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
    }
}