92 lines
3.1 KiB
Java
92 lines
3.1 KiB
Java
// understandingcollections/SimpleHashMap.java
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// (c)2016 MindView LLC: see Copyright.txt
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// We make no guarantees that this code is fit for any purpose.
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// Visit http://OnJava8.com for more book information.
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// A demonstration hashed Map
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import java.util.*;
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import onjava.*;
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public
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class SimpleHashMap<K, V> extends AbstractMap<K, V> {
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// Choose a prime number for the hash table
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// size, to achieve a uniform distribution:
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static final int SIZE = 997;
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// You can't have a physical array of generics,
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// but you can upcast to one:
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@SuppressWarnings("unchecked")
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LinkedList<MapEntry<K, V>>[] buckets =
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new LinkedList[SIZE];
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@Override
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public V put(K key, V value) {
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V oldValue = null;
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int index = Math.abs(key.hashCode()) % SIZE;
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if(buckets[index] == null)
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buckets[index] = new LinkedList<>();
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LinkedList<MapEntry<K, V>> bucket = buckets[index];
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MapEntry<K, V> pair = new MapEntry<>(key, value);
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boolean found = false;
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ListIterator<MapEntry<K, V>> it =
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bucket.listIterator();
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while(it.hasNext()) {
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MapEntry<K, V> iPair = it.next();
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if(iPair.getKey().equals(key)) {
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oldValue = iPair.getValue();
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it.set(pair); // Replace old with new
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found = true;
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break;
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}
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}
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if(!found)
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buckets[index].add(pair);
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return oldValue;
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}
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@Override
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public V get(Object key) {
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int index = Math.abs(key.hashCode()) % SIZE;
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if(buckets[index] == null) return null;
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for(MapEntry<K, V> iPair : buckets[index])
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if(iPair.getKey().equals(key))
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return iPair.getValue();
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return null;
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}
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@Override
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public Set<Map.Entry<K, V>> entrySet() {
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Set<Map.Entry<K, V>> set= new HashSet<>();
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for(LinkedList<MapEntry<K, V>> bucket : buckets) {
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if(bucket == null) continue;
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for(MapEntry<K, V> mpair : bucket)
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set.add(mpair);
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}
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return set;
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}
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public static void main(String[] args) {
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SimpleHashMap<String,String> m =
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new SimpleHashMap<>();
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m.putAll(Countries.capitals(25));
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System.out.println(m);
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System.out.println(m.get("ERITREA"));
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System.out.println(m.entrySet());
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}
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}
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/* Output:
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{CAPE VERDE=Praia, ANGOLA=Luanda, ETHIOPIA=Addis Ababa,
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BENIN=Porto-Novo, CONGO=Brazzaville, LESOTHO=Maseru,
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CENTRAL AFRICAN REPUBLIC=Bangui, EQUATORIAL GUINEA=Malabo,
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ERITREA=Asmara, COMOROS=Moroni, BURKINA FASO=Ouagadougou,
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GABON=Libreville, THE GAMBIA=Banjul, GUINEA=Conakry,
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EGYPT=Cairo, BURUNDI=Bujumbura, ALGERIA=Algiers,
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CAMEROON=Yaounde, GHANA=Accra, KENYA=Nairobi, COTE D'IVOIR
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(IVORY COAST)=Yamoussoukro, BISSAU=Bissau,
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DJIBOUTI=Dijibouti, CHAD=N'djamena, BOTSWANA=Gaberone}
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Asmara
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[CAPE VERDE=Praia, ANGOLA=Luanda, ETHIOPIA=Addis Ababa,
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BENIN=Porto-Novo, CONGO=Brazzaville, LESOTHO=Maseru,
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CENTRAL AFRICAN REPUBLIC=Bangui, EQUATORIAL GUINEA=Malabo,
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ERITREA=Asmara, COMOROS=Moroni, BURKINA FASO=Ouagadougou,
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GABON=Libreville, THE GAMBIA=Banjul, GUINEA=Conakry,
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EGYPT=Cairo, BURUNDI=Bujumbura, ALGERIA=Algiers,
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CAMEROON=Yaounde, GHANA=Accra, KENYA=Nairobi, COTE D'IVOIR
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(IVORY COAST)=Yamoussoukro, BISSAU=Bissau,
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DJIBOUTI=Dijibouti, CHAD=N'djamena, BOTSWANA=Gaberone]
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*/
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