public Set<String> stringPropertyNames()Returns a set of keys in <FONT color=magenta>this property list</FONT> where the key and its corresponding value are strings, including distinct keys in <STRONG><FONT style="BACKGROUND-COLOR: white" color=indigo>the default property list</FONT></STRONG> if a key of the same name has not already been found from the main properties list. Properties whose key or value is not of type String are omitted.
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这里所说的主属性列表this property list 就是我们后来添加的那些个属性,而所谓的默认的属性列表the default property list 就是构造函数里面添加的那些个属性列表Properties defaults。