Java集合(3)：使用Abstract类

每个java.util容器都有其自己的Abstract类，它们提供了该容器接口的部分实现。下面是一个定制自己的Map的例子（List set就省略了）：

定制自己的Map实现AbstractMap-->Map，需要实现[Set<Map.Entry<K,V>> entrySet()]方法

实现[Set<Map.Entry<K,V>> entrySet()]方法分两步：

(1) 实现Set<E>接口

(2) 实现Map.Entry<K,V>接口

 import java.util.AbstractMap;
 import java.util.AbstractSet;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Hashtable;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeMap;
 import java.util.TreeSet;

 class Countries {
     public static final String[][] DATA = {
             // Africa
             { "SOUTH AFRICA", "Cape Town" }, { "SUDAN", "Khartoum" },
             // Asia
             { "CHINA", "Beijing" }, { "JAPAN", "Tokyo" }, { "SOUTH KOREA", "Seoul" },
             // Australia and Oceania
             { "AUSTRALIA", "Canberra" }, { "NEW ZEALAND", "Wellington" },
             // Europe
             { "UNITED KINGDOM", "London" }, { "FRANCE", "Paris" }, { "GERMANY", "Berlin" }, { "ITALY", "Rome" },
             { "SPAIN", "Madrid" },
             // North and Central America
             { "UNITED STATES OF AMERICA", "Washington, D.C." }, { "CANADA", "Ottawa" },
             // South America
             { "BRAZIL", "Brasilia" }, { "ARGENTINA", "Buenos Aires" } };

     private static class FlyweightMap extends AbstractMap<String, String> {

         private final int dataLength;

         private static Set<Map.Entry<String, String>> entries = new EntrySet(DATA.length);

         public FlyweightMap() {
             dataLength = 0;
         }

         public FlyweightMap(int dataLength) {
             this.dataLength = dataLength;
         }

         @Override
         public Set<Map.Entry<String, String>> entrySet() {
             if (dataLength > 0) {
                 return new EntrySet(dataLength);
             }
             return entries;
         }

         // (1) 实现Set<E>接口
         // 定制自己的Set实现AbstractSet(AbstractCollection)-->Set， 需要实现[Iterator<E> iterator() & int size()]方法
         private static class EntrySet extends AbstractSet<Map.Entry<String, String>> {
             private int size;

             EntrySet(int size) {
                 this.size = size < 0 ? (this.size = 0)
                         : (size > DATA.length ? (this.size = DATA.length) : (this.size = size));
             }

             @Override
             public int size() {
                 return size;
             }

             @Override
             public Iterator<Map.Entry<String, String>> iterator() {
                 return new Iterator<Map.Entry<String, String>>() {
                     // Only one Entry object per Iterator:
                     private Entry entry = new Entry(-1);

                     @Override
                     public boolean hasNext() {
                         return entry.index < size - 1;
                     }

                     @Override
                     public java.util.Map.Entry<String, String> next() {
                         entry.index++;
                         return entry;
                     }
                 };
             }
         }

         // (2) 实现Map.Entry<K,V>接口
         // 定制自己的Map.Entry实现Map.Entry<K, V>接口， 需要实现下面的方法
         // 每个Map.Entry对象都只存了它们的索引，而不是实际的键值。当调用getKey()， getValue()时，才会用索引返回恰当的元素
         private static class Entry implements Map.Entry<String, String> {
             int index;

             Entry(int index) {
                 this.index = index;
             }

             @Override
             public boolean equals(Object o) {
                 return DATA[index][0].equals(o);
             }

             @Override
             public String getKey() {
                 return DATA[index][0];
             }

             @Override
             public String getValue() {
                 return DATA[index][1];
             }

             @Override
             public String setValue(String value) {
                 throw new UnsupportedOperationException();
             }

             @Override
             public int hashCode() {
                 return DATA[index][0].hashCode();
             }
         }
     }

     // 取Map全部内容
     public static Map<String, String> capitals() {
         return selectAll();
     }

     // 取Map全部内容的key
     public static List<String> names() {
         return new ArrayList<String>(capitals().keySet());
     }

     // 取Map部分内容
     public static Map<String, String> capitals(final int size) {
         return select(size);
     }

     // 取Map部分内容的key
     public static List<String> names(int size) {
         return new ArrayList<String>(select(size).keySet());
     }

     private static Map<String, String> selectAll() {
         return new FlyweightMap();
     }

     private static Map<String, String> select(final int size) {
         return new FlyweightMap(size);
     }
 }

 public class Test4 {
     public static void main(String[] args) {
         System.out.println(Countries.capitals(5)); // {SOUTH AFRICA=Cape Town, SUDAN=Khartoum, CHINA=Beijing, JAPAN=Tokyo, SOUTH KOREA=Seoul}
         System.out.println(Countries.names(5)); // [SOUTH AFRICA, SUDAN, CHINA, JAPAN, SOUTH KOREA]
         System.out.println(new HashMap<String, String>(Countries.capitals(3))); // {SUDAN=Khartoum, CHINA=Beijing, SOUTH AFRICA=Cape Town}
         System.out.println(new LinkedHashMap<String, String>(Countries.capitals(3))); // {SOUTH AFRICA=Cape Town, SUDAN=Khartoum, CHINA=Beijing}
         System.out.println(new TreeMap<String, String>(Countries.capitals(3))); // {CHINA=Beijing, SOUTH AFRICA=Pretoria/Cape Town, SUDAN=Khartoum}
         System.out.println(new Hashtable<String, String>(Countries.capitals(3))); // {SUDAN=Khartoum, SOUTH AFRICA=Pretoria/Cape Town, CHINA=Beijing}
         System.out.println(new HashSet<String>(Countries.names(4))); // [SUDAN, CHINA, SOUTH AFRICA, JAPAN]
         System.out.println(new LinkedHashSet<String>(Countries.names(4))); // [SOUTH AFRICA, SUDAN, CHINA, JAPAN]
         System.out.println(new TreeSet<String>(Countries.names(4))); // [CHINA, JAPAN, SOUTH AFRICA, SUDAN]
         System.out.println(new ArrayList<String>(Countries.names(4))); // [SOUTH AFRICA, SUDAN, CHINA, JAPAN]
         System.out.println(new LinkedList<String>(Countries.names(4))); // [SOUTH AFRICA, SUDAN, CHINA, JAPAN]
         System.out.println(Countries.capitals().get("BRAZIL")); // Brasilia
     }
 }