How to get a HashMap value with three values

问题

If I have a HashMap with a such key: [pubDate, title, link] and such value(example):

[Thu, 03 Jan 2013 21:50:02 +0100, Transferts - YBX : ''Je change de dimension'', http://www.link.fr/info/link_informations.html]

Can I retrieve the link http://www.link.fr/info/link_informations.html ? Code:

    for (int i = 0; i < nl.getLength(); i++) {
                    // creating new HashMap
                    map = new HashMap<String, String>();
                    Element e = (Element) nl.item(i);
                    // adding each child node to HashMap key => value
                    map.put(KEY_TITLE, parser.getValue(e, KEY_TITLE));
                    map.put(KEY_LINK, parser.getValue(e, KEY_LINK));

                    map.put(KEY_DATE, parser.getValue(e, KEY_DATE));
                    //map.put(KEY_DESC, parser.getValue(e, KEY_DESC));

                    // adding HashList to ArrayList
                    menuItems.add(map);
 }

回答1:

I just faced the same issue and decided to map my key to an Entry. This allows for the same indexing features provided by a map while having more than one property associated with a key. I think it's a much neater solution that creating a separate class or nesting maps.

Map<String, Entry<Action, Boolean>> actionMap = new HashMap<String, Entry<Action, Boolean>>();

actionMap.put("action_name", new SimpleEntry(action, true));

To use the data later:

Entry<Action, Boolean> actionMapEntry = actionMap.get("action_name");

if(actionMapEntry.value()) actionMapEntry.key().applyAction();

My personal use of this was a map of named functions. Having selected the function by name, the function would be run and the boolean would determine whether or not cleanup was needed in processing.

回答2:

You create an object that holds all three subkeys of the key as attributes. Make sure to implement equals and hashCode properly.

public class MyKey {
  public MyKey(String subkey1, String subkey2, String subkey3) {
    ...
  }
}

Use such object as a key of your map:

Map<MyKey, String> myMap = ....;
myMap.get(new MyKey("Thu", "03 Jan 2013 21:50:02 +0100", "Transferts - YBX"));

Have fun!

回答3:

You could also try to use a nested hashmap, of the type HashMap<String, HashMap<String, HashMap<String, String>>>, where you would then formulate your query as map.get(date).get(title).get(link)

The best solution would then be to encapsulate this nested hashmap in your own class as such:

public class NestedHashmap {
    private Map<String, Map<String, Map<String, String>>> map;

    public NestedHashMap() { 
        map = new HashMap<String, HashMap<String, HashMap<String, String>>>;
    }

    public put(String date, String title, String link, String value){
        if(map.get(date) == null) { 
            map.put(date, new HashMap<String, HashMap<String, String>>;
        }
        if(map.get(date).get(title) == null){
            map.get(date).put(new HashMap<String, String>);
        }
        map.get(date).get(title).put(link, value);
    }

    public get(String date, String title, String link) {
        // ...mostly analogous to put...
    }
}

回答4:

You could create a compound key which contains all of the values you want in the key. A simple way to do this could be to concatenate the together the values that make up your key, delimited by a value you are sure won't appear in the key values.

E.g.:

String makeCompoundKey(String pubDate, String title) {
  return pubDate + "|" + title;
}

HashMap<String,String> map = new HashMap<String,String>();

map.put(makeComoundKey(pubDate,title), link)

To avoid the problem of having to choose a character which doesn't appear in any of the key values, to handle non-String keys and likely for better performance, you can declare a new class to contain your key values and override the equals and hashCode methods:

final class DateAndTitle {
  private final Date pubDate;
  private final String title;

  @Overrde
  boolean equals(Object rhs) {
    // let eclipse generate this for you, but it will probably look like ...
    return rhs != null && rhs.getClass() == getClass() &&
      pubDate.equals(rhs.pubDate) && title.equals(rhs.title);
  }

  @Overrde
  int hashCode(Object) {
    // let eclipse generate this for you...
    ...
  }
}

Then you can define your map like:

HashMap

and use DateAndTitle object to index your map.

回答5:

In my case, I found another solution: is to create an ArrayList and put on it all the links on every for loop. That's a lot easier.

来源：https://stackoverflow.com/questions/14148331/how-to-get-a-hashmap-value-with-three-values

标签

java

hashmap