[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： ```java // 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 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 Map invert(Map 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 INVMAP = invert(MAP); // Look up RGB value given a name: public static Integer rgb(String colorName) { return INVMAP.get(colorName); } public static final List LIST = Arrays.stream(ARRAY) .map(item -> (String)item[1]) .collect(Collectors.toList()); public static final List RGBLIST = Arrays.stream(ARRAY) .map(item -> (Integer)item[0]) .collect(Collectors.toList()); public static void show(Map.Entry e) { System.out.format( "0x%06X: %s%n", e.getKey(), e.getValue()); } public static void show(Map m, int count) { m.entrySet().stream() .limit(count) .forEach(e -> show(e)); } public static void show(Map m) { show(m, m.size()); } public static void show(Collection lst, int count) { lst.stream() .limit(count) .forEach(System.out::println); } public static void show(Collection lst) { show(lst, lst.size()); } public static void showrgb(Collection lst, int count) { lst.stream() .limit(count) .forEach(n -> System.out.format("0x%06X%n", n)); } public static void showrgb(Collection lst) { showrgb(lst, lst.size()); } public static void showInv(Map 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 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： ```java // 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： ```java // 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 LIST = HTMLColors.LIST.subList(0, 10); private static boolean b; private static String s; private static int i; private static Iterator it; private static ListIterator lit; public static void basicTest(List 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 a) { ListIterator 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 a) { ListIterator 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 a) { System.out.println(a); List 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 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 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： ```java // 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 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 set = (Set) 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： ```java // 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** ： ```java // 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 COLORS = new ConcurrentSkipListMap<>(MAP); public static void main(String[] args) { show(COLORS.firstEntry()); border(); show(COLORS.lastEntry()); border(); NavigableMap 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： ```java // 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 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： ```java // collectiontopics/SuppliersCollectionTest.java import java.util.*; import java.util.function.*; import java.util.stream.*; import onjava.*; class Government implements Supplier { 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 set = Suppliers.create( LinkedHashSet::new, new Government(), 15); System.out.println(set); List 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）： ```java // onjava/Pair.java package onjava; public class Pair { 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 Pair make(K k, V v) { return new Pair(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： ```java // collectiontopics/StreamFillMaps.java import java.util.*; import java.util.function.*; import java.util.stream.*; import onjava.*; class Letters implements Supplier> { private int number = 1; private char letter = 'A'; @Override public Pair get() { return new Pair<>(number++, "" + letter++); } } public class StreamFillMaps { public static void main(String[] args) { Map 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 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 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： ```java // onjava/FillMap.java package onjava; import java.util.*; import java.util.function.*; import java.util.stream.*; public class FillMap { public static Map basic(Supplier> pairGen, int size) { return Stream.generate(pairGen) .limit(size) .collect(Collectors .toMap(Pair::key, Pair::value)); } public static Map basic(Supplier keyGen, Supplier valueGen, int size) { return Stream.generate( () -> Pair.make(keyGen.get(), valueGen.get())) .limit(size) .collect(Collectors .toMap(Pair::key, Pair::value)); } public static > M create(Supplier keyGen, Supplier valueGen, Supplier 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： ```java // 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 mcs = FillMap.basic( new Rand.String(4), new Count.Integer(), 7); System.out.println(mcs); HashMap hashm = FillMap.create(new Rand.String(4), new Count.Integer(), HashMap::new, 7); System.out.println(hashm); LinkedHashMap 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()**： ```java // onjava/CountingIntegerList.java // List of any length, containing sample data // {java onjava.CountingIntegerList} package onjava; import java.util.*; public class CountingIntegerList extends AbstractList { 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 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： ```java // 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 { 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 { 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> 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： ```java // 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 { private static class Entry implements Map.Entry { 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> { 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> { // 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 next() { entry.index++; return entry; } @Override public void remove() { throw new UnsupportedOperationException(); } } @Override public Iterator> iterator() { return new Iter(); } } private static Set> entries = new EntrySet(DATA.length); @Override public Set> entrySet() { return entries; } } // Create a partial map of 'size' countries: static Map select(final int size) { return new FlyweightMap() { @Override public Set> entrySet() { return new EntrySet(size); } }; } static Map map = new FlyweightMap(); public static Map capitals() { return map; // The entire map } public static Map capitals(int size) { return select(size); // A partial map } static List names = new ArrayList<>(map.keySet()); // All the names: public static List names() { return names; } // A partial list: public static List 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