常用操作集：
contains() : find out whether an element is part of a list
remove() : remove an object , take the reference of an object as parameter
indexOf() : discover the index number where that object is located in the list
*** these three methods above uses equals() function, so if elements are Strings, equal compares the content of two Strings***
subList() : this method allows you to easily create a slice out of a larger list
retainAll() : is effectively a "set intersection" operation. Again the resulting behavior depends on the equals().
set() : replace the element at the first argument with second argument
addAll() : allows to insert new list in the middle of the original list instead of just appending it to the end. It is effectively a "set union" operation.
removeAll() : removes all the objects from the list that are in the argument list. It is effectively a "set differ" operation.
Collections.sort() Collections.shuffle() : these operations will change the original lists.
toArray() : can convert any Collection to an array