Incorrect equals() implementation...

[RAGU KANNAN]

Hello Guru�s,

Why this equals method implemented incorrectly? The following code compares string literals against obj values. So what is the problem? Pls explain to me.

Thanks, Raghu.K

public class BetterString
{
String str;
public BetterString(String s)
{
str = s;
}
public boolean equals(Object obj)
{
if(obj instanceof BetterString)
{
if(obj == this)
return true;
else
return str.equals(obj);
}
else if(obj instanceof String)
{
return str.equals(obj);
}
return false;
}
}

[Keith Lynn]

Look at this line.

return str.equals(obj);

If you send a BetterString object to the equals method, then this line is going to return false no matter what.

Remmber that if you call the equals method on a String object and send a reference to an object that isn't a String to it, the result is false.

[wise owen]

return str.equals(obj);

And also the str may be null.

[Pad Ven]

But I got true for the following code. That equals() method does not return false always.

public class BetterString
{
String str;
public BetterString(String s)
{
str = s;
}
public boolean equals(Object obj)
{
if(obj instanceof BetterString)
{
if(obj == this)
return true;
else
return str.equals(obj);
}
else if(obj instanceof String)
{
return str.equals(obj);
}
return false;
}

public static void main (String[] args) {
BetterString bs = new BetterString ("OK");
System.out.println (bs.equals (new String ("OK")));
}
}

[Suhas Wadadekar]

System.out.println (bs.equals (new String ("OK")));
}
}

1. If you pass a String object then str.equals(obj) will be true if and only if in the above quote you write "OK".
2. Also u can pass only a String object to make the line str.equals(obj) work, passing a BetterString will always return false

[Vipin Das]

If you send a BetterString object to the equals method, then this line is going to return false no matter what.

Just to argue it wont be false all time, the following code will also return true. Append the following line to the padmanabhan's code.

 
 public static void main (String[] args) {
        System.out.println (bs.equals ("OK"));
        System.out.println (bs.equals(bs));
 
}
  

[Vijay Raj]

He's calling the equals() method on the String object. So passing anything other than a String reference, it will return false.

What you are doing is passing the BetterString object to BetterString's over-ridden equals() method. What Keith Lynn is saying that passing BetterString object to String's equals() method will always return false.

regards,
vijay.

[Keith Lynn]

 
 public static void main (String[] args) {
        System.out.println (bs.equals ("OK"));
        System.out.println (bs.equals(bs));
 
}
  

[/qb]<hr></blockquote>

That is a different situation.

The reason that works is because of this statement.

else if(obj instanceof String)
{
return str.equals(obj);

The condition in the else if will only be checked if you send a String object to the equals method.

But that is not the purpose of the equals method of BetterString.

The intention is to determine when one BetterString object is equal to another.

bs.equals(bs) only returns true because of this.

if(obj == this)
return true;

which does not test the contents of the BetterString object.
[ July 12, 2006: Message edited by: Keith Lynn ]