ListIndexedMap

A simple adapter class for making a Map view over an underlying List. The ListIndexedMap wraps the List and presents it as a Map with Integer keys and values of the type of the list. The Integer keys correspond to the indexes of the values in the list.

List<String> list = new ArrayList<>();
list.add("a");
list.add("b");
list.add("c");

ListIndexedMap listIndexedMap = ListIndexedMap.of(list);
listIndexedMap.size(); // 3
listIndexedMap.get(0); // "a"
listIndexedMap.get(1); // "b"
listIndexedMap.get(2); // "c"

The ListIndexedMap offers the ability to present a List as a Map without needing to allocate an entirely new block of storage and copy all of the value references. Changes to the ListIndexedMap write through to the underlying list and the ListIndexedMap is constrained by the invariants of the list. Passing an unmodifiable List will result in an unmodifiable ListIndexedMap. (Unfortunately, this does not apply to synchronization.)

Implementation Details and Quirks

The current implementation makes some assumptions about how to handle edge cases. For example, adding far past the end of the list will expand the list with null values in between:

List<String> list = new ArrayList<>();
list.add("a");
list.add("b");
list.add("c");

ListIndexedMap listIndexedMap = ListIndexedMap.of(list);
listIndexedMap.put(9, "d");
listIndexedMap.size(); // 10
listIndexedMap.get(6); // null
listIndexedMap.containsKey(6); // true

When removing, the ListIndexedMap will nullify the value at a given position instead of removing it from the List and shifting all values.

List<String> list = new ArrayList<>();
list.add("a");
list.add("b");
list.add("c");

ListIndexedMap listIndexedMap = ListIndexedMap.of(list);
listIndexedMap.remove(1);
listIndexedMap.size(); // 3
listIndexedMap.get(1); // null
listIndexedMap.containsKey(1); // true

The ListIndexedMap assumes that a key exists so long as it is positive and less than the size of the List. Keys that are removed and indexes with null values are still returned as valid keys, and will be iterated over by the key, value, and entry Iterators