1. java in the length property for the array is to say, for example, declare an array you want to know the length of the array is used the length of this property.
2. java The length() method is for the string to say, if want to see the length of the string is used in length() this method.
3. java The size() method is for the generic collection to say, if want to see this generic how many elements, it calls this method to see!