如何创建由 Map 支持的可变列表

Question

How can I return a List<E> backed by Map<K, E> if I have a method for mapping Map values to keys valueToKeyMapper ?

class Foo {
   private Map<Integer, String> backedMap = new HashMap<Integer, String>();

   public List<String> getList() {
      // NEED TO RETURN LIST BACKED BY MAP
   }

   public String getById(Integer id) {
      return backedMap.get(id);
   }

   private static Integer valueToKeyMapper(String value) {
      // just as exampl. In reality value will be POJO with method like getId()
      return value.hashCode();
   }
}

The code should work like this (simple example):

Foo foo = new Foo();
foo.getList().add("something");
String value = foo.getById("something".hashCode());
// I should see something

Notes:

• List should be mutable and changes must be propagated in underlying Map
• I can't modify public methods of object Foo , because it's used in other projects
• I don't care about order in List
• The getById method should continue to work fast

Answer 1

try something like this :---

---

List<String> list = new ArrayList<>();  

Set keys = backedMap.keySet();
for (Iterator i = keys.iterator(); i.hasNext(); ) {
           Integer key = (Integer) i.next();
           String value = (String) backedMap.get(key);
           list.add(value);
       }


 //To print Values in ArrayList :--    
    for(String s : list) {
              System.out.println("printing element -- " + s);
          }

Answer 2

An attempt using this List on github: ListBackedByMap.java

It usesForwardingList to maintain internal copy of the list in the map.

There are two issues with this impl I found:

1. It maintains additional copy of elements in memory
2. It doesn't throws ConcurrentModificationException on attempt to modify list in iterator.

Tests avaliable here .