如何使用一对 object ZA2F2ED4F8EBC2CBB4C21A29DC40AB6 将 HashMap 中的值复制到 ArrayList

Question

So I am trying to figure out how to move my Pair<Key, Value> from the HashMap that they are originally in, into an ArrayList. Nothing I am doing seems to be working and I am at a loss for how to copy the Keys & Values over.

The Pair class:

public class Pair<K,V> {
    private K key;
    private V value;
    public Pair(K key, V value)
    { this.key = key; this.value = value; }
    public K getKey() { return key; }
    public V getValue() { return value; }
}

And here is the ArrayList that I am attempting to put them into: ArrayList<Pair<String,Integer>> wcList = new ArrayList<>();

For the above ArrayList I am given a HashMap<String, Integer> as a parameter. The return must be an ArrayList<Pair<String, Integer>>

** Using an entrySet does not work for my test cases **

Answer 1

This is a one-liner. Stream out the map's entry set, and convert each entry to a Pair . Then collect them into a list.

yourMap.entrySet().stream().map(e->new Pair(e.getKey(), e.getValue())).collect(Collectors.toList());

Answer 2

tl;dr

The existing Map.Entry interface does the job of your Pair class, including getKey & getValue methods. In your case, use Map.Entry < String, Integer > .

Example: From a Map of student name (key) to their grade (value), produce a List of the key-value pairings, sorted first by their grade (highest first), then by their name alphabetically.

var entriesSorted =
        new TreeSet <>(
                Comparator
                        .comparing( Map.Entry < String, Integer > :: getValue ).reversed()  // Reversed for highest grades first.
                        .thenComparing( Map.Entry < String, Integer > :: getKey )  // Names sort alphabetically.
        );
entriesSorted.addAll(
        Map
                .of(
                        "Alice" , 4 ,  // Student, Grade
                        "Bobby" , 2 ,
                        "Carol" , 3 ,
                        "Davis" , 4 ,
                        "Edith" , 1 ,
                        "Frank" , 3
                )
                .entrySet()
);
var list = List.copyOf( entriesSorted );

list.toString() = [Alice=4, Davis=4, Carol=3, Frank=3, Bobby=2, Edith=1]

Or, if you insist upon your Pair class, use Stream#map to transfer the key & value from each Map.Entry to a new Pair object.

List < Pair > list =
        Map
                .of(
                        "Alice" , 4 ,  // Student, Grade
                        "Bobby" , 2 ,
                        "Carol" , 3 ,
                        "Davis" , 4 ,
                        "Edith" , 1 ,
                        "Frank" , 3
                )
                .entrySet()  // Returns a `Set` of `Map.Entry` objects.
                .stream()
                .map( entry -> new Pair( entry.getKey() , entry.getValue() ) )
                .toList();

list = [Pair{key=Edith, value=1}, Pair{key=Alice, value=4}, Pair{key=Carol, value=3}, Pair{key=Frank, value=3}, Pair{key=Bobby, value=2}, Pair{key=Davis, value=4}]

Map.Entry instead of your Pair

A Map already provides what you want.

You imply that you already have a Map < String, Integer > , using HashMap as your implementation.

You want a List of the key-value pairs contained in that Map. To represent those key-value pairs, you created a Pair class. But there is no need for your Pair class . The Map.Entry interface already provides the key-value pairing object you desire. To quote the Javadoc:

Map.Entry<K,V> … A map entry (key-value pair).

A Map.Entry object offers getKey and getValue methods, to access their contents.

You can easily produce a Set of those Entry objects, with a built-in method on every Map .

Map< String , Integer > myMap = new HashMap<>() ;
…
Set< String , Integer > pairs = myMap.entrySet() ;

A Set is appropriate here, because your HashMap by definition has no promised order. So a List , which keeps elements in a sequence, makes no sense.

Example code

Here is some example code, using student name mapped to their class grade.

Instead of a HashMap , I use an unspecified Map implementation produced by the convenient Map.of method.

Map < String, Integer > studentGrades =
        Map.of(
                "Alice" , 4 ,
                "Bob" , 2 ,
                "Carol" , 3 ,
                "Davis" , 4 ,
                "Edith" , 1 ,
                "Frank" , 3
        );
Set < Map.Entry < String, Integer > > entries = studentGrades.entrySet();
entries.forEach( System.out :: println );  // Call `toString` on each `Entry` object.

When run:

Davis=4
Edith=1
Alice=4
Bob=2
Carol=3
Frank=3

You needn't work out that exact type of Set < Map.Entry < String, Integer > > entries ; just use var to let the compiler figure it out.

var entries = studentGrades.entrySet();

Ordered set

var entries = studentGrades.entrySet();

If you want to keep those entries in a specific order, use a NavigableSet (or SortedSet ) such as TreeSet . You will need to provide the rule by which to order them.

Let's say we want them sorted by the student's grades, highest grades first.

NavigableSet < Map.Entry < String, Integer > > entriesByGrade =
        new TreeSet <>(
                Comparator
                        .comparing( Map.Entry < String, Integer > :: getValue ).reversed()
                        .thenComparing( Map.Entry < String, Integer > :: getKey )
        );
entriesByGrade.addAll( entries );

entriesByGrade.toString() = [Alice=4, Davis=4, Carol=3, Frank=3, Bob=2, Edith=1]

If you really need a List , you can make one from the NavigableSet .

List < Map.Entry < String, Integer > > listOfStudentsByGrade = List.copyOf( entriesByGrade );

Or more simply, use var .

var listOfStudentsByGrade = List.copyOf( entriesByGrade );

Either way, when run:

listOfStudentsByGrade.toString() = [Alice=4, Davis=4, Carol=3, Frank=3, Bob=2, Edith=1]

Using your Pair class

As discussed in this Answer, and in the Comments, your Pair class is utterly superfluous, with Map.Entry already doing its job.

But if you insist, you could easily pull the key & value out of each entry, then put that same key and value into an object of your Pair class. Silly, but easy.

List < Pair > list =
        Map
                .of(
                        "Alice" , 4 ,  // Student, Grade
                        "Bobby" , 2 ,
                        "Carol" , 3 ,
                        "Davis" , 4 ,
                        "Edith" , 1 ,
                        "Frank" , 3
                )
                .entrySet()
                .stream()
                .map( entry -> new Pair( entry.getKey() , entry.getValue() ) )
                .toList();

Answer 3

Here's a bit of code that populates a Map, and then prints out the map contents:

Map<String, Integer> map = new HashMap<>();
map.put("aardvark", 1);
map.put("bear", 2);
map.put("cat", 3);
System.out.println("map: " + map);

map: {cat=3, aardvark=1, bear=2}

Using that map, here's a bit of code that:

• gets each key from the map
• for each key, gets the associated value
• for each key-value pair, it creates a new Pair object (using the Pair code you posted)
• each Pair is then added to a List
• at the end, it prints out the contents of the list
• also, I added a toString() method to the Pair class (posted below) so that the list printing is more informative

List<Pair<String, Integer>> list = new ArrayList<>();
for (String key : map.keySet()) {
    Integer value = map.get(key);
    Pair<String, Integer> pair = new Pair<>(key, value);
    list.add(pair);
}
System.out.println("list: " + list);

list: [Pair{key=cat, value=3}, Pair{key=aardvark, value=1}, Pair{key=bear, value=2}]

Here's the addition to Pair so that the list contents can be printed nicely:

@Override
public String toString() {
    return "Pair{" +
            "key=" + key +
            ", value=" + value +
            '}';
}

You could also solve it with streams:

List<Pair<String, Integer>> list = map
        .keySet()
        .stream()
        .map(key -> new Pair<>(key, map.get(key)))
        .toList();

Answer 4

The below code seperates the keys and values from HashMap and creates 2 ArrayLists to hold them.

Please refer the extract method.

For the above ArrayList I am given a HashMap<String, Integer> as a parameter. The return must be an ArrayList<Pair<String, Integer>>

public static void main(String[] args) {
    HashMap<String, Integer> map = new HashMap<String, Integer>();
    map.put("one", 1);
    map.put("two", 2);
    map.put("three", 3);
    extract(map);
}

public static ArrayList<Pair<String, Integer>> extract(HashMap map)
{
    ArrayList<Pair<String, Integer>> list = new ArrayList<Pair<String,Integer>>();
    ArrayList<String> mapKeys = new ArrayList<String>(map.keySet());
    ArrayList<Integer> mapValues = new ArrayList<Integer>(map.values());
    Pair p;
    for(int i=0; i<map.size(); i++) {
        p = new Pair(mapKeys.get(i), mapValues.get(i));
        list.add(p);
    }
    
    list.stream().forEach(e ->System.out.println(e.toString()));
    return list;
}