容器深入研究--使用Abstract类

Leefs 2020-01-09 AM 3997℃ 2条

# 容器深入研究--使用Abstract类

### 前言

本篇将要讲述《Java》编程思想第17.2.3小节，使用Abstract类

### 概述

对于产生用于容器的测试数据问题，另一个解放方式是创建定制的Collection和Map实现。每个**java.util**容器都有自己的**Abstract**类，它们提供了该容器的部分实现，因此你必须做的只是去实现那些产生想要容器所必需的方法。

**享元模式：**可在普通解决方案需要过多对象，或者产生普通对象太占用空间时使用享元。享元模式使得对象的一部分可以被具体化，因此，与对象中的所有事物都包含在对象内部不同，可以在更加高效的外部表中查找对象的一部分或整体。

### 示例代码

下面演示通过继承java.util.Abstract来创建定制的Map和Collection。为了创建只读的Map，可以继承Abstractmap并实现entrySet。为了创建只读的Set，可以继承AbstractSet并实现iterator和size。

```java
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> {// 需要实现抽象方法 entrySet()

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(0);
            }

@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();
            }

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;
                else if (size > DATA.length)
                    this.size = DATA.length;
                else
                    this.size = size;
            }

@Override
            public Iterator<Map.Entry<String, String>> iterator() {
                return new Iter();
            }

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

private class Iter implements Iterator<Map.Entry<String,String>>{
                // Only one Entry object per Iterator:每个迭代器只有一个Entry对象
                private Entry entry = new Entry(-1);
                @Override
                public boolean hasNext() {
                    return entry.index < size -1;
                }

@Override
                public Map.Entry<String, String> next() {
                    entry.index++;
                    return entry;
                }
                public void remove(){
                    throw new UnsupportedOperationException();
                }
            }
        }
        private static Set<Map.Entry<String,String>> entries = new EntrySet(DATA.length);

@Override
        public Set<Map.Entry<String,String>> entrySet(){
            return entries;
        }

}
    //创建 部分 固定size国家的map
    static Map<String,String> select(final int size){
        return new FlyweightMap(){
            @Override
            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;// 整个map
    }

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

public static List<String> names(int size){
        return new ArrayList<String>(select(size).keySet());
    }

public static void main(String[] args) {
        System.out.println(capitals(3));
        System.out.println(names(3));
        System.out.println(new HashMap<>(capitals(3)));
        System.out.println(new LinkedHashMap<>(capitals(3)));
        System.out.println(new TreeMap<>(capitals(3)));
        System.out.println(new Hashtable<>(capitals(3)));
        System.out.println(new HashSet<String>(names(6)));
        System.out.println(new LinkedHashSet<String>(names(6)));
        System.out.println(new TreeSet<>(names(6)));
        System.out.println(new ArrayList<>(names(6)));
        System.out.println(new LinkedList<>(names(6)));
        System.out.println(capitals().get("BRAZIL"));
    }
}
```

> 运行结果

```java
{ALGERIA=Algiers, ANGOLA=Luanda, BENIN=Porto-Novo}
[ALGERIA, ANGOLA, BENIN]
{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}
[BENIN, BOTSWANA, ANGOLA, BURKINA FASO, ALGERIA, BURUNDI]
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BURKINA FASO, BURUNDI]
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BURKINA FASO, BURUNDI]
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BURKINA FASO, BURUNDI]
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BURKINA FASO, BURUNDI]
Brasilia
```

FlyweightMap必须实现entrySet方法，它需要定制set实现和定制Map.Entry类。这里正是享元部分：每个Map.Entry对象都只存储了它的索引，而不是实际的键和值。当你调用getKey和getValue时，它们会使用该索引来返回恰当的DATA元素。EntrySet可以确保它的size不会大于DATA。

可以在EntrySet.Iterator中看到享元其它部分的实现。与为DATA中的每个数据对都创建Map,Entry对象不同，每个迭代器只有一个Map.Entry。Entry对象被用作数据的视窗，它只包含在静态字符数组中的索引。每次调用迭代器的nest方法时，Entry中的index都会递增，使其指向下一个元素，然后从next返回该Iterator所持有的单一Entry对象。

select方法将产生一个包含制定尺寸的EntrySet的FlyweightMap，它会被用于重载过的capitals和names方法。
       Countries的尺寸受限会成为问题。可以采用与产生定制容器相同的方法来解决，其中定制容器是经过初始化的，并且具有任意尺寸的数据集。下面的类是一个list，它可以具有任意尺寸，并且用Integer数据进行了预初始化：

```java
public class CountingIntegerList extends AbstractList<Integer> {
    private int size;
    public CountingIntegerList(int size){
        this.size=size<0?0:size;
    }
    @Override
    public Integer get(int index) {
        return Integer.valueOf(index);
    }

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

public static void main(String[] args) {
        System.out.println(new CountingIntegerList(30));
    }
}
```

> 运行结果

```java
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]
```

当我从看到输出的结果时头上出现了三个大大的问号？

我没有使用迭代器，也没有进行任何遍历操作，怎么输出来的是一个数组。

从我作为小白级别的编程经验来判断，要想寻找答案还得DEBUG调试去看源码

**问题解析**

在代码中通过继承AbstractList抽象类并重写了get()和size()方法，通过这两个方法也不能解释数据的遍历操作，

在debug模式下进行调试时发现`System.out.println(new CountingIntegerList(30))`在创建完对象进行输出时会先主动去调用AbstractCollection中的toString()方法

那么问题又来了，AbstractCollection又是从哪冒出来的？

**源码**

```java
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
```

抽象类AbstractList继承了AbstractCollection并对其方法中的iterator()方法进行了实现

```java
//AbstractList.java
public Iterator<E> iterator() {
    return new Itr();//调用成员内部类的构造器
}

private class Itr implements Iterator<E> {
    /**
         * Index of element to be returned by subsequent call to next.
         */
    int cursor = 0;//代表下一次调用next将会返回的元素索引

/**
         * Index of element returned by most recent call to next or
         * previous.  Reset to -1 if this element is deleted by a call
         * to remove.如果lastRet索引元素已经被删除，那么lastRet会置为-1
         */
    int lastRet = -1;//代表上一次调用next或previous返回的元素索引

/**
         * The modCount value that the iterator believes that the backing
         * List should have.  If this expectation is violated, the iterator
         * has detected concurrent modification.
         */
    int expectedModCount = modCount;//获得外部类list对象的modCount成员，代表被修改的次数

public boolean hasNext() {
        return cursor != size();//调用了例子代码重写的size方法。size刚好为最大索引+1，所以等于size时应该返回false
    }

public E next() {
        checkForComodification();
        try {
            int i = cursor;//既然cursor是下一次next返回的元素索引，那么next里就使用这个索引
            E next = get(i);//调用了例子代码重写的get方法。这句是可能抛出IndexOutOfBoundsException的，当然本文例子不会抛出
            lastRet = i;//next执行后，cursor就会变成last return
            cursor = i + 1;
            return next;
        } catch (IndexOutOfBoundsException e) {
            checkForComodification();
            throw new NoSuchElementException();
        }
    }

public void remove() {
        if (lastRet < 0)//如果自己已经remove过了，那么再次remove就会抛异常
            throw new IllegalStateException();
        checkForComodification();//如果别的线程也remove外部类list对象了，那么也会抛出异常
        try {
            AbstractList.this.remove(lastRet);//调用外部类对象的remove方法，但AbstractList里只是默认实现直接抛异常
            if (lastRet < cursor)
                cursor--;
            lastRet = -1;//删除后就置lastRet为-1
            expectedModCount = modCount;//再次获得外部类对象的modCount成员，因为remove后modCount会发生改变
        } catch (IndexOutOfBoundsException e) {
            throw new ConcurrentModificationException();
        }
    }

final void checkForComodification() {
        if (modCount != expectedModCount)//比较迭代器自己存的count，和外部类对象存的count
            throw new ConcurrentModificationException();
    }
}
```

发现原来有个成员内部类`Itr`已经实现了Iterator接口，并在`iterator()`方法返回`return new Itr()`，这样每个迭代器对象都会持有一个外部类list对象的引用，而且list对象和迭代器对象的关系是一对多的。

接下来我们再来看一下AbstractCollection中的toString()方法源码

```java
//AbstractCollection.java
public String toString() {
    Iterator<E> it = iterator();//返回一个迭代器，用Iterator接口来接的
    if (! it.hasNext())//如果迭代器刚开始就不能移动
        return "[]";

StringBuilder sb = new StringBuilder();
    sb.append('[');
    for (;;) {
        E e = it.next();//此时已经判断过hasNext可以返回true了
        sb.append(e == this ? "(this Collection)" : e);//加入StringBuilder
        if (! it.hasNext())//如果hasNext可以返回false，那么退出该方法
            return sb.append(']').toString();
        sb.append(',').append(' ');//如果hasNext可以返回true，那么先提前加好逗号空格
    }
}
```

通过上方的源码可以了解到在toString()方法中对输出进行循环了遍历操作。

> 通过上方的简单的小示例就可以想像出这一章的真正威力，和之前的持有对象相比难度大了很多

下面是包含经过预处初始化，并且都是唯一的Integer和String对的Map,它可以具有任意尺寸：

```java
public class CountingMapData extends AbstractMap<Integer, String> {
    private int size;
    private static String[] chars = "A B C D E F G H I J K L M N O P Q R S T U V W X Y Z".split(" ");

public CountingMapData(int size) {
        if(size < 0) {
            this.size = 0;
        }

this.size = size;
    }

public Set<java.util.Map.Entry<Integer, String>> entrySet() {
        LinkedHashSet entries = new LinkedHashSet();

for(int i = 0; i < this.size; ++i) {
            entries.add(new CountingMapData.Entry(i));
        }

return entries;
    }

public static void main(String[] args) {
        System.out.println(new CountingMapData(60));
    }

private static class Entry implements java.util.Map.Entry<Integer, String> {
        int index;

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

public boolean equals(Object o) {
            return Integer.valueOf(this.index).equals(o);
        }

public Integer getKey() {
            return Integer.valueOf(this.index);
        }

public String getValue() {
            return CountingMapData.chars[this.index % CountingMapData.chars.length] + Integer.toString(this.index / CountingMapData.chars.length);
        }

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

public int hashCode() {
            return Integer.valueOf(this.index).hashCode();
        }
    }
}
```

> 运行结果

```java
{0=A0, 1=B0, 2=C0, 3=D0, 4=E0, 5=F0, 6=G0, 7=H0, 8=I0, 9=J0, 10=K0, 11=L0, 12=M0, 13=N0, 14=O0, 15=P0, 16=Q0, 17=R0, 18=S0, 19=T0, 20=U0, 21=V0, 22=W0, 23=X0, 24=Y0, 25=Z0, 26=A1, 27=B1, 28=C1, 29=D1, 30=E1, 31=F1, 32=G1, 33=H1, 34=I1, 35=J1, 36=K1, 37=L1, 38=M1, 39=N1, 40=O1, 41=P1, 42=Q1, 43=R1, 44=S1, 45=T1, 46=U1, 47=V1, 48=W1, 49=X1, 50=Y1, 51=Z1, 52=A2, 53=B2, 54=C2, 55=D2, 56=E2, 57=F2, 58=G2, 59=H2}
```

这里使用的是LinkedHashSet,而不是定制的Set类，因此享元并未完全实现。

标签: Java, Java编程思想, 容器深入研究

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