Enthuware Map question

[John Stark]

Hi,

One question in Enthuware is:

Which of these statements concerning maps are true?

One option is:

It is permissible for a map to contain itself as a key.

In the review it says this option is wrong 'But it can contain itself as a value'.
I don't really know what that means. Can anybody explain?

Thanks,

John

[Riza Aktunc]

A map has two attributes - key and value.
Judging by what you have posted; a map can contain itself as a value but not a key.

String x="";
HashMap hm=new HashMap();
hm.put(hm,x); //not ok
hm.put(x,hm); //ok

But, when I try it, compiler does not complain! In runtime hm.put(hm,x); gives StackOverFlowError!

[John Stark]

So the StackOverFlowError occurs because the map contains itself which contains itself contains itself ...?
Why does this not occur when adding the map as a value?

Thanks,

John