番石榴Multimap。把价值放在最后和开始

Question

I use Guava Multimap:

Multimap<Integer, String> commandMap = LinkedHashMultimap.create();
...
actionMap.put(index, "string"); // Put value at the end of list.

This command put value at the end of list. But I need to be able to add both to the end and beginning. Is there a way to solve this?

Answer 1

A linked hashmap doesn't work as a list because it's just a regular map where the order with which the nodes were added is kept, for you to use later (with an iterator for example). That's why you don't have any function to add an element with an index.

If you want to add an element to the beggining of a LinkedHashMultimap you will need to create a new one and add all elements of the old LinkedHashMultimap to the new one:

Multimap<Integer, String> newMap = LinkedHashMultimap.create();
newMap.put(key,valueForTheFirstIndex); // first (and only) object of new map
newMap.putAll(commandMap); // adds with the order of commandMap 
commandMap = newMap;

the add all will add all other elements to the newMap making that valueForTheFirstIndex actually stay in the first index. Note that you are loosing the advantages of using a map if you do this, because if always add to the begining of the array your complexity will be O(n^2). If you want to add to indexes you should use a list while you are adding stuff and then convert to a linkedhashmap for fast accessing.

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(out of question scope)

The value that you have there named index is not an index but is actually a key. You don't have indexes in maps.

actionMap.put(index, "string"); 

As you can read in the documentation: http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/LinkedHashMultimap.html

put(K key, V value) // you don't see any reference to index there

Answer 2

This isn't a ListMultimap , it's a SetMultimap . If you want a ListMultimap , use ArrayListMultimap or LinkedListMultimap .