About Question enthuware.ocpjp.v7.2.1380 :

[Alexey Berezkin]

Why the first option incorrect? Please try the following:

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Map<Map<?, ?>, String> m = new HashMap<Map<?, ?>, String>();
m.put(m, "value");
System.out.println(m);

The output is:

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{(this Map)=value}

You see, this is considered even a special case and properly handled in AbstractMap.toString().

[admin]

Well, as per JavaDoc API description of Map

The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key.