Java – Looking a data structure that is a Map but in which keys can be values, values can be keys

Speaking as someone who spent 18 months working at a mapping company, which included working on the routing algorithm... yes, Dijkstra's does work, with a couple of modifications:

• Instead of doing Dijkstra's once from source to dest, you start at each end, and expand both sides until they meet in the middle. This eliminates roughly half the work (2*pi*(r/2)^2 vs pi*r^2).
• To avoid exploring the back-alleys of every city between your source and destination, you can have several layers of map data: A 'highways' layer that contains only highways, a 'secondary' layer that contains only secondary streets, and so forth. Then, you explore only smaller sections of the more detailed layers, expanding as necessary. Obviously this description leaves out a lot of detail, but you get the idea.

With modifications along those lines, you can do even cross-country routing in a very reasonable timeframe.

AbstractMap.SimpleEntry

Easy you are looking for this:

java.util.List<java.util.Map.Entry<String,Integer>> pairList= new java.util.ArrayList<>();

How can you fill it?

java.util.Map.Entry<String,Integer> pair1=new java.util.AbstractMap.SimpleEntry<>("Not Unique key1",1);
java.util.Map.Entry<String,Integer> pair2=new java.util.AbstractMap.SimpleEntry<>("Not Unique key2",2);
pairList.add(pair1);
pairList.add(pair2);

This simplifies to:

Entry<String,Integer> pair1=new SimpleEntry<>("Not Unique key1",1);
Entry<String,Integer> pair2=new SimpleEntry<>("Not Unique key2",2);
pairList.add(pair1);
pairList.add(pair2);

And, with the help of a createEntry method, can further reduce the verbosity to:

pairList.add(createEntry("Not Unique key1", 1));
pairList.add(createEntry("Not Unique key2", 2));

Since ArrayList isn't final, it can be subclassed to expose an of method (and the aforementioned createEntry method), resulting in the syntactically terse:

TupleList<java.util.Map.Entry<String,Integer>> pair = new TupleList<>();
pair.of("Not Unique key1", 1);
pair.of("Not Unique key2", 2);