About wrap class and ==,equals;

[Ronyn Woods]

Could you tell me the answers and why? thanks a lot.
Integer i = new Integer("42");
Long l = new Long("42L");
Double d = new Double("42.0");
Which returns true ?(choose one or more)
a)i==d;
b)i==l;
c)l==d;
d)i.equals(d);
e)d.equals(i);
f)i.equals("42");

[Mikael Jonasson]

I believe that only f is coorect here. The wraperclasses doesn't have converters.
The == operator isn't defined for the wrapperclasses, so it tries to compare the objects themselves, which will result in conversion error.
/Mike

[Ravindra Mohan]

Hi Folks,
The answer is none of these. You can compile the code
and check the same. If you see the API , then you can see that
each Wrapper class overrides the equals() of the Object and returns "true" if and only if the argument is not null and is a corresponding wrapper class object that represents the same wrapper class object value as the object being compared.
for example lets take a boolean primitive and its corresponding
wrapper class Boolean:

boolean b = false;
Boolean boolObj= Boolean.valueOf("false");    
boolean boolTest  = boolObj.equals("false");  // returns "false"

In case you want to compare the primitive values and the corresponding wrapper class object's value , then you should
use the typeValue() of the corresponding wrapper class.
Lets take the boolean as the example, viz., usage of booleaValue() method of Boolean Class :

boolean b = false;
Boolean boolObj= Boolean.valueOf("false");    
b == boolObj.booleanValue();  // returns "true"

Hope your doubts are cleared now.
Ravindra Mohan.

[This message has been edited by Ravindra Mohan (edited May 22, 2001).]

[Susan Delph]

I wrote a short program to test these. Here are the results.
Options a,b,c all give compiler errors:

TestIt.java:9: incomparable types: java.lang.Integer and java.lang.Double
    if (i==d)
         ^
TestIt.java:11: incomparable types: java.lang.Integer and java.lang.Long
    if (i==l)
         ^
TestIt.java:13: incomparable types: java.lang.Long and java.lang.Double
    if (l==d)
         ^   

I got rid of these lines. The program then compiled OK, but gave me NumberFormatExeception on the line:
Long l = new Long("42L");
So I changed this line to read:
Long l = new Long("42");
The program then compiled and ran. The remaining tests all gave 'false' results:
d) i.equals(d);
e) d.equals(i);
f) i.equals("42");
Ravindra's message explained why these gave 'false' results.
Susan

[This message has been edited by Susan Delph (edited May 22, 2001).]