Returns true if this map contains a mapping for the specified key. More formally, returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k)). (There can be at most one such mapping.)
@param key key whose presence in this map is to be tested
@return true if this map contains a mapping for the specifiedkey
@throws ClassCastException if the key is of an inappropriate type forthis map (optional)
@throws NullPointerException if the specified key is null and this mapdoes not permit null keys (optional)
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