这段代码做了什么？

Question

我一直在阅读＆＃34;思考Java＆＃34; Bruce Eckel和我无法理解这段代码在做什么：

  static Map<String,String> select(final int size) {
    return new FlyweightMap() {
      public Set<Map.Entry<String,String>> entrySet() {
        return new EntrySet(size);
      }
    };
  }

程序的完整列表：

package net.mindview.util;
import java.util.*;
import static net.mindview.util.Print.*;

public class Countries {
  public static final String[][] DATA = {
    // this is a big array "Country-Capital"
    {"ALGERIA","Algiers"}, {"ANGOLA","Luanda"},
    {"URUGUAY","Montevideo"}, {"VENEZUELA","Caracas"},
    // ...
  };
  // Use AbstractMap by implementing entrySet()
  private static class FlyweightMap
  extends AbstractMap<String,String> {
    private static class Entry
    implements Map.Entry<String,String> {
      int index;
      Entry(int index) { this.index = index; }
      public boolean equals(Object o) {
        return DATA[index][0].equals(o);
      }
      public String getKey() { return DATA[index][0]; }
      public String getValue() { return DATA[index][1]; }
      public String setValue(String value) {
        throw new UnsupportedOperationException();
      }
      public int hashCode() {
        return DATA[index][0].hashCode();
      }
    }
    // Use AbstractSet by implementing size() & iterator()
    static class EntrySet
    extends AbstractSet<Map.Entry<String,String>> {
      private int size;
      EntrySet(int size) {
        if(size < 0)
          this.size = 0;
        // Can't be any bigger than the array:
        else if(size > DATA.length)
          this.size = DATA.length;
        else
          this.size = size;
      }
      public int size() { return size; }
      private class Iter
      implements Iterator<Map.Entry<String,String>> {
        // Only one Entry object per Iterator:
        private Entry entry = new Entry(-1);
        public boolean hasNext() {
          return entry.index < size - 1;
        }
        public Map.Entry<String,String> next() {
          entry.index++;
          return entry;
        }
        public void remove() {
          throw new UnsupportedOperationException();
        }
      }
      public
      Iterator<Map.Entry<String,String>> iterator() {
        return new Iter();
      }
    }
    private static Set<Map.Entry<String,String>> entries =
      new EntrySet(DATA.length);
    public Set<Map.Entry<String,String>> entrySet() {
      return entries;
    }
  }
  // Create a partial map of 'size' countries:
  static Map<String,String> select(final int size) {
    return new FlyweightMap() {
      public Set<Map.Entry<String,String>> entrySet() {
        return new EntrySet(size);
      }
    };
  }
  static Map<String,String> map = new FlyweightMap();
  public static Map<String,String> capitals() {
    return map; // The entire map
  }
  public static Map<String,String> capitals(int size) {
    return select(size); // A partial map
  }
  static List<String> names =
    new ArrayList<String>(map.keySet());
  // All the names:
  public static List<String> names() { return names; }
  // A partial list:
  public static List<String> names(int size) {
    return new ArrayList<String>(select(size).keySet());
  }
  public static void main(String[] args) {
    print(capitals(10));
    print(names(10));
    print(new HashMap<String,String>(capitals(3)));
    print(new LinkedHashMap<String,String>(capitals(3)));
    print(new TreeMap<String,String>(capitals(3)));
    print(new Hashtable<String,String>(capitals(3)));
    print(new HashSet<String>(names(6)));
    print(new LinkedHashSet<String>(names(6)));
    print(new TreeSet<String>(names(6)));
    print(new ArrayList<String>(names(6)));
    print(new LinkedList<String>(names(6)));
    print(capitals().get("BRAZIL"));
  }
} /* Output:
{ALGERIA=Algiers, ANGOLA=Luanda, BENIN=Porto-Novo, BOTSWANA=Gaberone, BULGARIA=Sofia, BURKINA FASO=Ouagadougou, BURUNDI=Bujumbura, CAMEROON=Yaounde, CAPE VERDE=Praia, CENTRAL AFRICAN REPUBLIC=Bangui}
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BULGARIA, BURKINA FASO, BURUNDI, CAMEROON, CAPE VERDE, CENTRAL AFRICAN REPUBLIC]
{BENIN=Porto-Novo, ANGOLA=Luanda, ALGERIA=Algiers}
{ALGERIA=Algiers, ANGOLA=Luanda, BENIN=Porto-Novo}
{ALGERIA=Algiers, ANGOLA=Luanda, BENIN=Porto-Novo}
{ALGERIA=Algiers, ANGOLA=Luanda, BENIN=Porto-Novo}
[BULGARIA, BURKINA FASO, BOTSWANA, BENIN, ANGOLA, ALGERIA]
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BULGARIA, BURKINA FASO]
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BULGARIA, BURKINA FASO]
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BULGARIA, BURKINA FASO]
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BULGARIA, BURKINA FASO]
Brasilia
*///:~

Answer 1

它返回FlyweightMap {{1}}覆盖anonymous instance