OnJava8-Examples/collectionsindepth/SimpleHashMap.java

// collectionsindepth/SimpleHashMap.java
// (c)2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://mindviewinc.com/Books/OnJava/ for more book information.
// A demonstration hashed Map
import java.util.*;
import onjava.*;

public
class SimpleHashMap<K, V> extends AbstractMap<K, V> {
  // Choose a prime number for the hash table
  // size, to achieve a uniform distribution:
  static final int SIZE = 997;
  // You can't have a physical array of generics,
  // but you can upcast to one:
  @SuppressWarnings("unchecked")
  LinkedList<MapEntry<K, V>>[] buckets =
    new LinkedList[SIZE];
  @Override
  public V put(K key, V value) {
    V oldValue = null;
    int index = Math.abs(key.hashCode()) % SIZE;
    if(buckets[index] == null)
      buckets[index] = new LinkedList<>();
    LinkedList<MapEntry<K, V>> bucket = buckets[index];
    MapEntry<K, V> pair = new MapEntry<>(key, value);
    boolean found = false;
    ListIterator<MapEntry<K, V>> it =
      bucket.listIterator();
    while(it.hasNext()) {
      MapEntry<K, V> iPair = it.next();
      if(iPair.getKey().equals(key)) {
        oldValue = iPair.getValue();
        it.set(pair); // Replace old with new
        found = true;
        break;
      }
    }
    if(!found)
      buckets[index].add(pair);
    return oldValue;
  }
  @Override
  public V get(Object key) {
    int index = Math.abs(key.hashCode()) % SIZE;
    if(buckets[index] == null) return null;
    for(MapEntry<K, V> iPair : buckets[index])
      if(iPair.getKey().equals(key))
        return iPair.getValue();
    return null;
  }
  @Override
  public Set<Map.Entry<K, V>> entrySet() {
    Set<Map.Entry<K, V>> set= new HashSet<>();
    for(LinkedList<MapEntry<K, V>> bucket : buckets) {
      if(bucket == null) continue;
      for(MapEntry<K, V> mpair : bucket)
        set.add(mpair);
    }
    return set;
  }
  public static void main(String[] args) {
    SimpleHashMap<String,String> m =
      new SimpleHashMap<>();
    m.putAll(Countries.capitals(25));
    System.out.println(m);
    System.out.println(m.get("ERITREA"));
    System.out.println(m.entrySet());
  }
}
/* Output:
{CAPE VERDE=Praia, ANGOLA=Luanda, ETHIOPIA=Addis Ababa,
BENIN=Porto-Novo, CONGO=Brazzaville, LESOTHO=Maseru,
CENTRAL AFRICAN REPUBLIC=Bangui, EQUATORIAL GUINEA=Malabo,
ERITREA=Asmara, COMOROS=Moroni, BURKINA FASO=Ouagadougou,
GABON=Libreville, THE GAMBIA=Banjul, GUINEA=Conakry,
EGYPT=Cairo, BURUNDI=Bujumbura, ALGERIA=Algiers,
CAMEROON=Yaounde, GHANA=Accra, KENYA=Nairobi, COTE D'IVOIR
(IVORY COAST)=Yamoussoukro, BISSAU=Bissau,
DJIBOUTI=Dijibouti, CHAD=N'djamena, BOTSWANA=Gaberone}
Asmara
[CAPE VERDE=Praia, ANGOLA=Luanda, ETHIOPIA=Addis Ababa,
BENIN=Porto-Novo, CONGO=Brazzaville, LESOTHO=Maseru,
CENTRAL AFRICAN REPUBLIC=Bangui, EQUATORIAL GUINEA=Malabo,
ERITREA=Asmara, COMOROS=Moroni, BURKINA FASO=Ouagadougou,
GABON=Libreville, THE GAMBIA=Banjul, GUINEA=Conakry,
EGYPT=Cairo, BURUNDI=Bujumbura, ALGERIA=Algiers,
CAMEROON=Yaounde, GHANA=Accra, KENYA=Nairobi, COTE D'IVOIR
(IVORY COAST)=Yamoussoukro, BISSAU=Bissau,
DJIBOUTI=Dijibouti, CHAD=N'djamena, BOTSWANA=Gaberone]
*/