The Collection views do not support element addition under any circumstances. (Yet again, this assumes that the backing Map supports element removal.) The Collection views support element removal in all its many forms В— remove, removeAll, retainAll, and clear operations, as well as the Iterator.remove operation. Note that these are the only safe ways to modify a Map during iteration the behavior is unspecified if the underlying Map is modified in any other way while the iteration is in progress. With the entrySet view, it is also possible to change the value associated with a key by calling a Map.Entry's setValue method during iteration (again, assuming the Map supports value modification to begin with). This is illustrated by the preceding filtering idiom. With all three Collection views, calling an Iterator's remove operation removes the associated entry from the backing Map, assuming that the backing Map supports element removal to begin with. This is precisely what all the Map implementations in java.util do. Rest easy: There's no reason that a Map cannot always return the same object each time it is asked for a given Collection view. (e.getKey() + ': ' + e.getValue()) ДЄt first, many people worry that these idioms may be slow because the Map has to create a new Collection instance each time a Collection view operation is called. The following one-liner creates a new HashMap initially containing all of the same key-value mappings as m. For example, suppose you have a Map, named m. This standard Map conversion constructor is entirely analogous to the standard Collection constructor: It allows the caller to create a Map of a desired implementation type that initially contains all of the mappings in another Map, regardless of the other Map's implementation type. Two Map instances are equal if they represent the same key-value mappings.Д«y convention, all general-purpose Map implementations provide constructors that take a Map object and initialize the new Map to contain all the key-value mappings in the specified Map. Listinterfaces, Map strengthens the requirements on the equals and hashCode methods so that two Map objects can be compared for logical equality without regard to their implementation types. This flexibility provides a potent illustration of the power of an interface-based framework.

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