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第二十章 泛型 翻译更新-泛型方法
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1326670425
@@ -570,6 +570,520 @@ extends Fibonacci implements Iterable<Integer> {
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## 泛型方法
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到目前为止，我们已经研究了参数化整个类。其实还可以参数化类中的方法。类本身可能是泛型的，也可能不是，不过这与它的方法是否是泛型的并没有什么关系。
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泛型方法独立于类而改变方法。作为准则，请“尽可能”使用泛型方法。通常将单个方法泛型化要比将整个类泛型化更清晰易懂。
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如果方法是 **static** 的，则无法访问该类的泛型类型参数，因此，如果使用了泛型类型参数，则它必须是泛型方法。
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要定义泛型方法，请将泛型参数列表放置在返回值之前，如下所示：
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```java
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// generics/GenericMethods.java
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public class GenericMethods {
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public <T> void f(T x) {
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System.out.println(x.getClass().getName());
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}
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public static void main(String[] args) {
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GenericMethods gm = new GenericMethods();
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gm.f("");
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gm.f(1);
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gm.f(1.0);
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gm.f(1.0F);
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gm.f('c');
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gm.f(gm);
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}
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}
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/* Output:
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java.lang.String
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java.lang.Integer
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java.lang.Double
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java.lang.Float
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java.lang.Character
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GenericMethods
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*/
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```
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尽管可以同时对类及其方法进行参数化，但这里未将 **GenericMethods** 类参数化。只有方法 `f()` 具有类型参数，该参数由方法返回类型之前的参数列表指示。
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对于泛型类，必须在实例化该类时指定类型参数。使用泛型方法时，通常不需要指定参数类型，因为编译器会找出这些类型。 这称为 *类型参数推断*。因此，对`f()` 的调用看起来像普通的方法调用，并且 `f()` 看起来像被重载了无数次一样。它甚至会接受 **GenericMethods** 类型的参数。
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如果使用基本类型调用 `f()` ，自动装箱就开始起作用，自动将基本类型包装在它们对应的包装类型中。
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<!-- Varargs and Generic Methods -->
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### 变量和泛型方法
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泛型方法和变长参数列表可以很好地共存：
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```java
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// generics/GenericVarargs.java
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import java.util.ArrayList;
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import java.util.List;
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public class GenericVarargs {
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@SafeVarargs
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public static <T> List<T> makeList(T... args) {
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List<T> result = new ArrayList<>();
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for (T item : args)
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result.add(item);
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return result;
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}
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public static void main(String[] args) {
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List<String> ls = makeList("A");
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System.out.println(ls);
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ls = makeList("A", "B", "C");
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System.out.println(ls);
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ls = makeList(
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"ABCDEFFHIJKLMNOPQRSTUVWXYZ".split(""));
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System.out.println(ls);
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}
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}
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/* Output:
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[A]
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[A, B, C]
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[A, B, C, D, E, F, F, H, I, J, K, L, M, N, O, P, Q, R,
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S, T, U, V, W, X, Y, Z]
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*/
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```
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此处显示的 `makeList()` 方法产生的功能与标准库的 `java.util.Arrays.asList()` 方法相同。
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`@SafeVarargs` 注解保证我们不会对变长参数列表进行任何修改，这是正确的，因为我们只从中读取。如果没有此注解，编译器将无法知道这些并会发出警告。
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<!-- A General-Purpose Supplier -->
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### 一个泛型的Supplier
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这是一个为任意具有无参构造方法的类生成 **Supplier** 的类。为了减少键入，它还包括一个用于生成 **BasicSupplier** 的泛型方法：
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```java
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// onjava/BasicSupplier.java
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// Supplier from a class with a no-arg constructor
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package onjava;
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import java.util.function.Supplier;
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public class BasicSupplier<T> implements Supplier<T> {
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private Class<T> type;
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public BasicSupplier(Class<T> type) {
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this.type = type;
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}
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@Override
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public T get() {
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try {
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// Assumes type is a public class:
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return type.newInstance();
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} catch (InstantiationException |
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IllegalAccessException e) {
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throw new RuntimeException(e);
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}
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}
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// Produce a default Supplier from a type token:
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public static <T> Supplier<T> create(Class<T> type) {
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return new BasicSupplier<>(type);
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}
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}
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```
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此类提供了产生以下对象的基本实现：
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1. 是 **public** 的。 因为 **BasicSupplier** 在单独的包中，所以相关的类必须具有 **public** 权限，而不仅仅是包级访问权限。
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2. 具有无参构造方法。要创建一个这样的 **BasicSupplier** 对象，请调用 `create()` 方法，并将要生成类型的类型令牌传递给它。通用的 `create()` 方法提供了 `BasicSupplier.create(MyType.class)` 这种较简洁的语法来代替较笨拙的 `new BasicSupplier <MyType>(MyType.class)`。
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例如，这是一个具有无参构造方法的简单类：
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```java
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// generics/CountedObject.java
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public class CountedObject {
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private static long counter = 0;
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private final long id = counter++;
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public long id() {
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return id;
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}
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@Override
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public String toString() {
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return "CountedObject " + id;
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}
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}
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```
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**CountedObject** 类可以跟踪自身创建了多少个实例，并通过 `toString()` 报告这些实例的数量。 **BasicSupplier** 可以轻松地为 **CountedObject** 创建 **Supplier**：
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```java
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// generics/BasicSupplierDemo.java
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import onjava.BasicSupplier;
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import java.util.stream.Stream;
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public class BasicSupplierDemo {
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public static void main(String[] args) {
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Stream.generate(
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BasicSupplier.create(CountedObject.class))
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.limit(5)
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.forEach(System.out::println);
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}
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}
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/* Output:
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CountedObject 0
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CountedObject 1
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CountedObject 2
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CountedObject 3
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CountedObject 4
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*/
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```
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泛型方法减少了产生 **Supplier** 对象所需的代码量。 Java 泛型强制传递 **Class** 对象，以便在 `create()` 方法中将其用于类型推断。
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<!-- Simplifying Tuple Use -->
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### 简化元组的使用
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使用类型参数推断和静态导入，我们将把早期的元组重写为更通用的库。在这里，我们使用重载的静态方法创建元组：
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```java
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// onjava/Tuple.java
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// Tuple library using type argument inference
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package onjava;
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public class Tuple {
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public static <A, B> Tuple2<A, B> tuple(A a, B b) {
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return new Tuple2<>(a, b);
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}
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public static <A, B, C> Tuple3<A, B, C>
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tuple(A a, B b, C c) {
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return new Tuple3<>(a, b, c);
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}
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public static <A, B, C, D> Tuple4<A, B, C, D>
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tuple(A a, B b, C c, D d) {
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return new Tuple4<>(a, b, c, d);
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}
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public static <A, B, C, D, E>
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Tuple5<A, B, C, D, E> tuple(A a, B b, C c, D d, E e) {
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return new Tuple5<>(a, b, c, d, e);
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}
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}
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```
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我们修改 **TupleTest.java** 来测试 **Tuple.java** :
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```java
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// generics/TupleTest2.java
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import onjava.Tuple2;
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import onjava.Tuple3;
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import onjava.Tuple4;
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import onjava.Tuple5;
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import static onjava.Tuple.tuple;
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public class TupleTest2 {
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static Tuple2<String, Integer> f() {
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return tuple("hi", 47);
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}
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static Tuple2 f2() {
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return tuple("hi", 47);
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}
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static Tuple3<Amphibian, String, Integer> g() {
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return tuple(new Amphibian(), "hi", 47);
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}
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static Tuple4<Vehicle, Amphibian, String, Integer> h() {
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return tuple(
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new Vehicle(), new Amphibian(), "hi", 47);
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}
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static Tuple5<Vehicle, Amphibian,
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String, Integer, Double> k() {
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return tuple(new Vehicle(), new Amphibian(),
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"hi", 47, 11.1);
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}
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public static void main(String[] args) {
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Tuple2<String, Integer> ttsi = f();
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System.out.println(ttsi);
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System.out.println(f2());
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System.out.println(g());
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System.out.println(h());
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System.out.println(k());
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}
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}
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/* Output:
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(hi, 47)
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(hi, 47)
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(Amphibian@14ae5a5, hi, 47)
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(Vehicle@135fbaa4, Amphibian@45ee12a7, hi, 47)
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(Vehicle@4b67cf4d, Amphibian@7ea987ac, hi, 47, 11.1)
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*/
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```
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请注意，`f()` 返回一个参数化的 **Tuple2** 对象，而 `f2()` 返回一个未参数化的 **Tuple2** 对象。编译器不会在这里警告 `f2()` ，因为返回值未以参数化方式使用。从某种意义上说，它被“向上转型”为一个未参数化的 **Tuple2** 。 但是，如果如果尝试将 `f2()` 的结果放入到参数化的 **Tuple2** 中，则编译器将发出警告。
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<!-- A Set Utility -->
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### 一个Set工具
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对于泛型方法的另一个示例，请考虑由 **Set** 表示的数学关系。这些被方便地定义为可用于所有不同类型的泛型方法：
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```java
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// onjava/Sets.java
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package onjava;
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import java.util.HashSet;
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import java.util.Set;
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public class Sets {
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public static <T> Set<T> union(Set<T> a, Set<T> b) {
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Set<T> result = new HashSet<>(a);
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result.addAll(b);
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return result;
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}
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public static <T>
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Set<T> intersection(Set<T> a, Set<T> b) {
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Set<T> result = new HashSet<>(a);
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result.retainAll(b);
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return result;
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}
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// Subtract subset from superset:
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public static <T> Set<T>
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difference(Set<T> superset, Set<T> subset) {
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Set<T> result = new HashSet<>(superset);
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result.removeAll(subset);
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return result;
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}
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// Reflexive--everything not in the intersection:
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public static <T> Set<T> complement(Set<T> a, Set<T> b) {
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return difference(union(a, b), intersection(a, b));
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}
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}
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```
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前三个方法通过将第一个参数的引用复制到新的 **HashSet** 对象中来复制第一个参数，因此不会直接修改参数集合。因此，返回值是一个新的 **Set** 对象。
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这四种方法代表数学集合操作： `union()` 返回一个包含两个参数并集的 **Set** ， `intersection()` 返回一个包含两个参数集合交集的 **Set** ， `difference()` 从 **superset** 中减去 **subset** 的元素 ，而 `complement()` 返回所有不在交集中的元素的 **Set**。作为显示这些方法效果的简单示例的一部分，下面是一个包含不同水彩名称的 **enum** ：
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```java
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// generics/watercolors/Watercolors.java
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package watercolors;
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public enum Watercolors {
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ZINC, LEMON_YELLOW, MEDIUM_YELLOW, DEEP_YELLOW,
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ORANGE, BRILLIANT_RED, CRIMSON, MAGENTA,
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ROSE_MADDER, VIOLET, CERULEAN_BLUE_HUE,
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PHTHALO_BLUE, ULTRAMARINE, COBALT_BLUE_HUE,
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PERMANENT_GREEN, VIRIDIAN_HUE, SAP_GREEN,
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YELLOW_OCHRE, BURNT_SIENNA, RAW_UMBER,
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BURNT_UMBER, PAYNES_GRAY, IVORY_BLACK
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}
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```
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为了方便起见（不必全限定所有名称），将其静态导入到以下示例中。本示例使用 **EnumSet** 轻松从 **enum** 中创建 **Set** 。（可以在[第二十二章 枚举](https://github.com/LingCoder/OnJava8/blob/master/docs/book/22-Enumerations.md)一章中了解有关 **EnumSet** 的更多信息。）在这里，静态方法 `EnumSet.range()` 要求提供所要在结果 **Set** 中创建的元素范围的第一个和最后一个元素：
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```java
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// generics/WatercolorSets.java
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import watercolors.*;
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import java.util.EnumSet;
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import java.util.Set;
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import static watercolors.Watercolors.*;
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import static onjava.Sets.*;
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public class WatercolorSets {
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public static void main(String[] args) {
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Set<Watercolors> set1 =
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EnumSet.range(BRILLIANT_RED, VIRIDIAN_HUE);
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Set<Watercolors> set2 =
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EnumSet.range(CERULEAN_BLUE_HUE, BURNT_UMBER);
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System.out.println("set1: " + set1);
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System.out.println("set2: " + set2);
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System.out.println(
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"union(set1, set2): " + union(set1, set2));
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Set<Watercolors> subset = intersection(set1, set2);
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System.out.println(
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"intersection(set1, set2): " + subset);
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System.out.println("difference(set1, subset): " +
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difference(set1, subset));
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System.out.println("difference(set2, subset): " +
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difference(set2, subset));
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System.out.println("complement(set1, set2): " +
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complement(set1, set2));
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}
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}
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/* Output:
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set1: [BRILLIANT_RED, CRIMSON, MAGENTA, ROSE_MADDER,
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VIOLET, CERULEAN_BLUE_HUE, PHTHALO_BLUE, ULTRAMARINE,
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COBALT_BLUE_HUE, PERMANENT_GREEN, VIRIDIAN_HUE]
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set2: [CERULEAN_BLUE_HUE, PHTHALO_BLUE, ULTRAMARINE,
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COBALT_BLUE_HUE, PERMANENT_GREEN, VIRIDIAN_HUE,
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SAP_GREEN, YELLOW_OCHRE, BURNT_SIENNA, RAW_UMBER,
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BURNT_UMBER]
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union(set1, set2): [BURNT_SIENNA, BRILLIANT_RED,
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YELLOW_OCHRE, MAGENTA, SAP_GREEN, CERULEAN_BLUE_HUE,
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ULTRAMARINE, VIRIDIAN_HUE, VIOLET, RAW_UMBER,
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ROSE_MADDER, PERMANENT_GREEN, BURNT_UMBER,
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PHTHALO_BLUE, CRIMSON, COBALT_BLUE_HUE]
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intersection(set1, set2): [PERMANENT_GREEN,
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CERULEAN_BLUE_HUE, ULTRAMARINE, VIRIDIAN_HUE,
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PHTHALO_BLUE, COBALT_BLUE_HUE]
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difference(set1, subset): [BRILLIANT_RED, MAGENTA,
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VIOLET, CRIMSON, ROSE_MADDER]
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difference(set2, subset): [BURNT_SIENNA, YELLOW_OCHRE,
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BURNT_UMBER, SAP_GREEN, RAW_UMBER]
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complement(set1, set2): [BURNT_SIENNA, BRILLIANT_RED,
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YELLOW_OCHRE, MAGENTA, SAP_GREEN, VIOLET, RAW_UMBER,
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ROSE_MADDER, BURNT_UMBER, CRIMSON]
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*/
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```
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接下来的例子使用 `Sets.difference()` 方法来展示 **java.util** 包中各种 **Collection** 和 **Map** 类之间的方法差异：
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```java
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// onjava/CollectionMethodDifferences.java
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// {java onjava.CollectionMethodDifferences}
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package onjava;
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import java.lang.reflect.Method;
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import java.util.*;
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import java.util.stream.Collectors;
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public class CollectionMethodDifferences {
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static Set<String> methodSet(Class<?> type) {
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return Arrays.stream(type.getMethods())
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.map(Method::getName)
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.collect(Collectors.toCollection(TreeSet::new));
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}
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static void interfaces(Class<?> type) {
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System.out.print("Interfaces in " +
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type.getSimpleName() + ": ");
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System.out.println(
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Arrays.stream(type.getInterfaces())
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.map(Class::getSimpleName)
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.collect(Collectors.toList()));
|
||||
}
|
||||
|
||||
static Set<String> object = methodSet(Object.class);
|
||||
|
||||
static {
|
||||
object.add("clone");
|
||||
}
|
||||
|
||||
static void
|
||||
difference(Class<?> superset, Class<?> subset) {
|
||||
System.out.print(superset.getSimpleName() +
|
||||
" extends " + subset.getSimpleName() +
|
||||
", adds: ");
|
||||
Set<String> comp = Sets.difference(
|
||||
methodSet(superset), methodSet(subset));
|
||||
comp.removeAll(object); // Ignore 'Object' methods
|
||||
System.out.println(comp);
|
||||
interfaces(superset);
|
||||
}
|
||||
|
||||
public static void main(String[] args) {
|
||||
System.out.println("Collection: " +
|
||||
methodSet(Collection.class));
|
||||
interfaces(Collection.class);
|
||||
difference(Set.class, Collection.class);
|
||||
difference(HashSet.class, Set.class);
|
||||
difference(LinkedHashSet.class, HashSet.class);
|
||||
difference(TreeSet.class, Set.class);
|
||||
difference(List.class, Collection.class);
|
||||
difference(ArrayList.class, List.class);
|
||||
difference(LinkedList.class, List.class);
|
||||
difference(Queue.class, Collection.class);
|
||||
difference(PriorityQueue.class, Queue.class);
|
||||
System.out.println("Map: " + methodSet(Map.class));
|
||||
difference(HashMap.class, Map.class);
|
||||
difference(LinkedHashMap.class, HashMap.class);
|
||||
difference(SortedMap.class, Map.class);
|
||||
difference(TreeMap.class, Map.class);
|
||||
}
|
||||
}
|
||||
/* Output:
|
||||
Collection: [add, addAll, clear, contains, containsAll,
|
||||
equals, forEach, hashCode, isEmpty, iterator,
|
||||
parallelStream, remove, removeAll, removeIf, retainAll,
|
||||
size, spliterator, stream, toArray]
|
||||
Interfaces in Collection: [Iterable]
|
||||
Set extends Collection, adds: []
|
||||
Interfaces in Set: [Collection]
|
||||
HashSet extends Set, adds: []
|
||||
Interfaces in HashSet: [Set, Cloneable, Serializable]
|
||||
LinkedHashSet extends HashSet, adds: []
|
||||
Interfaces in LinkedHashSet: [Set, Cloneable,
|
||||
Serializable]
|
||||
TreeSet extends Set, adds: [headSet,
|
||||
descendingIterator, descendingSet, pollLast, subSet,
|
||||
floor, tailSet, ceiling, last, lower, comparator,
|
||||
pollFirst, first, higher]
|
||||
Interfaces in TreeSet: [NavigableSet, Cloneable,
|
||||
Serializable]
|
||||
List extends Collection, adds: [replaceAll, get,
|
||||
indexOf, subList, set, sort, lastIndexOf, listIterator]
|
||||
Interfaces in List: [Collection]
|
||||
ArrayList extends List, adds: [trimToSize,
|
||||
ensureCapacity]
|
||||
Interfaces in ArrayList: [List, RandomAccess,
|
||||
Cloneable, Serializable]
|
||||
LinkedList extends List, adds: [offerFirst, poll,
|
||||
getLast, offer, getFirst, removeFirst, element,
|
||||
removeLastOccurrence, peekFirst, peekLast, push,
|
||||
pollFirst, removeFirstOccurrence, descendingIterator,
|
||||
pollLast, removeLast, pop, addLast, peek, offerLast,
|
||||
addFirst]
|
||||
Interfaces in LinkedList: [List, Deque, Cloneable,
|
||||
Serializable]
|
||||
Queue extends Collection, adds: [poll, peek, offer,
|
||||
element]
|
||||
Interfaces in Queue: [Collection]
|
||||
PriorityQueue extends Queue, adds: [comparator]
|
||||
Interfaces in PriorityQueue: [Serializable]
|
||||
Map: [clear, compute, computeIfAbsent,
|
||||
computeIfPresent, containsKey, containsValue, entrySet,
|
||||
equals, forEach, get, getOrDefault, hashCode, isEmpty,
|
||||
keySet, merge, put, putAll, putIfAbsent, remove,
|
||||
replace, replaceAll, size, values]
|
||||
HashMap extends Map, adds: []
|
||||
Interfaces in HashMap: [Map, Cloneable, Serializable]
|
||||
LinkedHashMap extends HashMap, adds: []
|
||||
Interfaces in LinkedHashMap: [Map]
|
||||
SortedMap extends Map, adds: [lastKey, subMap,
|
||||
comparator, firstKey, headMap, tailMap]
|
||||
Interfaces in SortedMap: [Map]
|
||||
TreeMap extends Map, adds: [descendingKeySet,
|
||||
navigableKeySet, higherEntry, higherKey, floorKey,
|
||||
subMap, ceilingKey, pollLastEntry, firstKey, lowerKey,
|
||||
headMap, tailMap, lowerEntry, ceilingEntry,
|
||||
descendingMap, pollFirstEntry, lastKey, firstEntry,
|
||||
floorEntry, comparator, lastEntry]
|
||||
Interfaces in TreeMap: [NavigableMap, Cloneable,
|
||||
Serializable]
|
||||
*/
|
||||
```
|
||||
|
||||
在[第十二章 集合](https://github.com/LingCoder/OnJava8/blob/master/docs/book/12-Collections.md)的[本章小结](https://github.com/LingCoder/OnJava8/blob/master/docs/book/12-Collections.md#本章小结)部分将会用到这里的输出结果。
|
||||
|
||||
<!-- Building Complex Models -->
|
||||
## 复杂模型构建
|
||||
|
||||
Reference in New Issue
Block a user