系统研究Java--容器（二）--使用Abstract类

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本文展示了一个使用Java创建只读Map的示例，通过继承`AbstractMap`并实现`entrySet()`方法，提供了不可修改的国家及其首都数据映射。

这个示例在于演示通过继承java.util.Abstract来创建定制的Map和Collection，实现一个只读的Map。

//: net/mindview/util/Countries.java // "Flyweight" Maps and Lists of sample data. package net.mindview.util; import java.util.*; import static net.mindview.util.Print.*; public class Countries { public static final String[][] DATA = { // Africa {"ALGERIA","Algiers"}, {"ANGOLA","Luanda"}, {"BENIN","Porto-Novo"}, {"BOTSWANA","Gaberone"}, {"BURKINA FASO","Ouagadougou"}, {"BURUNDI","Bujumbura"}, {"CAMEROON","Yaounde"}, {"CAPE VERDE","Praia"}, {"CENTRAL AFRICAN REPUBLIC","Bangui"}, {"CHAD","N'djamena"}, {"COMOROS","Moroni"}, {"CONGO","Brazzaville"}, {"DJIBOUTI","Dijibouti"}, {"EGYPT","Cairo"}, {"EQUATORIAL GUINEA","Malabo"}, {"ERITREA","Asmara"}, {"ETHIOPIA","Addis Ababa"}, {"GABON","Libreville"}, {"THE GAMBIA","Banjul"}, {"GHANA","Accra"}, {"GUINEA","Conakry"}, {"BISSAU","Bissau"}, {"COTE D'IVOIR (IVORY COAST)","Yamoussoukro"}, {"KENYA","Nairobi"}, {"LESOTHO","Maseru"}, {"LIBERIA","Monrovia"}, {"LIBYA","Tripoli"}, {"MADAGASCAR","Antananarivo"}, {"MALAWI","Lilongwe"}, {"MALI","Bamako"}, {"MAURITANIA","Nouakchott"}, {"MAURITIUS","Port Louis"}, {"MOROCCO","Rabat"}, {"MOZAMBIQUE","Maputo"}, {"NAMIBIA","Windhoek"}, {"NIGER","Niamey"}, {"NIGERIA","Abuja"}, {"RWANDA","Kigali"}, {"SAO TOME E PRINCIPE","Sao Tome"}, {"SENEGAL","Dakar"}, {"SEYCHELLES","Victoria"}, {"SIERRA LEONE","Freetown"}, {"SOMALIA","Mogadishu"}, {"SOUTH AFRICA","Pretoria/Cape Town"}, {"SUDAN","Khartoum"}, {"SWAZILAND","Mbabane"}, {"TANZANIA","Dodoma"}, {"TOGO","Lome"}, {"TUNISIA","Tunis"}, {"UGANDA","Kampala"}, {"DEMOCRATIC REPUBLIC OF THE CONGO (ZAIRE)", "Kinshasa"}, {"ZAMBIA","Lusaka"}, {"ZIMBABWE","Harare"}, // Asia {"AFGHANISTAN","Kabul"}, {"BAHRAIN","Manama"}, {"BANGLADESH","Dhaka"}, {"BHUTAN","Thimphu"}, {"BRUNEI","Bandar Seri Begawan"}, {"CAMBODIA","Phnom Penh"}, {"CHINA","Beijing"}, {"CYPRUS","Nicosia"}, {"INDIA","New Delhi"}, {"INDONESIA","Jakarta"}, {"IRAN","Tehran"}, {"IRAQ","Baghdad"}, {"ISRAEL","Jerusalem"}, {"JAPAN","Tokyo"}, {"JORDAN","Amman"}, {"KUWAIT","Kuwait City"}, {"LAOS","Vientiane"}, {"LEBANON","Beirut"}, {"MALAYSIA","Kuala Lumpur"}, {"THE MALDIVES","Male"}, {"MONGOLIA","Ulan Bator"}, {"MYANMAR (BURMA)","Rangoon"}, {"NEPAL","Katmandu"}, {"NORTH KOREA","P'yongyang"}, {"OMAN","Muscat"}, {"PAKISTAN","Islamabad"}, {"PHILIPPINES","Manila"}, {"QATAR","Doha"}, {"SAUDI ARABIA","Riyadh"}, {"SINGAPORE","Singapore"}, {"SOUTH KOREA","Seoul"}, {"SRI LANKA","Colombo"}, {"SYRIA","Damascus"}, {"TAIWAN (REPUBLIC OF CHINA)","Taipei"}, {"THAILAND","Bangkok"}, {"TURKEY","Ankara"}, {"UNITED ARAB EMIRATES","Abu Dhabi"}, {"VIETNAM","Hanoi"}, {"YEMEN","Sana'a"}, // Australia and Oceania {"AUSTRALIA","Canberra"}, {"FIJI","Suva"}, {"KIRIBATI","Bairiki"}, {"MARSHALL ISLANDS","Dalap-Uliga-Darrit"}, {"MICRONESIA","Palikir"}, {"NAURU","Yaren"}, {"NEW ZEALAND","Wellington"}, {"PALAU","Koror"}, {"PAPUA NEW GUINEA","Port Moresby"}, {"SOLOMON ISLANDS","Honaira"}, {"TONGA","Nuku'alofa"}, {"TUVALU","Fongafale"}, {"VANUATU","< Port-Vila"}, {"WESTERN SAMOA","Apia"}, // Eastern Europe and former USSR {"ARMENIA","Yerevan"}, {"AZERBAIJAN","Baku"}, {"BELARUS (BYELORUSSIA)","Minsk"}, {"BULGARIA","Sofia"}, {"GEORGIA","Tbilisi"}, {"KAZAKSTAN","Almaty"}, {"KYRGYZSTAN","Alma-Ata"}, {"MOLDOVA","Chisinau"}, {"RUSSIA","Moscow"}, {"TAJIKISTAN","Dushanbe"}, {"TURKMENISTAN","Ashkabad"}, {"UKRAINE","Kyiv"}, {"UZBEKISTAN","Tashkent"}, // Europe {"ALBANIA","Tirana"}, {"ANDORRA","Andorra la Vella"}, {"AUSTRIA","Vienna"}, {"BELGIUM","Brussels"}, {"BOSNIA","-"}, {"HERZEGOVINA","Sarajevo"}, {"CROATIA","Zagreb"}, {"CZECH REPUBLIC","Prague"}, {"DENMARK","Copenhagen"}, {"ESTONIA","Tallinn"}, {"FINLAND","Helsinki"}, {"FRANCE","Paris"}, {"GERMANY","Berlin"}, {"GREECE","Athens"}, {"HUNGARY","Budapest"}, {"ICELAND","Reykjavik"}, {"IRELAND","Dublin"}, {"ITALY","Rome"}, {"LATVIA","Riga"}, {"LIECHTENSTEIN","Vaduz"}, {"LITHUANIA","Vilnius"}, {"LUXEMBOURG","Luxembourg"}, {"MACEDONIA","Skopje"}, {"MALTA","Valletta"}, {"MONACO","Monaco"}, {"MONTENEGRO","Podgorica"}, {"THE NETHERLANDS","Amsterdam"}, {"NORWAY","Oslo"}, {"POLAND","Warsaw"}, {"PORTUGAL","Lisbon"}, {"ROMANIA","Bucharest"}, {"SAN MARINO","San Marino"}, {"SERBIA","Belgrade"}, {"SLOVAKIA","Bratislava"}, {"SLOVENIA","Ljuijana"}, {"SPAIN","Madrid"}, {"SWEDEN","Stockholm"}, {"SWITZERLAND","Berne"}, {"UNITED KINGDOM","London"}, {"VATICAN CITY","---"}, // North and Central America {"ANTIGUA AND BARBUDA","Saint John's"}, {"BAHAMAS","Nassau"}, {"BARBADOS","Bridgetown"}, {"BELIZE","Belmopan"}, {"CANADA","Ottawa"}, {"COSTA RICA","San Jose"}, {"CUBA","Havana"}, {"DOMINICA","Roseau"}, {"DOMINICAN REPUBLIC","Santo Domingo"}, {"EL SALVADOR","San Salvador"}, {"GRENADA","Saint George's"}, {"GUATEMALA","Guatemala City"}, {"HAITI","Port-au-Prince"}, {"HONDURAS","Tegucigalpa"}, {"JAMAICA","Kingston"}, {"MEXICO","Mexico City"}, {"NICARAGUA","Managua"}, {"PANAMA","Panama City"}, {"ST. KITTS","-"}, {"NEVIS","Basseterre"}, {"ST. LUCIA","Castries"}, {"ST. VINCENT AND THE GRENADINES","Kingstown"}, {"UNITED STATES OF AMERICA","Washington, D.C."}, // South America {"ARGENTINA","Buenos Aires"}, {"BOLIVIA","Sucre (legal)/La Paz(administrative)"}, {"BRAZIL","Brasilia"}, {"CHILE","Santiago"}, {"COLOMBIA","Bogota"}, {"ECUADOR","Quito"}, {"GUYANA","Georgetown"}, {"PARAGUAY","Asuncion"}, {"PERU","Lima"}, {"SURINAME","Paramaribo"}, {"TRINIDAD AND TOBAGO","Port of Spain"}, {"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 *///:~

源码源于《Thinking in Java》