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[TOC]

附录:集合主题

本附录是一些比第十二章 集合中介绍的更高级的内容。

示例数据

这里创建一些样本数据用于集合示例。 以下数据将颜色名称与HTML颜色的RGB值相关联。请注意，每个键和值都是唯一的：

// onjava/HTMLColors.java
// Sample data for collection examples
package onjava;
import java.util.*;
import java.util.stream.*;
import java.util.concurrent.*;

public class HTMLColors {
  public static final Object[][] ARRAY = {
    { 0xF0F8FF, "AliceBlue" },
    { 0xFAEBD7, "AntiqueWhite" },
    { 0x7FFFD4, "Aquamarine" },
    { 0xF0FFFF, "Azure" },
    { 0xF5F5DC, "Beige" },
    { 0xFFE4C4, "Bisque" },
    { 0x000000, "Black" },
    { 0xFFEBCD, "BlanchedAlmond" },
    { 0x0000FF, "Blue" },
    { 0x8A2BE2, "BlueViolet" },
    { 0xA52A2A, "Brown" },
    { 0xDEB887, "BurlyWood" },
    { 0x5F9EA0, "CadetBlue" },
    { 0x7FFF00, "Chartreuse" },
    { 0xD2691E, "Chocolate" },
    { 0xFF7F50, "Coral" },
    { 0x6495ED, "CornflowerBlue" },
    { 0xFFF8DC, "Cornsilk" },
    { 0xDC143C, "Crimson" },
    { 0x00FFFF, "Cyan" },
    { 0x00008B, "DarkBlue" },
    { 0x008B8B, "DarkCyan" },
    { 0xB8860B, "DarkGoldenRod" },
    { 0xA9A9A9, "DarkGray" },
    { 0x006400, "DarkGreen" },
    { 0xBDB76B, "DarkKhaki" },
    { 0x8B008B, "DarkMagenta" },
    { 0x556B2F, "DarkOliveGreen" },
    { 0xFF8C00, "DarkOrange" },
    { 0x9932CC, "DarkOrchid" },
    { 0x8B0000, "DarkRed" },
    { 0xE9967A, "DarkSalmon" },
    { 0x8FBC8F, "DarkSeaGreen" },
    { 0x483D8B, "DarkSlateBlue" },
    { 0x2F4F4F, "DarkSlateGray" },
    { 0x00CED1, "DarkTurquoise" },
    { 0x9400D3, "DarkViolet" },
    { 0xFF1493, "DeepPink" },
    { 0x00BFFF, "DeepSkyBlue" },
    { 0x696969, "DimGray" },
    { 0x1E90FF, "DodgerBlue" },
    { 0xB22222, "FireBrick" },
    { 0xFFFAF0, "FloralWhite" },
    { 0x228B22, "ForestGreen" },
    { 0xDCDCDC, "Gainsboro" },
    { 0xF8F8FF, "GhostWhite" },
    { 0xFFD700, "Gold" },
    { 0xDAA520, "GoldenRod" },
    { 0x808080, "Gray" },
    { 0x008000, "Green" },
    { 0xADFF2F, "GreenYellow" },
    { 0xF0FFF0, "HoneyDew" },
    { 0xFF69B4, "HotPink" },
    { 0xCD5C5C, "IndianRed" },
    { 0x4B0082, "Indigo" },
    { 0xFFFFF0, "Ivory" },
    { 0xF0E68C, "Khaki" },
    { 0xE6E6FA, "Lavender" },
    { 0xFFF0F5, "LavenderBlush" },
    { 0x7CFC00, "LawnGreen" },
    { 0xFFFACD, "LemonChiffon" },
    { 0xADD8E6, "LightBlue" },
    { 0xF08080, "LightCoral" },
    { 0xE0FFFF, "LightCyan" },
    { 0xFAFAD2, "LightGoldenRodYellow" },
    { 0xD3D3D3, "LightGray" },
    { 0x90EE90, "LightGreen" },
    { 0xFFB6C1, "LightPink" },
    { 0xFFA07A, "LightSalmon" },
    { 0x20B2AA, "LightSeaGreen" },
    { 0x87CEFA, "LightSkyBlue" },
    { 0x778899, "LightSlateGray" },
    { 0xB0C4DE, "LightSteelBlue" },
    { 0xFFFFE0, "LightYellow" },
    { 0x00FF00, "Lime" },
    { 0x32CD32, "LimeGreen" },
    { 0xFAF0E6, "Linen" },
    { 0xFF00FF, "Magenta" },
    { 0x800000, "Maroon" },
    { 0x66CDAA, "MediumAquaMarine" },
    { 0x0000CD, "MediumBlue" },
    { 0xBA55D3, "MediumOrchid" },
    { 0x9370DB, "MediumPurple" },
    { 0x3CB371, "MediumSeaGreen" },
    { 0x7B68EE, "MediumSlateBlue" },
    { 0x00FA9A, "MediumSpringGreen" },
    { 0x48D1CC, "MediumTurquoise" },
    { 0xC71585, "MediumVioletRed" },
    { 0x191970, "MidnightBlue" },
    { 0xF5FFFA, "MintCream" },
    { 0xFFE4E1, "MistyRose" },
    { 0xFFE4B5, "Moccasin" },
    { 0xFFDEAD, "NavajoWhite" },
    { 0x000080, "Navy" },
    { 0xFDF5E6, "OldLace" },
    { 0x808000, "Olive" },
    { 0x6B8E23, "OliveDrab" },
    { 0xFFA500, "Orange" },
    { 0xFF4500, "OrangeRed" },
    { 0xDA70D6, "Orchid" },
    { 0xEEE8AA, "PaleGoldenRod" },
    { 0x98FB98, "PaleGreen" },
    { 0xAFEEEE, "PaleTurquoise" },
    { 0xDB7093, "PaleVioletRed" },
    { 0xFFEFD5, "PapayaWhip" },
    { 0xFFDAB9, "PeachPuff" },
    { 0xCD853F, "Peru" },
    { 0xFFC0CB, "Pink" },
    { 0xDDA0DD, "Plum" },
    { 0xB0E0E6, "PowderBlue" },
    { 0x800080, "Purple" },
    { 0xFF0000, "Red" },
    { 0xBC8F8F, "RosyBrown" },
    { 0x4169E1, "RoyalBlue" },
    { 0x8B4513, "SaddleBrown" },
    { 0xFA8072, "Salmon" },
    { 0xF4A460, "SandyBrown" },
    { 0x2E8B57, "SeaGreen" },
    { 0xFFF5EE, "SeaShell" },
    { 0xA0522D, "Sienna" },
    { 0xC0C0C0, "Silver" },
    { 0x87CEEB, "SkyBlue" },
    { 0x6A5ACD, "SlateBlue" },
    { 0x708090, "SlateGray" },
    { 0xFFFAFA, "Snow" },
    { 0x00FF7F, "SpringGreen" },
    { 0x4682B4, "SteelBlue" },
    { 0xD2B48C, "Tan" },
    { 0x008080, "Teal" },
    { 0xD8BFD8, "Thistle" },
    { 0xFF6347, "Tomato" },
    { 0x40E0D0, "Turquoise" },
    { 0xEE82EE, "Violet" },
    { 0xF5DEB3, "Wheat" },
    { 0xFFFFFF, "White" },
    { 0xF5F5F5, "WhiteSmoke" },
    { 0xFFFF00, "Yellow" },
    { 0x9ACD32, "YellowGreen" },
  };
  public static final Map<Integer,String> MAP =
    Arrays.stream(ARRAY)
      .collect(Collectors.toMap(
        element -> (Integer)element[0],
        element -> (String)element[1],
        (v1, v2) -> { // Merge function
          throw new IllegalStateException();
        },
        LinkedHashMap::new
      ));
  // Inversion only works if values are unique:
  public static <V, K> Map<V, K>
  invert(Map<K, V> map) {
    return map.entrySet().stream()
      .collect(Collectors.toMap(
        Map.Entry::getValue,
        Map.Entry::getKey,
        (v1, v2) -> {
          throw new IllegalStateException();
        },
        LinkedHashMap::new
      ));
  }
  public static final Map<String,Integer>
    INVMAP = invert(MAP);
  // Look up RGB value given a name:
  public static Integer rgb(String colorName) {
    return INVMAP.get(colorName);
  }
  public static final List<String> LIST =
    Arrays.stream(ARRAY)
      .map(item -> (String)item[1])
      .collect(Collectors.toList());
  public static final List<Integer> RGBLIST =
    Arrays.stream(ARRAY)
      .map(item -> (Integer)item[0])
      .collect(Collectors.toList());
  public static
  void show(Map.Entry<Integer,String> e) {
    System.out.format(
      "0x%06X: %s%n", e.getKey(), e.getValue());
  }
  public static void
  show(Map<Integer,String> m, int count) {
    m.entrySet().stream()
      .limit(count)
      .forEach(e -> show(e));
  }
  public static void show(Map<Integer,String> m) {
    show(m, m.size());
  }
  public static
  void show(Collection<String> lst, int count) {
    lst.stream()
      .limit(count)
      .forEach(System.out::println);
  }
  public static void show(Collection<String> lst) {
    show(lst, lst.size());
  }
  public static
  void showrgb(Collection<Integer> lst, int count) {
    lst.stream()
      .limit(count)
      .forEach(n -> System.out.format("0x%06X%n", n));
  }
  public static void showrgb(Collection<Integer> lst) {
    showrgb(lst, lst.size());
  }
  public static
  void showInv(Map<String,Integer> m, int count) {
    m.entrySet().stream()
      .limit(count)
      .forEach(e ->
        System.out.format(
          "%-20s  0x%06X%n", e.getKey(), e.getValue()));
  }
  public static void showInv(Map<String,Integer> m) {
    showInv(m, m.size());
  }
  public static void border() {
    System.out.println(
      "******************************");
  }
}

MAP 是使用Streams（第十四章 流式编程）创建的。 二维数组 ARRAY 作为流传输到 Map 中，但请注意我们不仅仅是使用简单版本的 Collectors.toMap() 。 那个版本生成一个 HashMap ，它使用散列函数来控制对键的排序。 为了保留原来的顺序，我们必须将键值对直接放入 TreeMap 中，这意味着我们需要使用更复杂的 Collectors.toMap() 版本。这需要两个函数从每个流元素中提取键和值，就像简单版本的Collectors.toMap() 一样。 然后它需要一个合并函数（merge function），它解决了与同一个键相关的两个值之间的冲突。这里的数据已经预先审查过，因此绝不会发生这种情况，如果有的话，这里会抛出异常。最后，传递生成所需类型的空map的函数，然后用流来填充它。

rgb() 方法是一个便捷函数（convenience function），它接受颜色名称 String 参数并生成其数字RGB值。为此，我们需要一个反转版本的 COLORS ，它接受一个 String键并查找RGB的 Integer 值。 这是通过 invert() 方法实现的，如果任何 COLORS 值不唯一，则抛出异常。

我们还创建包含所有名称的 LIST ，以及包含十六进制表示法的RGB值的 RGBLIST 。

第一个 show() 方法接受一个 Map.Entry 并显示以十六进制表示的键，以便轻松地对原始 ARRAY 进行双重检查。 名称以 show 开头的每个方法都会重载两个版本，其中一个版本采用 count 参数来指示要显示的元素数量，第二个版本显示序列中的所有元素。

这里是一个基本的测试：

// collectiontopics/HTMLColorTest.java
import static onjava.HTMLColors.*;

public class HTMLColorTest {
  static final int DISPLAY_SIZE = 20;
  public static void main(String[] args) {
    show(MAP, DISPLAY_SIZE);
    border();
    showInv(INVMAP, DISPLAY_SIZE);
    border();
    show(LIST, DISPLAY_SIZE);
    border();
    showrgb(RGBLIST, DISPLAY_SIZE);
  }
}
/* Output:
0xF0F8FF: AliceBlue
0xFAEBD7: AntiqueWhite
0x7FFFD4: Aquamarine
0xF0FFFF: Azure
0xF5F5DC: Beige
0xFFE4C4: Bisque
0x000000: Black
0xFFEBCD: BlanchedAlmond
0x0000FF: Blue
0x8A2BE2: BlueViolet
0xA52A2A: Brown
0xDEB887: BurlyWood
0x5F9EA0: CadetBlue
0x7FFF00: Chartreuse
0xD2691E: Chocolate
0xFF7F50: Coral
0x6495ED: CornflowerBlue
0xFFF8DC: Cornsilk
0xDC143C: Crimson
0x00FFFF: Cyan
******************************
AliceBlue             0xF0F8FF
AntiqueWhite          0xFAEBD7
Aquamarine            0x7FFFD4
Azure                 0xF0FFFF
Beige                 0xF5F5DC
Bisque                0xFFE4C4
Black                 0x000000
BlanchedAlmond        0xFFEBCD
Blue                  0x0000FF
BlueViolet            0x8A2BE2
Brown                 0xA52A2A
BurlyWood             0xDEB887
CadetBlue             0x5F9EA0
Chartreuse            0x7FFF00
Chocolate             0xD2691E
Coral                 0xFF7F50
CornflowerBlue        0x6495ED
Cornsilk              0xFFF8DC
Crimson               0xDC143C
Cyan                  0x00FFFF
******************************
AliceBlue
AntiqueWhite
Aquamarine
Azure
Beige
Bisque
Black
BlanchedAlmond
Blue
BlueViolet
Brown
BurlyWood
CadetBlue
Chartreuse
Chocolate
Coral
CornflowerBlue
Cornsilk
Crimson
Cyan
******************************
0xF0F8FF
0xFAEBD7
0x7FFFD4
0xF0FFFF
0xF5F5DC
0xFFE4C4
0x000000
0xFFEBCD
0x0000FF
0x8A2BE2
0xA52A2A
0xDEB887
0x5F9EA0
0x7FFF00
0xD2691E
0xFF7F50
0x6495ED
0xFFF8DC
0xDC143C
0x00FFFF
*/

可以看到，使用 LinkedHashMap 确实能够保留 HTMLColors.ARRAY 的顺序。

List行为

Lists 是存储和检索对象（次于数组）的最基本方法。基本列表操作包括：

  • add() 用于插入元素
  • get() 用于随机访问元素
  • iterator() 获取序列上的一个 Iterator
  • stream() 生成元素的一个 Stream

列表构造方法始终保留元素的添加顺序。

以下示例中的方法各自涵盖了一组不同的行为：每个 List 可以执行的操作（ basicTest() ），使用 Iterator （ iterMotion() ）遍历序列，使用 Iterator （ iterManipulation() ）更改内容，查看 List 操作（ testVisual() ）的效果，以及仅可用于 LinkedLists 的操作：

// collectiontopics/ListOps.java
// Things you can do with Lists
import java.util.*;
import onjava.HTMLColors;

public class ListOps {
  // Create a short list for testing:
  static final List<String> LIST =
    HTMLColors.LIST.subList(0, 10);
  private static boolean b;
  private static String s;
  private static int i;
  private static Iterator<String> it;
  private static ListIterator<String> lit;
  public static void basicTest(List<String> a) {
    a.add(1, "x"); // Add at location 1
    a.add("x"); // Add at end
    // Add a collection:
    a.addAll(LIST);
    // Add a collection starting at location 3:
    a.addAll(3, LIST);
    b = a.contains("1"); // Is it in there?
    // Is the entire collection in there?
    b = a.containsAll(LIST);
    // Lists allow random access, which is cheap
    // for ArrayList, expensive for LinkedList:
    s = a.get(1); // Get (typed) object at location 1
    i = a.indexOf("1"); // Tell index of object
    b = a.isEmpty(); // Any elements inside?
    it = a.iterator(); // Ordinary Iterator
    lit = a.listIterator(); // ListIterator
    lit = a.listIterator(3); // Start at location 3
    i = a.lastIndexOf("1"); // Last match
    a.remove(1); // Remove location 1
    a.remove("3"); // Remove this object
    a.set(1, "y"); // Set location 1 to "y"
    // Keep everything that's in the argument
    // (the intersection of the two sets):
    a.retainAll(LIST);
    // Remove everything that's in the argument:
    a.removeAll(LIST);
    i = a.size(); // How big is it?
    a.clear(); // Remove all elements
  }
  public static void iterMotion(List<String> a) {
    ListIterator<String> it = a.listIterator();
    b = it.hasNext();
    b = it.hasPrevious();
    s = it.next();
    i = it.nextIndex();
    s = it.previous();
    i = it.previousIndex();
  }
  public static void iterManipulation(List<String> a) {
    ListIterator<String> it = a.listIterator();
    it.add("47");
    // Must move to an element after add():
    it.next();
    // Remove the element after the new one:
    it.remove();
    // Must move to an element after remove():
    it.next();
    // Change the element after the deleted one:
    it.set("47");
  }
  public static void testVisual(List<String> a) {
    System.out.println(a);
    List<String> b = LIST;
    System.out.println("b = " + b);
    a.addAll(b);
    a.addAll(b);
    System.out.println(a);
    // Insert, remove, and replace elements
    // using a ListIterator:
    ListIterator<String> x =
      a.listIterator(a.size()/2);
    x.add("one");
    System.out.println(a);
    System.out.println(x.next());
    x.remove();
    System.out.println(x.next());
    x.set("47");
    System.out.println(a);
    // Traverse the list backwards:
    x = a.listIterator(a.size());
    while(x.hasPrevious())
      System.out.print(x.previous() + " ");
    System.out.println();
    System.out.println("testVisual finished");
  }
  // There are some things that only LinkedLists can do:
  public static void testLinkedList() {
    LinkedList<String> ll = new LinkedList<>();
    ll.addAll(LIST);
    System.out.println(ll);
    // Treat it like a stack, pushing:
    ll.addFirst("one");
    ll.addFirst("two");
    System.out.println(ll);
    // Like "peeking" at the top of a stack:
    System.out.println(ll.getFirst());
    // Like popping a stack:
    System.out.println(ll.removeFirst());
    System.out.println(ll.removeFirst());
    // Treat it like a queue, pulling elements
    // off the tail end:
    System.out.println(ll.removeLast());
    System.out.println(ll);
  }
  public static void main(String[] args) {
    // Make and fill a new list each time:
    basicTest(new LinkedList<>(LIST));
    basicTest(new ArrayList<>(LIST));
    iterMotion(new LinkedList<>(LIST));
    iterMotion(new ArrayList<>(LIST));
    iterManipulation(new LinkedList<>(LIST));
    iterManipulation(new ArrayList<>(LIST));
    testVisual(new LinkedList<>(LIST));
    testLinkedList();
  }
}
/* Output:
[AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet]
b = [AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet]
[AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet,
AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet,
AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet]
[AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet,
AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige, one,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet,
AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet]
Bisque
Black
[AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet,
AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige, one,
47, BlanchedAlmond, Blue, BlueViolet, AliceBlue,
AntiqueWhite, Aquamarine, Azure, Beige, Bisque, Black,
BlanchedAlmond, Blue, BlueViolet]
BlueViolet Blue BlanchedAlmond Black Bisque Beige Azure
Aquamarine AntiqueWhite AliceBlue BlueViolet Blue
BlanchedAlmond 47 one Beige Azure Aquamarine
AntiqueWhite AliceBlue BlueViolet Blue BlanchedAlmond
Black Bisque Beige Azure Aquamarine AntiqueWhite
AliceBlue
testVisual finished
[AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue, BlueViolet]
[two, one, AliceBlue, AntiqueWhite, Aquamarine, Azure,
Beige, Bisque, Black, BlanchedAlmond, Blue, BlueViolet]
two
two
one
BlueViolet
[AliceBlue, AntiqueWhite, Aquamarine, Azure, Beige,
Bisque, Black, BlanchedAlmond, Blue]
*/

在 basicTest() 和 iterMotion() 中，方法调用是为了展示正确的语法，尽管获取了返回值，但不会使用它。在某些情况下，根本不会去获取返回值。在使用这些方法之前，请查看JDK文档中这些方法的完整用法。

Set行为

Set 的主要用处是测试成员身份，不过也可以将其用作删除重复元素的工具。如果不关心元素顺序或并发性， HashSet 总是最好的选择，因为它是专门为了快速查找而设计的（这里使用了在附录：理解equals和hashCode方法章节中探讨的散列函数）。

其它的 Set 实现产生不同的排序行为：

// collectiontopics/SetOrder.java
import java.util.*;
import onjava.HTMLColors;

public class SetOrder {
  static String[] sets = {
    "java.util.HashSet",
    "java.util.TreeSet",
    "java.util.concurrent.ConcurrentSkipListSet",
    "java.util.LinkedHashSet",
    "java.util.concurrent.CopyOnWriteArraySet",
  };
  static final List<String> RLIST =
    new ArrayList<>(HTMLColors.LIST);
  static {
    Collections.reverse(RLIST);
  }
  public static void
  main(String[] args) throws Exception {
    for(String type: sets) {
      System.out.format("[-> %s <-]%n",
        type.substring(type.lastIndexOf('.') + 1));
      @SuppressWarnings("unchecked")
      Set<String> set = (Set<String>)
        Class.forName(type).newInstance();
      set.addAll(RLIST);
      set.stream()
        .limit(10)
        .forEach(System.out::println);
    }
  }
}
/* Output:
[-> HashSet <-]
MediumOrchid
PaleGoldenRod
Sienna
LightSlateGray
DarkSeaGreen
Black
Gainsboro
Orange
LightCoral
DodgerBlue
[-> TreeSet <-]
AliceBlue
AntiqueWhite
Aquamarine
Azure
Beige
Bisque
Black
BlanchedAlmond
Blue
BlueViolet
[-> ConcurrentSkipListSet <-]
AliceBlue
AntiqueWhite
Aquamarine
Azure
Beige
Bisque
Black
BlanchedAlmond
Blue
BlueViolet
[-> LinkedHashSet <-]
YellowGreen
Yellow
WhiteSmoke
White
Wheat
Violet
Turquoise
Tomato
Thistle
Teal
[-> CopyOnWriteArraySet <-]
YellowGreen
Yellow
WhiteSmoke
White
Wheat
Violet
Turquoise
Tomato
Thistle
Teal
*/

这里需要使用 @SuppressWarnings(“unchecked”) ，因为这里将一个 String （可能是任何东西）传递给了 Class.forName(type).newInstance() 。编译器并不能保证这是一次成功的操作。

RLIST 是 HTMLColors.LIST 的反转版本。因为 Collections.reverse() 是通过修改参数来执行反向操作，而不是返回包含反向元素的新 List ，所以该调用在 static 块内执行。 RLIST 可以防止我们意外地认为 Set 对其结果进行了排序。

HashSet 的输出结果似乎没有可辨别的顺序，因为它是基于散列函数的。 TreeSet 和 ConcurrentSkipListSet 都对它们的元素进行了排序，它们都实现了 SortedSet 接口来标识这个特点。因为实现该接口的 Set 按顺序排列，所以该接口还有一些其他的可用操作。 LinkedHashSet 和 CopyOnWriteArraySet 尽管没有用于标识的接口，但它们还是保留了元素的插入顺序。

ConcurrentSkipListSet 和 CopyOnWriteArraySet 是线程安全的。

在附录的最后，我们将了解在非 HashSet 实现的 Set 上添加额外排序的性能成本，以及不同实现中的任何其他功能的成本。

在Map中使用函数式操作

与 Collection 接口一样，forEach() 也内置在 Map 接口中。但是如果想要执行任何其他的基本功能操作，比如 map() ，flatMap() ，reduce() 或 filter() 时，该怎么办？ 查看 Map 接口发现并没有这些。

可以通过 entrySet() 连接到这些方法，该方法会生成一个由 Map.Entry 对象组成的 Set 。这个 Set 包含 stream() 和 parallelStream() 方法。只需要记住一件事，这里正在使用的是 Map.Entry 对象：

// collectiontopics/FunctionalMap.java
// Functional operations on a Map
import java.util.*;
import java.util.stream.*;
import java.util.concurrent.*;
import static onjava.HTMLColors.*;

public class FunctionalMap {
  public static void main(String[] args) {
    MAP.entrySet().stream()
      .map(Map.Entry::getValue)
      .filter(v -> v.startsWith("Dark"))
      .map(v -> v.replaceFirst("Dark", "Hot"))
      .forEach(System.out::println);
  }
}
/* Output:
HotBlue
HotCyan
HotGoldenRod
HotGray
HotGreen
HotKhaki
HotMagenta
HotOliveGreen
HotOrange
HotOrchid
HotRed
HotSalmon
HotSeaGreen
HotSlateBlue
HotSlateGray
HotTurquoise
HotViolet
*/

生成 Stream 后，所有的基本功能方法，甚至更多就都可以使用了。

选择Map片段

由 TreeMap 和 ConcurrentSkipListMap 实现的 NavigableMap 接口解决了需要选择Map片段的问题。下面是一个示例，使用了 HTMLColors ：

// collectiontopics/NavMap.java
// NavigableMap produces pieces of a Map
import java.util.*;
import java.util.concurrent.*;
import static onjava.HTMLColors.*;

public class NavMap {
  public static final
  NavigableMap<Integer,String> COLORS =
    new ConcurrentSkipListMap<>(MAP);
  public static void main(String[] args) {
    show(COLORS.firstEntry());
    border();
    show(COLORS.lastEntry());
    border();
    NavigableMap<Integer, String> toLime =
      COLORS.headMap(rgb("Lime"), true);
    show(toLime);
    border();
    show(COLORS.ceilingEntry(rgb("DeepSkyBlue") - 1));
    border();
    show(COLORS.floorEntry(rgb("DeepSkyBlue") - 1));
    border();
    show(toLime.descendingMap());
    border();
    show(COLORS.tailMap(rgb("MistyRose"), true));
    border();
    show(COLORS.subMap(
      rgb("Orchid"), true,
      rgb("DarkSalmon"), false));
  }
}
/* Output:
0x000000: Black
******************************
0xFFFFFF: White
******************************
0x000000: Black
0x000080: Navy
0x00008B: DarkBlue
0x0000CD: MediumBlue
0x0000FF: Blue
0x006400: DarkGreen
0x008000: Green
0x008080: Teal
0x008B8B: DarkCyan
0x00BFFF: DeepSkyBlue
0x00CED1: DarkTurquoise
0x00FA9A: MediumSpringGreen
0x00FF00: Lime
******************************
0x00BFFF: DeepSkyBlue
******************************
0x008B8B: DarkCyan
******************************
0x00FF00: Lime
0x00FA9A: MediumSpringGreen
0x00CED1: DarkTurquoise
0x00BFFF: DeepSkyBlue
0x008B8B: DarkCyan
0x008080: Teal
0x008000: Green
0x006400: DarkGreen
0x0000FF: Blue
0x0000CD: MediumBlue
0x00008B: DarkBlue
0x000080: Navy
0x000000: Black
******************************
0xFFE4E1: MistyRose
0xFFEBCD: BlanchedAlmond
0xFFEFD5: PapayaWhip
0xFFF0F5: LavenderBlush
0xFFF5EE: SeaShell
0xFFF8DC: Cornsilk
0xFFFACD: LemonChiffon
0xFFFAF0: FloralWhite
0xFFFAFA: Snow
0xFFFF00: Yellow
0xFFFFE0: LightYellow
0xFFFFF0: Ivory
0xFFFFFF: White
******************************
0xDA70D6: Orchid
0xDAA520: GoldenRod
0xDB7093: PaleVioletRed
0xDC143C: Crimson
0xDCDCDC: Gainsboro
0xDDA0DD: Plum
0xDEB887: BurlyWood
0xE0FFFF: LightCyan
0xE6E6FA: Lavender
*/

在 main() 方法中可以看到 NavigableMap 的各种功能。 因为 NavigableMap 具有键顺序，所以它使用了 firstEntry() 和 lastEntry() 的概念。调用 headMap() 会生成一个 NavigableMap ，其中包含了从 Map 的开头到 headMap() 参数中所指向的一组元素，其中 boolean 值指示结果中是否包含该参数。调用 tailMap() 执行了类似的操作，只不过是从参数开始到 Map 的末尾。 subMap() 则允许生成 Map 中间的一部分。

ceilingEntry() 从当前键值对向上搜索下一个键值对，floorEntry() 则是向下搜索。 descendingMap() 反转了 NavigableMap 的顺序。

如果需要通过分割 Map 来简化所正在解决的问题，则 NavigableMap 可以做到。具有类似的功能的其它集合实现也可以用来帮助解决问题。

填充集合

与 Arrays 一样，这里有一个名为 Collections 的伴随类（companion class），包含了一些 static 的实用方法，其中包括一个名为 fill() 的方法。 fill() 只复制整个集合中的单个对象引用。此外，它仅适用于 List 对象，但结果列表可以传递给构造方法或 addAll() 方法：

// collectiontopics/FillingLists.java
// Collections.fill() & Collections.nCopies()
import java.util.*;

class StringAddress {
  private String s;
  StringAddress(String s) { this.s = s; }
  @Override
  public String toString() {
    return super.toString() + " " + s;
  }
}

public class FillingLists {
  public static void main(String[] args) {
    List<StringAddress> list = new ArrayList<>(
      Collections.nCopies(4,
        new StringAddress("Hello")));
    System.out.println(list);
    Collections.fill(list,
      new StringAddress("World!"));
    System.out.println(list);
  }
}
/* Output:
[StringAddress@15db9742 Hello, StringAddress@15db9742
Hello, StringAddress@15db9742 Hello,
StringAddress@15db9742 Hello]
[StringAddress@6d06d69c World!, StringAddress@6d06d69c
World!, StringAddress@6d06d69c World!,
StringAddress@6d06d69c World!]
*/

这个示例展示了两种使用对单个对象的引用来填充 Collection 的方法。 第一个： Collections.nCopies() ，创建一个 List，并传递给 ArrayList 的构造方法，进而填充了 ArrayList 。

StringAddress 中的 toString() 方法调用了 Object.toString() ，它先生成类名，后跟着对象的哈希码的无符号十六进制表示（哈希吗由 hashCode() 方法生成）。 输出显示所有的引用都指向同一个对象。调用第二个方法 Collections.fill() 后也是如此。 fill() 方法的用处非常有限，它只能替换 List 中已有的元素,而且不会添加新元素，

使用 Suppliers 填充集合

第二十章 泛型章节中介绍的 onjava.Suppliers 类为填充集合提供了通用解决方案。 这是一个使用 Suppliers 初始化几种不同类型的 Collection 的示例：

// collectiontopics/SuppliersCollectionTest.java
import java.util.*;
import java.util.function.*;
import java.util.stream.*;
import onjava.*;

class Government implements Supplier<String> {
  static String[] foundation = (
    "strange women lying in ponds " +
    "distributing swords is no basis " +
    "for a system of government").split(" ");
  private int index;
  @Override
  public String get() {
    return foundation[index++];
  }
}

public class SuppliersCollectionTest {
  public static void main(String[] args) {
    // Suppliers class from the Generics chapter:
    Set<String> set = Suppliers.create(
      LinkedHashSet::new, new Government(), 15);
    System.out.println(set);
    List<String> list = Suppliers.create(
      LinkedList::new, new Government(), 15);
    System.out.println(list);
    list = new ArrayList<>();
    Suppliers.fill(list, new Government(), 15);
    System.out.println(list);

    // Or we can use Streams:
    set = Arrays.stream(Government.foundation)
      .collect(Collectors.toSet());
    System.out.println(set);
    list = Arrays.stream(Government.foundation)
      .collect(Collectors.toList());
    System.out.println(list);
    list = Arrays.stream(Government.foundation)
      .collect(Collectors
        .toCollection(LinkedList::new));
    System.out.println(list);
    set = Arrays.stream(Government.foundation)
      .collect(Collectors
        .toCollection(LinkedHashSet::new));
    System.out.println(set);
  }
}
/* Output:
[strange, women, lying, in, ponds, distributing,
swords, is, no, basis, for, a, system, of, government]
[strange, women, lying, in, ponds, distributing,
swords, is, no, basis, for, a, system, of, government]
[strange, women, lying, in, ponds, distributing,
swords, is, no, basis, for, a, system, of, government]
[ponds, no, a, in, swords, for, is, basis, strange,
system, government, distributing, of, women, lying]
[strange, women, lying, in, ponds, distributing,
swords, is, no, basis, for, a, system, of, government]
[strange, women, lying, in, ponds, distributing,
swords, is, no, basis, for, a, system, of, government]
[strange, women, lying, in, ponds, distributing,
swords, is, no, basis, for, a, system, of, government]
*/

LinkedHashSet 中的的元素按插入顺序排列，因为它维护一个链表来保存该顺序。

但是请注意示例的第二部分：大多数情况下都可以使用 Stream 来创建和填充 Collection 。在本例中的 Stream 版本不需要声明 Supplier 所想要创建的元素数量;，它直接吸收了 Stream 中的所有元素。

尽可能优先选择 Stream 来解决问题。

Map Suppliers

使用 Supplier 来填充 Map 时需要一个 Pair 类，因为每次调用一个 Supplier 的 get() 方法时，都必须生成一对对象（一个键和一个值）：

// onjava/Pair.java
package onjava;

public class Pair<K, V> {
  public final K key;
  public final V value;
  public Pair(K k, V v) {
    key = k;
    value = v;
  }
  public K key() { return key; }
  public V value() { return value; }
  public static <K,V> Pair<K, V> make(K k, V v) {
    return new Pair<K,V>(k, v);
  }
}

Pair 是一个只读的 数据传输对象 （Data Transfer Object）或 信使 （Messenger）。 这与第二十章 泛型章节中的 Tuple2 基本相同，但名字更适合 Map 初始化。我还添加了静态的 make() 方法，以便为创建 Pair 对象提供一个更简洁的名字。

Java 8 的 Stream 提供了填充 Map 的便捷方法：

// collectiontopics/StreamFillMaps.java
import java.util.*;
import java.util.function.*;
import java.util.stream.*;
import onjava.*;

class Letters
implements Supplier<Pair<Integer,String>> {
  private int number = 1;
  private char letter = 'A';
  @Override
  public Pair<Integer,String> get() {
    return new Pair<>(number++, "" + letter++);
  }
}

public class StreamFillMaps {
  public static void main(String[] args) {
    Map<Integer,String> m =
      Stream.generate(new Letters())
      .limit(11)
      .collect(Collectors
        .toMap(Pair::key, Pair::value));
    System.out.println(m);

    // Two separate Suppliers:
    Rand.String rs = new Rand.String(3);
    Count.Character cc = new Count.Character();
    Map<Character,String> mcs = Stream.generate(
      () -> Pair.make(cc.get(), rs.get()))
      .limit(8)
      .collect(Collectors
        .toMap(Pair::key, Pair::value));
    System.out.println(mcs);

    // A key Supplier and a single value:
    Map<Character,String> mcs2 = Stream.generate(
      () -> Pair.make(cc.get(), "Val"))
      .limit(8)
      .collect(Collectors
        .toMap(Pair::key, Pair::value));
    System.out.println(mcs2);
  }
}
/* Output:
{1=A, 2=B, 3=C, 4=D, 5=E, 6=F, 7=G, 8=H, 9=I, 10=J,
11=K}
{b=btp, c=enp, d=ccu, e=xsz, f=gvg, g=mei, h=nne,
i=elo}
{p=Val, q=Val, j=Val, k=Val, l=Val, m=Val, n=Val,
o=Val}
*/

上面的示例中出现了一个模式，可以使用它来创建一个自动创建和填充 Map 的工具：

// onjava/FillMap.java
package onjava;
import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class FillMap {
  public static <K, V> Map<K,V>
  basic(Supplier<Pair<K,V>> pairGen, int size) {
    return Stream.generate(pairGen)
      .limit(size)
      .collect(Collectors
        .toMap(Pair::key, Pair::value));
  }
  public static <K, V> Map<K,V>
  basic(Supplier<K> keyGen,
        Supplier<V> valueGen, int size) {
    return Stream.generate(
      () -> Pair.make(keyGen.get(), valueGen.get()))
      .limit(size)
      .collect(Collectors
        .toMap(Pair::key, Pair::value));
  }
  public static <K, V, M extends Map<K,V>>
  M create(Supplier<K> keyGen,
           Supplier<V> valueGen,
           Supplier<M> mapSupplier, int size) {
    return Stream.generate( () ->
      Pair.make(keyGen.get(), valueGen.get()))
        .limit(size)
        .collect(Collectors
          .toMap(Pair::key, Pair::value,
                 (k, v) -> k, mapSupplier));
  }
}

basic() 方法生成一个默认的 Map ，而 create() 方法允许指定一个确切的 Map 类型，并返回那个确切的类型。

下面是一个测试：

// collectiontopics/FillMapTest.java
import java.util.*;
import java.util.function.*;
import java.util.stream.*;
import onjava.*;

public class FillMapTest {
  public static void main(String[] args) {
    Map<String,Integer> mcs = FillMap.basic(
      new Rand.String(4), new Count.Integer(), 7);
    System.out.println(mcs);
    HashMap<String,Integer> hashm =
      FillMap.create(new Rand.String(4),
        new Count.Integer(), HashMap::new, 7);
    System.out.println(hashm);
    LinkedHashMap<String,Integer> linkm =
      FillMap.create(new Rand.String(4),
        new Count.Integer(), LinkedHashMap::new, 7);
    System.out.println(linkm);
  }
}
/* Output:
{npcc=1, ztdv=6, gvgm=3, btpe=0, einn=4, eelo=5,
uxsz=2}
{npcc=1, ztdv=6, gvgm=3, btpe=0, einn=4, eelo=5,
uxsz=2}
{btpe=0, npcc=1, uxsz=2, gvgm=3, einn=4, eelo=5,
ztdv=6}
*/

使用享元（Flyweight）自定义Collection和Map

本节介绍如何创建自定义 Collection 和 Map 实现。每个 java.util 中的集合都有自己的 Abstract 类，它提供了该集合的部分实现，因此只需要实现必要的方法来生成所需的集合。你将看到通过继承 java.util.Abstract 类来创建自定义 Map 和 Collection 是多么简单。例如，要创建一个只读的 Set ，则可以从 AbstractSet 继承并实现 iterator() 和 size() 。最后一个示例是生成测试数据的另一种方法。生成的集合通常是只读的，并且所提供的方法最少。

该解决方案还演示了 享元 （Flyweight）设计模式。当普通解决方案需要太多对象时，或者当生成普通对象占用太多空间时，可以使用享元。享元设计模式将对象的一部分外部化（externalizes）。相比于把对象的所有内容都包含在对象中，这样做使得对象的部分或者全部可以在更有效的外部表中查找，或通过一些节省空间的其他计算生成。

下面是一个可以是任何大小的 List ，并且（有效地）使用 Integer 数据进行预初始化。要从 AbstractList 创建只读 List ，必须实现 get() 和 size()：

// onjava/CountingIntegerList.java
// List of any length, containing sample data
// {java onjava.CountingIntegerList}
package onjava;
import java.util.*;

public class CountingIntegerList
extends AbstractList<Integer> {
  private int size;
  public CountingIntegerList() { size = 0; }
  public CountingIntegerList(int size) {
    this.size = size < 0 ? 0 : size;
  }
  @Override
  public Integer get(int index) {
    return index;
  }
  @Override
  public int size() { return size; }
  public static void main(String[] args) {
    List<Integer> cil =
      new CountingIntegerList(30);
    System.out.println(cil);
    System.out.println(cil.get(500));
  }
}
/* Output:
[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]
500
*/

只有当想要限制 List 的长度时， size 值才是重要的，就像在 main() 中那样。即使在这种情况下， get() 也会产生任何值。

这个类是享元模式的一个简洁的例子。当需要的时候， get() “计算”所需的值，因此没必要存储和初始化实际的底层 List 结构。

在大多数程序中，这里所保存的存储结构永远都不会改变。但是，它允许用非常大的 index 来调用 List.get() ，而 List 并不需要填充到这么大。此外，还可以在程序中大量使用 CountingIntegerLists 而无需担心存储问题。实际上，享元的一个好处是它允许使用更好的抽象而不用担心资源。

可以使用享元设计模式来实现具有任何大小数据集的其他“初始化”自定义集合。下面是一个 Map ，它为每一个 Integer 键产生唯一的值：

// onjava/CountMap.java
// Unlimited-length Map containing sample data
// {java onjava.CountMap}
package onjava;
import java.util.*;
import java.util.stream.*;

public class CountMap
extends AbstractMap<Integer,String> {
  private int size;
  private static char[] chars =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
  private static String value(int key) {
    return
      chars[key % chars.length] +
      Integer.toString(key / chars.length);
  }
  public CountMap(int size) {
    this.size = size < 0 ? 0 : size;
  }
  @Override
  public String get(Object key) {
    return value((Integer)key);
  }
  private static class Entry
  implements Map.Entry<Integer,String> {
    int index;
    Entry(int index) { this.index = index; }
    @Override
    public boolean equals(Object o) {
      return o instanceof Entry &&
        Objects.equals(index, ((Entry)o).index);
    }
    @Override
    public Integer getKey() { return index; }
    @Override
    public String getValue() {
      return value(index);
    }
    @Override
    public String setValue(String value) {
      throw new UnsupportedOperationException();
    }
    @Override
    public int hashCode() {
      return Objects.hashCode(index);
    }
  }
  @Override
  public Set<Map.Entry<Integer,String>> entrySet() {
    // LinkedHashSet retains initialization order:
    return IntStream.range(0, size)
      .mapToObj(Entry::new)
      .collect(Collectors
        .toCollection(LinkedHashSet::new));
  }
  public static void main(String[] args) {
    final int size = 6;
    CountMap cm = new CountMap(60);
    System.out.println(cm);
    System.out.println(cm.get(500));
    cm.values().stream()
      .limit(size)
      .forEach(System.out::println);
    System.out.println();
    new Random(47).ints(size, 0, 1000)
      .mapToObj(cm::get)
      .forEach(System.out::println);
  }
}
/* Output:
{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}
G19
A0
B0
C0
D0
E0
F0

Y9
J21
R26
D33
Z36
N16
*/

要创建一个只读的 Map ，则从 AbstractMap 继承并实现 entrySet() 。私有的 value() 方法计算任何键的值，并在 get() 和 Entry.getValue() 中使用。可以忽略 CountMap 的大小。

这里是使用了 LinkedHashSet 而不是创建自定义 Set 类，因此并未完全实现享元。只有在调用 entrySet() 时才会生成此对象。

现在创建一个更复杂的享元。这个示例中的数据集是世界各国及其首都的 Map 。 capitals() 方法生成一个国家和首都的 Map 。 names() 方法生成一个由国家名字组成的 List 。 当给定了表示所需大小的 int 参数时，两种方法都生成对应大小的列表片段：

// onjava/Countries.java
// "Flyweight" Maps and Lists of sample data
// {java onjava.Countries}
package onjava;
import java.util.*;

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","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 AND 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; }
      @Override
      public boolean equals(Object o) {
        return o instanceof FlyweightMap &&
          Objects.equals(DATA[index][0], o);
      }
      @Override
      public int hashCode() {
        return Objects.hashCode(DATA[index][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();
      }
    }
    // Implement size() & iterator() for AbstractSet:
    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;
      }
      @Override
      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);
        @Override
        public boolean hasNext() {
          return entry.index < size - 1;
        }
        @Override
        public Map.Entry<String,String> next() {
          entry.index++;
          return entry;
        }
        @Override
        public void remove() {
          throw new UnsupportedOperationException();
        }
      }
      @Override
      public
      Iterator<Map.Entry<String,String>> iterator() {
        return new Iter();
      }
    }
    private static
    Set<Map.Entry<String,String>> entries =
      new EntrySet(DATA.length);
    @Override
    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() {
      @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; // The entire map
  }
  public static Map<String,String> capitals(int size) {
    return select(size); // A partial map
  }
  static List<String> names =
    new ArrayList<>(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<>(select(size).keySet());
  }
  public static void main(String[] args) {
    System.out.println(capitals(10));
    System.out.println(names(10));
    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<>(names(6)));
    System.out.println(new LinkedHashSet<>(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"));
  }
}
/* Output:
{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}
[ALGERIA, ANGOLA, BENIN, BOTSWANA, BURKINA FASO,
BURUNDI, CAMEROON, CAPE VERDE, CENTRAL AFRICAN
REPUBLIC, CHAD]
{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
*/

二维数组 String DATA 是 public 的，因此可以在别处使用。 FlyweightMap 必须实现 entrySet() 方法，该方法需要一个自定义 Set 实现和一个自定义 Map.Entry 类。这是实现享元的另一种方法：每个 Map.Entry 对象存储它自身的索引，而不是实际的键和值。当调用 getKey() 或 getValue() 时，它使用索引返回相应的 DATA 元素。 EntrySet 确保它的 size 不大于 DATA 。

享元的另一部分在 EntrySet.Iterator 中实现。相比于为 DATA 中的每个数据对创建一个 Map.Entry 对象，这里每个迭代器只有一个 Map.Entry 对象。 Entry 对象作为数据的窗口，它只包含 String 静态数组的索引。每次为迭代器调用 next() 时，Entry 中的索引都会递增，因此它会指向下一个数据对，然后从 next() 返回 Iterators 的单个 Entry 对象。

select() 方法生成一个包含所需大小的 EntrySet 的 FlyweightMap ，这用于在 main() 中演示的重载的 capitals() 和 names() 方法。

集合功能

可选操作

Set和存储顺序

队列

理解Map

集合工具类

持有引用

避免旧式类库

本章小结