Asked  7 Months ago    Answers:  5   Viewed   27 times

Is it possible for us to implement a HashMap with one key and two values. Just as HashMap?

Please do help me, also by telling (if there is no way) any other way to implement the storage of three values with one as the key?

 Answers

63

You could:

  1. Use a map that has a list as the value. Map<KeyType, List<ValueType>>.
  2. Create a new wrapper class and place instances of this wrapper in the map. Map<KeyType, WrapperType>.
  3. Use a tuple like class (saves creating lots of wrappers). Map<KeyType, Tuple<Value1Type, Value2Type>>.
  4. Use mulitple maps side-by-side.

Examples

1. Map with list as the value

// create our map
Map<String, List<Person>> peopleByForename = new HashMap<>();    

// populate it
List<Person> people = new ArrayList<>();
people.add(new Person("Bob Smith"));
people.add(new Person("Bob Jones"));
peopleByForename.put("Bob", people);

// read from it
List<Person> bobs = peopleByForename["Bob"];
Person bob1 = bobs[0];
Person bob2 = bobs[1];

The disadvantage with this approach is that the list is not bound to exactly two values.

2. Using wrapper class

// define our wrapper
class Wrapper {
    public Wrapper(Person person1, Person person2) {
       this.person1 = person1;
       this.person2 = person2;
    }

    public Person getPerson1 { return this.person1; }
    public Person getPerson2 { return this.person2; }

    private Person person1;
    private Person person2;
}

// create our map
Map<String, Wrapper> peopleByForename = new HashMap<>();

// populate it
Wrapper people = new Wrapper();
peopleByForename.put("Bob", new Wrapper(new Person("Bob Smith"),
                                        new Person("Bob Jones"));

// read from it
Wrapper bobs = peopleByForename.get("Bob");
Person bob1 = bobs.getPerson1;
Person bob2 = bobs.getPerson2;

The disadvantage to this approach is that you have to write a lot of boiler-plate code for all of these very simple container classes.

3. Using a tuple

// you'll have to write or download a Tuple class in Java, (.NET ships with one)

// create our map
Map<String, Tuple2<Person, Person> peopleByForename = new HashMap<>();

// populate it
peopleByForename.put("Bob", new Tuple2(new Person("Bob Smith",
                                       new Person("Bob Jones"));

// read from it
Tuple<Person, Person> bobs = peopleByForename["Bob"];
Person bob1 = bobs.Item1;
Person bob2 = bobs.Item2;

This is the best solution in my opinion.

4. Multiple maps

// create our maps
Map<String, Person> firstPersonByForename = new HashMap<>();
Map<String, Person> secondPersonByForename = new HashMap<>();

// populate them
firstPersonByForename.put("Bob", new Person("Bob Smith"));
secondPersonByForename.put("Bob", new Person("Bob Jones"));

// read from them
Person bob1 = firstPersonByForename["Bob"];
Person bob2 = secondPersonByForename["Bob"];

The disadvantage of this solution is that it's not obvious that the two maps are related, a programmatic error could see the two maps get out of sync.

Tuesday, June 1, 2021
 
OMGKurtNilsen
answered 7 Months ago
17

There is an issue already filed about this at the OpenJFX docs.

While it hasn't been resolved yet, there is a possible workaround, based on:

NetBeans only adds javadoc/source jars for a jar with the exact same name and -javadoc/-source suffix

So here are the steps to solve it:

  • Install NetBeans 10 and JDK 11.0.2.

  • Clone the HelloFX sample for NetBeans and Maven, from the OpenJFX samples.

  • Update the JavaFX dependencies to 11.0.2.

    <dependency>
        <groupId>org.openjfx</groupId>
        <artifactId>javafx-controls</artifactId>
        <version>11.0.2</version>
    </dependency>
    <dependency>
        <groupId>org.openjfx</groupId>
        <artifactId>javafx-fxml</artifactId>
        <version>11.0.2</version>
    </dependency>
    
  • Run it:

    mvn clean compile exec:java
    
  • Check that the JavaFX dependencies have been downloaded to your local m2 repository. Under <user home>/.m2/repository/org/openjfx/javafx-base/11.0.2 for instance you will find javafx-base-11.0.2.jar and javafx-base-mac-11.0.2.jar (or win, or linux based on your platform).

  • Back on NetBeans, right click in the Dependencies folder and select Download Sources (see the task progress in the bottom right taskbar), and then Download Javadoc(see the task progress).

  • Go to your m2 repository and verify that there are now -source and -javadoc jar files.

However, this won't solve the issue yet, there is an extra step:

  • In your m2 repository, manually rename the -source and -javadoc jar files using your platform classifier, to -mac-source and -mac-javadoc (or win, or linux based on your platform). Do this for the different JavaFX modules:

Back to NetBeans, check that now you have JavaDoc, or if you press Ctrl/CMD+Click you can access the source.

Note that this fix has to be done only once, the rest of your Maven projects should pick JavaDoc and Sources.

Thursday, August 5, 2021
 
spender
answered 4 Months ago
68

if you are going to step down, then change your project's source to 1.7 as well,

right click on your Project -> Properties -> Sources window 

and set 1.7 here

note: however I would suggest you to figure out why it doesn't work on 1.8

Saturday, September 4, 2021
 
Kenny
answered 3 Months ago
33

You can use the Guava MapMaker for this:

ConcurrentMap<Long, CustomObject> graphs = new MapMaker()
   .weakValues()
   .makeMap();

You can even include the computation part by replacing makeMap() with this:

   .makeComputingMap(
       new Function<Long, CustomObject>() {
         public CustomObject apply(Long id) {
           return getObjectFromPersistence(id);
         }
       });

Since what you are writing looks a lot like a cache, the newer, more specialized Cache (built via a CacheBuilder) might be even more relevant to you. It doesn't implement the Map interface directly, but provides even more controls that you might want for a cache.

You can refer to this for a detailed how to work for CacheBuilder and here is an example for fast access:

LoadingCache<Integer, String> cache = CacheBuilder.newBuilder()
   .maximumSize(100)
   .expireAfterWrite(10, TimeUnit.MINUTES)
   .build(
       new CacheLoader<Integer, String>() {
           @Override
           public String load(Integer id) throws Exception {
               return "value";
           }
       }
   ); 
Wednesday, October 6, 2021
 
Zizoo
answered 2 Months ago
70

If you want to check for the existance of your entry, you can use a Set (a useful concrete implementation is HashSet.

final Set<List<Integer>> population;

population = new HashSet<List<Integer>>();
population.add(Arrays.asList(1, 2));

if (population.contains(Arrays.asList(1, 2)))
{
  ...
}

You can use an List as I have done above - but that doesn't guarantee that all your lists are exactly two elements long (if that is indeed a constraint). To make it a bit more robust, you could create your own class to represent the tuple. If you do, make sure you implement equals() and hashCode() (here's an article explaining good practice).

Arrays.asList() is a useful way of creating a list in-line in the code. A more general list is an ArrayList.

Monday, November 29, 2021
 
M.P. Korstanje
answered 1 Day ago
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