In Java 8 how do I transform a Map<K,V> to another Map<K,V> using a lambda?

问题

I've just started looking at Java 8 and to try out lambdas I thought I'd try to rewrite a very simple thing I wrote recently. I need to turn a Map of String to Column into another Map of String to Column where the Column in the new Map is a defensive copy of the Column in the first Map. Column has a copy constructor. The closest I've got so far is:

    Map<String, Column> newColumnMap= new HashMap<>();
    originalColumnMap.entrySet().stream().forEach(x -> newColumnMap.put(x.getKey(), new Column(x.getValue())));

but I'm sure there must be a nicer way to do it and I'd be grateful for some advice.

回答1:

You could use a Collector:

import java.util.*;
import java.util.stream.Collectors;

public class Defensive {

  public static void main(String[] args) {
    Map<String, Column> original = new HashMap<>();
    original.put("foo", new Column());
    original.put("bar", new Column());

    Map<String, Column> copy = original.entrySet()
        .stream()
        .collect(Collectors.toMap(Map.Entry::getKey,
                                  e -> new Column(e.getValue())));

    System.out.println(original);
    System.out.println(copy);
  }

  static class Column {
    public Column() {}
    public Column(Column c) {}
  }
}

回答2:

Map<String, Integer> map = new HashMap<>();
map.put("test1", 1);
map.put("test2", 2);

Map<String, Integer> map2 = new HashMap<>();
map.forEach(map2::put);

System.out.println("map: " + map);
System.out.println("map2: " + map2);
// Output:
// map:  {test2=2, test1=1}
// map2: {test2=2, test1=1}

You can use the forEach method to do what you want.

What you're doing there is:

map.forEach(new BiConsumer<String, Integer>() {
    @Override
    public void accept(String s, Integer integer) {
        map2.put(s, integer);     
    }
});

Which we can simplify into a lambda:

map.forEach((s, integer) ->  map2.put(s, integer));

And because we're just calling an existing method we can use a method reference, which gives us:

map.forEach(map2::put);

回答3:

The way without re-inserting all entries into the new map should be the fastest it won't because HashMap.clone internally performs rehash as well.

Map<String, Column> newColumnMap = originalColumnMap.clone();
newColumnMap.replaceAll((s, c) -> new Column(c));

回答4:

Keep it Simple and use Java 8:-

 Map<String, AccountGroupMappingModel> mapAccountGroup=CustomerDAO.getAccountGroupMapping();
 Map<String, AccountGroupMappingModel> mapH2ToBydAccountGroups = 
              mapAccountGroup.entrySet().stream()
                         .collect(Collectors.toMap(e->e.getValue().getH2AccountGroup(),
                                                   e ->e.getValue())
                                  );

回答5:

Here is another way that gives you access to the key and the value at the same time, in case you have to do some kind of transformation.

Map<String, Integer> pointsByName = new HashMap<>();
Map<String, Integer> maxPointsByName = new HashMap<>();

Map<String, Double> gradesByName = pointsByName.entrySet().stream()
        .map(entry -> new AbstractMap.SimpleImmutableEntry<>(
                entry.getKey(), ((double) entry.getValue() /
                        maxPointsByName.get(entry.getKey())) * 100d))
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

回答6:

If you use Guava (v11 minimum) in your project you can use Maps::transformValues.

Map<String, Column> newColumnMap = Maps.transformValues(
  originalColumnMap,
  Column::new // equivalent to: x -> new Column(x) 
)

Note: The values of this map are evaluated lazily. If the transformation is expensive you can copy the result to a new map like suggested in the Guava docs.

To avoid lazy evaluation when the returned map doesn't need to be a view, copy the returned map into a new map of your choosing.

回答7:

If you don't mind using 3rd party libraries, my cyclops-react lib has extensions for all JDK Collection types, including Map. You can directly use the map or bimap methods to transform your Map. A MapX can be constructed from an existing Map eg.

  MapX<String, Column> y = MapX.fromMap(orgColumnMap)
                               .map(c->new Column(c.getValue());

If you also wish to change the key you can write

  MapX<String, Column> y = MapX.fromMap(orgColumnMap)
                               .bimap(this::newKey,c->new Column(c.getValue());

bimap can be used to transform the keys and values at the same time.

As MapX extends Map the generated map can also be defined as

  Map<String, Column> y

来源：https://stackoverflow.com/questions/22742974/in-java-8-how-do-i-transform-a-mapk-v-to-another-mapk-v-using-a-lambda