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Googleguava工具类中Lists、Maps、Sets简单使用

时间：2023-07-30
Google guava

Guava是对Java API的补充，对Java开发中常用功能进行更优雅的实现，使得编码更加轻松，代码容易理解。Guava使用了多种设计模式，同时经过了很多测试，得到了越来越多开发团队的青睐。Java最新版本的API采纳了Guava的部分功能，但依旧无法替代。

特点

高效设计良好的API，被Google的开发者设计，实现和使用遵循高效的java语法实践使代码更刻度，简洁，简单节约时间，资源，提高生产力  Guava工程包含了若干被Google的 Java项目广泛依赖的核心库

核心库例如：

集合 [collections]缓存 [caching]原生类型支持 [primitives support]并发库 [concurrency libraries]通用注解 [common annotations]字符串处理 [string processing]I/O 等等。        github地址

https://github.com/google/guavahttps://github.com/google/guava

使用         依赖引入

com.google.guava guava 28.1-jre

 Guava 集合工具类 Lists

        官网文档：

Lists (Guava: Google Core Libraries for Java 27.0.1-jre API)https://guava.dev/releases/27.0.1-jre/api/docs/com/google/common/collect/Lists.html对应于List集合接口, 在com.google.common.collect包下

 Lists 接口的声明如下：

@GwtCompatible(emulated=true)public final class Lists extends Object

 Lists 类主要提供了对List类的子类构造以及操作的静态方法。在Lists类中支持构造 ArrayList、linkedList 以及 newCopyOnWriteArrayList 对象的方法。其中提供了以下构造ArrayList的函数：下面四个构造一个 ArrayList 对象，但是不显式的给出申请空间的大小:

newArrayList()newArrayList(E..、elements)newArrayList(Iterable<? extends E> elements)newArrayList(Iterator<? extends E> elements)

 测试

ArrayList objects = Lists.newArrayList(); objects.add("张三"); objects.add(20); System.out.println("--- newArrayList test ---"); System.out.println(objects); System.out.println("--- newArrayList test ---"); ArrayList objects1 = Lists.newArrayList(objects); System.out.println(objects1); System.out.println("--- newArrayList test ---"); ArrayList strings = Lists.newArrayList("张三", "北京市海淀区"); System.out.println(strings);

以下两个函数在构造 ArrayList 对象的时候给出了需要分配空间的大小:

newArrayListWithCapacity(int initialArraySize)newArrayListWithExpectedSize(int estimatedSize)

//如果你事先知道元素的个数，可以用 newArrayListWithCapacity 函数；如果你不能确定元素的个数，可以用newArrayListWithExpectedSize函数 //这个方法就是直接返回一个10的数组。 ArrayList objects = Lists.newArrayListWithCapacity(10); objects.add("123"); System.out.println(objects); ArrayList objects1 = Lists.newArrayListWithExpectedSize(10); objects1.add("123"); System.out.println(objects1);

在 newArrayListWithExpectedSize 函数里面调用了 computeArrayListCapacity(int arraySize) 函数，其实现如下：

@VisibleForTesting static int computeArrayListCapacity(int arraySize) { checkArgument(arraySize >= 0);　　　　　// TODO(kevinb): Figure out the right behavior, and document it return Ints.saturatedCast(5L + arraySize + (arraySize / 10));}

         返回的容量大小为 5L + arraySize + (arraySize / 10)，当 arraySize 比较大的时候，给定大小和真正分配的容量之比为 10/11。

Lists 类还支持构造 linkedList、newCopyOnWriteArrayList 对象，其函数接口为：

newlinkedList()newlinkedList(Iterable<? extends E> elements)　　　newCopyonWriteArrayList()newCopyonWriteArrayList(Iterable<? extends E> elements)

linkedList objects = Lists.newlinkedList(); objects.add("张三"); objects.add("李四"); System.out.println("--- newlinkedList ---"); System.out.println(objects); ArrayList objects1 = Lists.newArrayList(objects); System.out.println("--- newlinkedList ---"); System.out.println(objects1); System.out.println("--- newCopyonWriteArrayList ---"); CopyOnWriteArrayList objects2 = Lists.newCopyonWriteArrayList(); objects2.add("王五"); objects2.add("张三"); System.out.println(objects2); System.out.println("--- newCopyonWriteArrayList ---"); CopyOnWriteArrayList objects3 = Lists.newCopyonWriteArrayList(objects2); System.out.println(objects3);

 

我们还可以将两个（或三个）类型相同的数据存放在一个list中，这样可以传入到只有一个参数的函数或者需要减少参数的函数中，这些函数如下：

asList(@Nullable E first, E[] rest)asList(@Nullable E first, @Nullable E second, E[] rest)

String str = "i love u"; String[] strs = {"i like u", "i miss u"}; List strings = Lists.asList(str, strs); System.out.println(strings);

 

 Lists 类中 transform 函数可以根据传进来的 function 对 fromList 进行相应的处理，并将处理得到的结果存入到新的list对象中，这样有利于我们进行分析，函数接口如下：

public static List transform(List fromList, Function<? super F, ? extends T> function)

Function strlen = new Function() { public Integer apply(String from) { Preconditions.checkNotNull(from); return from.length(); } }; List from = Lists.newArrayList("abc", "defg", "hijkl"); List to = Lists.transform(from, strlen); for (int i = 0; i < from.size(); i++) { System.out.printf("%s has length %dn", from.get(i), to.get(i)); }

 

 

Maps

        官方文档

Maps (Guava: Google Core Libraries for Java 27.0.1-jre API)https://guava.dev/releases/27.0.1-jre/api/docs/com/google/common/collect/Maps.html com.google.common.collect.Maps 接口的声明：

@GwtCompatible(emulated=true)public final class Maps extends Object

newHashMapnewHashMapWithExpectedSizenewlinkedHashMap ImmutableMapdifferencetransformValues

测试

newHashMap

HashMap hashMap = Maps.newHashMap(); for (int i = 0; i < 10; i++) { hashMap.put(i, i); } System.out.println("hashMap：" + hashMap); HashMap hashMap1 = Maps.newHashMap(hashMap); System.out.println("hashMap1：" + hashMap1);

 newHashMapWithExpectedSize

Map map = Maps.newHashMapWithExpectedSize(3); map.put(1, 1); map.put(2, 2); map.put(3, 3); System.out.println("map：" + map); // map：{1=1, 2=2, 3=3}

 

 newlinkedHashMap

Map map = Maps.newlinkedHashMap(); map.put(11, 11); System.out.println("map：" + map); linkedHashMap map1 = Maps.newlinkedHashMap(map); System.out.println("map1：" + map1);

 

 ImmutableMap

ImmutableMap a = ImmutableMap.of("a", "1"); System.out.println(a);

 

difference

Map map = Maps.newHashMap(); map.put(10, 10); Map map1 = Maps.newHashMap(); map1.put(10, 10); map1.put(20, 20); MapDifference difference = Maps.difference(map, map1); System.out.println(difference.areEqual()); System.out.println(difference.entriesInCommon()); System.out.println(difference.entriesOnlyOnRight()); System.out.println(difference.entriesOnlyOnLeft()); System.out.println(difference.entriesDiffering()); System.out.println(difference);

transformValues

Map fromMap = Maps.newHashMap(); fromMap.put("key", true); fromMap.put("value", false); // 对传入的元素取反 System.out.println(Maps.transformValues(fromMap, (Function) input -> !input)); // value为假，则key变大写 Maps.EntryTransformer entryTransformer = (key, value) -> value ? key : key.toUpperCase(); System.out.println(Maps.transformEntries(fromMap, entryTransformer));

  Sets           官方文档

Sets (Guava: Google Core Libraries for Java 27.0.1-jre API)https://guava.dev/releases/27.0.1-jre/api/docs/com/google/common/collect/Sets.htmlcom.google.common.collect.Sets 接口的声明：

@GwtCompatible(emulated=true)public final class Sets extends Object

newHashSetfilterdifferencesymmetricDifferenceintersectionunioncartesianProductpowerSet

newHashSet

HashSet objects = Sets.newHashSet(); objects.add("张三"); objects.add("李四"); System.out.println(objects);

 

filter

@Test public void testFilter() { Set set = Sets.newHashSet("i like u", "i miss u", "i love u"); Predicate predicate = new Predicate() { @Override public boolean apply(String input) { //过滤包含字母l的元素 return input.contains("l"); } }; System.out.println(Sets.filter(set, predicate)); // [i like u, i love u] System.out.println(Sets.filter(set, input -> input.contains("l"))); // [i like u, i love u] }

difference

@Test public void testDifference() { Set set1 = Sets.newHashSet(1, 2, 3, 4, 5); Set set2 = Sets.newHashSet(1, 3, 5, 7, 9); System.out.println(Sets.difference(set1, set2)); // [2, 4] }

 

symmetricDifference

@Test public void testSymmetricDifference() { Set set1 = Sets.newHashSet(1, 2, 3, 4, 5); Set set2 = Sets.newHashSet(1, 3, 5, 7, 9); System.out.println(Sets.symmetricDifference(set1, set2)); // [2, 4, 9, 7] }

 

intersection

@Test public void testIntersection() { Set set1 = Sets.newHashSet(1, 2, 3, 4, 5); Set set2 = Sets.newHashSet(1, 3, 5, 7, 9); System.out.println(Sets.intersection(set1, set2)); // [1, 3, 5] }

Union

@Test public void testUnion() { Set set1 = Sets.newHashSet(1, 2, 3, 4, 5); Set set2 = Sets.newHashSet(1, 3, 5, 7, 9); System.out.println(Sets.union(set1, set2)); // [1, 2, 3, 4, 5, 9, 7] }

 cartesianProduct

@Test public void testCartesianProduct() { Set set1 = Sets.newHashSet("i love u", "i hate u"); Set set2 = Sets.newHashSet("tom", "jerry"); Set> sets = Sets.cartesianProduct(set1, set2); System.out.println(sets); // [[i hate u, tom], [i hate u, jerry], [i love u, tom], [i love u, jerry]] }

powerSet

@Test public void testPowerSet() { Set set1 = Sets.newHashSet("A", "B", "C"); Set> sets = Sets.powerSet(set1);// for (Set set : sets) {// System.out.println(set);// } System.out.println(Arrays.toString(sets.toArray())); // [[], [A], [B], [A, B], [C], [A, C], [B, C], [A, B, C]] }

 完整代码

        maven项目里用junit进行单元测试

import com.google.common.base.Predicate;import com.google.common.collect.Sets;import org.junit.Test;import java.util.Arrays;import java.util.List;import java.util.Set;public class TestSets { @Test public void testFilter() { Set set = Sets.newHashSet("i like u", "i miss u", "i love u"); Predicate predicate = new Predicate() { @Override public boolean apply(String input) { //过滤包含字母l的元素 return input.contains("l"); } }; System.out.println(Sets.filter(set, predicate)); // [i like u, i love u] System.out.println(Sets.filter(set, input -> input.contains("l"))); // [i like u, i love u] } @Test public void testDifference() { Set set1 = Sets.newHashSet(1, 2, 3, 4, 5); Set set2 = Sets.newHashSet(1, 3, 5, 7, 9); System.out.println(Sets.difference(set1, set2)); // [2, 4] } @Test public void testSymmetricDifference() { Set set1 = Sets.newHashSet(1, 2, 3, 4, 5); Set set2 = Sets.newHashSet(1, 3, 5, 7, 9); System.out.println(Sets.symmetricDifference(set1, set2)); // [2, 4, 9, 7] } @Test public void testIntersection() { Set set1 = Sets.newHashSet(1, 2, 3, 4, 5); Set set2 = Sets.newHashSet(1, 3, 5, 7, 9); System.out.println(Sets.intersection(set1, set2)); // [1, 3, 5] } @Test public void testUnion() { Set set1 = Sets.newHashSet(1, 2, 3, 4, 5); Set set2 = Sets.newHashSet(1, 3, 5, 7, 9); System.out.println(Sets.union(set1, set2)); // [1, 2, 3, 4, 5, 9, 7] } @Test public void testCartesianProduct() { Set set1 = Sets.newHashSet("i love u", "i hate u"); Set set2 = Sets.newHashSet("tom", "jerry"); Set> sets = Sets.cartesianProduct(set1, set2); System.out.println(sets); // [[i hate u, tom], [i hate u, jerry], [i love u, tom], [i love u, jerry]] } @Test public void testPowerSet() { Set set1 = Sets.newHashSet("A", "B", "C"); Set> sets = Sets.powerSet(set1);// for (Set set : sets) {// System.out.println(set);// } System.out.println(Arrays.toString(sets.toArray())); // [[], [A], [B], [A, B], [C], [A, C], [B, C], [A, B, C]] }}

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