03 Strategy策略

抽丝剥茧设计模式 之 Strategy策略 - 更多内容请见 目录

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一、Strategy策略

策略模式是一种设计模式，它定义了一系列的算法，并将每个算法封装起来，使它们可以互相替换。Java中的Comparable和Comparator两个接口确实是策略模式的典型应用。

在Go语言的基本类库中，也有一些接口和实现是基于策略模式的。例如，io.Reader和io.Writer就是这样的接口。这两个接口定义了读取和写入数据的通用方法，但具体的实现可以根据不同的需求进行替换。你可以使用内存中的字节切片作为数据源，也可以使用文件、网络连接或其他任何数据源作为数据源，只要实现io.Reader和io.Writer接口即可。

另一个例子是http.Handler接口，它定义了处理HTTP请求的方法。你可以编写自己的实现该接口的函数，然后将其注册到HTTP服务器上，以处理特定的URL路径或路由。

这些例子中，接口定义了一组通用的方法，而具体的实现可以根据不同的需求进行替换。这种模式使得代码更加灵活，易于扩展和维护，符合策略模式的思想。

二、Comparable和Comparator源码分析

以下代码分析，基于openjdk-jdk8-b120，我们可以注意到，我们自定义了一个Comparator的子类，用于封装一组可以对对象进行比较的算法。传给sort()方法作为参数，用于决定排序的实际策略。

使用案例

首先看两个使用案例，Arrays.sort和Collections.sort都可以接收一个自定义的Comparator，用于对数组排序。

// Arrays.sort
import java.util.Arrays;
import java.util.Comparator;public class Main {public static void main(String[] args) {Integer[] numbers = {3, 1, 2, 4};// 使用自定义的Comparator对数组进行排序Comparator<Integer> comparator = new MyComparator();Arrays.sort(numbers, comparator);System.out.println(Arrays.toString(numbers)); // 输出: [1, 2, 3, 4]}static class MyComparator implements Comparator<Integer> {@Overridepublic int compare(Integer o1, Integer o2) {return o1.compareTo(o2);}}
}

// Collections.sort
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;public class Main {public static void main(String[] args) {List<String> names = new ArrayList<>();names.add("Alice");names.add("Bob");names.add("Charlie");// 使用自定义的比较器对列表进行排序Comparator<String> comparator = new LengthComparator();Collections.sort(names, comparator);System.out.println(names); // 输出: [Charlie, Bob, Alice]}static class LengthComparator implements Comparator<String> {@Overridepublic int compare(String o1, String o2) {return Integer.compare(o1.length(), o2.length());}}
}

Arrays.sort源码

package java.util;public class Arrays {private Arrays() {}public static <T> void sort(T[] a, Comparator<? super T> c) {if (c == null)c = NaturalOrder.INSTANCE;if (LegacyMergeSort.userRequested)legacyMergeSort(a, c);elseTimSort.sort(a, 0, a.length, c, null, 0, 0);}
}

Collections.sort源码

package java.util;
public class Collections {// Suppresses default constructor, ensuring non-instantiability.private Collections() {}@SuppressWarnings({"unchecked", "rawtypes"})public static <T> void sort(List<T> list, Comparator<? super T> c) {Object[] a = list.toArray();Arrays.sort(a, (Comparator)c);ListIterator<T> i = list.listIterator();for (int j=0; j<a.length; j++) {i.next();i.set((T)a[j]);}}
}

Comparable源码

package java.lang;
import java.util.*;
public interface Comparable<T> {public int compareTo(T o);
}

Comparator源码

package java.util;import java.io.Serializable;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.function.ToDoubleFunction;
import java.util.Comparators;@FunctionalInterface
public interface Comparator<T> {int compare(T o1, T o2);boolean equals(Object obj);default Comparator<T> reversed() {return Collections.reverseOrder(this);}default Comparator<T> thenComparing(Comparator<? super T> other) {Objects.requireNonNull(other);return (Comparator<T> & Serializable) (c1, c2) -> {int res = compare(c1, c2);return (res != 0) ? res : other.compare(c1, c2);};}default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor,Comparator<? super U> keyComparator){return thenComparing(comparing(keyExtractor, keyComparator));}default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor){return thenComparing(comparing(keyExtractor));}default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor) {return thenComparing(comparingInt(keyExtractor));}default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor) {return thenComparing(comparingLong(keyExtractor));}default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) {return thenComparing(comparingDouble(keyExtractor));}public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {return Collections.reverseOrder();}@SuppressWarnings("unchecked")public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;}public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {return new Comparators.NullComparator<>(true, comparator);}public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {return new Comparators.NullComparator<>(false, comparator);}public static <T, U> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor,Comparator<? super U> keyComparator){Objects.requireNonNull(keyExtractor);Objects.requireNonNull(keyComparator);return (Comparator<T> & Serializable)(c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),keyExtractor.apply(c2));}public static <T, U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor){Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)(c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));}public static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor) {Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)(c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));}public static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor) {Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)(c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));}public static<T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor) {Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)(c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));}
}