.getName()));
> }

Here you are overloading the equals method (same name, different
parameter number / type).
You are not overriding it (same name, same parameter number / type).

Regards

Reply to this Message

Adam Maass - 06 Jan 2006 04:59 GMT

>> this is my equals() function
>>
[quoted text clipped - 6 lines]
> parameter number / type).
> You are not overriding it (same name, same parameter number / type).

IE, you need this (or something similar) instead (note that the type of the
parameter is Object, not SaleItem):

public boolean equals(Object other)
{
if(other instanceof SaleItem)
{
SaleItem saleItem = (SaleItem)other;
return this.name.equals(saleItem.name);
}
else
{
return false;
}
}

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Jean-Francois Briere - 04 Jan 2006 06:16 GMT

The equals method MUST have the following signature:

public boolean equals(Object obj)

Regards

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Chanchal - 04 Jan 2006 06:23 GMT

oooooooppppsssssssss, silly me,

thankx pal

Chanchal

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Thomas Hawtin - 04 Jan 2006 12:30 GMT

> The equals method MUST have the following signature:
>
> public boolean equals(Object obj)

Adding @Override to methods that are supposed to override will cause the
compiler to catch these sorts of errors.

Tom Hawtin

Signature

Unemployed English Java programmer
http://jroller.com/page/tackline/

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Chanchal - 05 Jan 2006 07:00 GMT

hi thomas

thanks for that tip. but where should i add @Override? should the
signature of function be

@override public boolean equals(Object obj) ? where can i find similar
swithces??

Chanchal

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Chanchal - 05 Jan 2006 07:00 GMT

hi thomas

thanks for that tip. but where should i add @Override? should the
signature of function be

@override public boolean equals(Object obj) ? where can i find similar
switches??

Chanchal

Reply to this Message

Jean-Francois Briere - 05 Jan 2006 09:07 GMT

The @Override annotation is not a part of the method signature.
It is just information to the compiler.
It's more like:

@Override
public boolean equals(Object obj)
{
...
}

Take a look at:
http://java.sun.com/docs/books/tutorial/java/javaOO/annotations.html

Regards

Reply to this Message

Thomas Hawtin - 05 Jan 2006 13:04 GMT

> The @Override annotation is not a part of the method signature.
> It is just information to the compiler.
[quoted text clipped - 5 lines]
> ...
> }

For @Override only, some people will write that as:

@Override public boolean equals(Object obj)

Not sure how widespread that is. We still haven't got everyone to put
opening braces at end of line.

> http://java.sun.com/docs/books/tutorial/java/javaOO/annotations.html

And the JavaDocs for @Override itself. Annotations are hidden right at
the bottom of the type lists for packages (far bottom left). Note, Java
is case sensitive, so always a capital O.

http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Override.html

Tom Hawtin

Signature

Unemployed English Java programmer
http://jroller.com/page/tackline/

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Problem with contains() method in ArrayList

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