Maintain java mapping from two different types of values in a single data structure

Question

I have a collection of objects that look something like

class Widget {
  String name;
  int id;
  // Intuitive constructor omitted
}

Sometimes I want to look up an item by name, and sometime I want to look it up by id. I can obviously do this by

Map<String, Widget> mapByName;
Map<Integer, Widget> mapById;

However, that requires maintaining two maps, and at some point, I will (or another user who is unfamiliar with the double map) will make a change to the code and only update one of the maps.

The obvious solution is to make a class to manage the two maps. Does such a class already exist, probably in a third party package?

I am looking for something that lets me do something along the lines of

DoubleMap<String, Integer, Widget> map = new DoubleMap<>();
Widget w = new Widget(3, "foo");
map.put(w.id, w.name, w);
map.get1(3); // returns w
map.get2("foo"); // returns w

Thomas Bosch · Accepted Answer · 2019-01-25 22:27:34Z

1

A simple solution could be, to write your own key class that includes both keys.

class WidgetKey {
  String id;
  String name;
  boolean equals() {...}
  boolean hashCode() {...}
}

Map<WidgetKey, Widget> yourMap;

Beware that you have to implement equals and hashCode in the WidgetKey class. Otherwise put/get and other map methods wouldn't work properly.

answered Jan 25, 2019 at 22:27

Thomas Bosch

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5 Comments

xehpuk Over a year ago

How would you implement them?

David Soroko Over a year ago

It sounds that the id and the name are unique. If this is the case just use Map<Object,Widget> and put each widget twice into that same map. You can have a class wrapping all this which exposes a single add(Widget) and two gets

Thomas Bosch Over a year ago

For "equals" and "hashCode" there are several ways to implement. 1. Use your IDE to generate them 2. Use 3rd party libs like commons.apache.org/proper/commons-lang with EqualsBuilder and HashCodeBuilder 3. Do it by hand

Troy Daniels Over a year ago

I fail to see how this would work. I suppose you could implement hashCode as return 0 and equals as return this.id.equals(other) || this.name.equals(other), but that is unorthodox. When I am trying to get something out, I only have one of the two key values.

Thomas Bosch Over a year ago

If you use HashMap as implementation of Map, implementing hashCode() return 0 is no good idea, as the hash value is used as primary key inside HashMap. Have you take a look at commons-lang library? Or if you don't want a new dependency have you tried to let it be generated by your IDE?

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