Guava的介绍与使用示例

Guava是什么

Guava，翻译是番石榴。这里指的是Google Core Libraries for Java， 即Google开发的Java核心库的扩展。
其提供新的集合类型（比如multimap 和 multiset）、不变集合（immutable collections）、并发库 （concurrency libraries）、I/O、图库、以及字符串等。

https://github.com/google/guava
https://guava.dev/releases/snapshot-jre/api/docs/

Guava提供了JRE和Android两个版本。

Guava导入

目前最新版本是28.1-jre，使用Maven导入方式如下：

		<dependency>
		    <groupId>com.google.guava</groupId>
		    <artifactId>guava</artifactId>
		    <version>28.1-jre</version>
		</dependency>

代码示例

/**
 * @Title: GuavaTests.java
 * @Package com.edu.badou.base
 * @Description: TODO
 * @author oscarchen
 * @date Oct 25, 2019 3:05:49 PM
 * @version V1.0
 */

package com.edu.badou.base;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.junit.jupiter.api.Test;

import com.google.common.base.CharMatcher;
import com.google.common.base.Joiner;
import com.google.common.base.Splitter;
import com.google.common.base.Strings;
import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Multimap;
import com.google.common.collect.Sets;

/**
 * @ClassName: GuavaTests
 * @Description: TODO
 * @author oscarchen
 */

public class GuavaTests {

    @Test // List使用对比
    public void list() {
        List<String> javaList = new ArrayList<String>();
        javaList.add("Java Standard List");
        List<String> guavaList = Lists.newArrayList();
        guavaList.add("Google Guava List");
        System.out.println(javaList.toString());
        System.out.println(guavaList.toString());
    }

    @SuppressWarnings("unused")
    @Test
    public void collections() {
        List<String> list = Lists.newArrayList();
        Set<String> set = Sets.newHashSet();
        Map<String, String> map = Maps.newHashMap();
    }

    @SuppressWarnings("unused")
    @Test // 不可变集合
    public void immutableCollections() {
        ImmutableList<String> iList = ImmutableList.of("a", "b", "c");
        ImmutableSet<String> iSet = ImmutableSet.of("a", "b", "c");
        ImmutableMap<String, String> iMap = ImmutableMap.of("key1", "value1", "key2", "value2");
    }

    @Test // Map的元素是List类型
    public void mapList() {
        // JDK标准处理
        Map<String, List<Integer>> map = new HashMap<String, List<Integer>>();
        List<Integer> list = new ArrayList<Integer>();
        list.add(1);
        list.add(2);
        map.put("test", list);

        // Guava处理
        Multimap<String, Integer> mapM = ArrayListMultimap.create();
        mapM.put("test", 1);
        mapM.put("test", 2);
    }

    @Test // Joiner 字符串连接
    public void stringJoin() {
        StringBuilder stringBuilder = new StringBuilder("开始：");
        Joiner joiner = Joiner.on("|").skipNulls(); // 设置连接符，忽略空对象
        stringBuilder = joiner.appendTo(stringBuilder, "1", "2", null, "3", "4", "5");
        System.out.println(stringBuilder.toString());
    }

    @Test // Map转换为字符串
    public void mapToString() {
        Map<String, String> map = Maps.newLinkedHashMap();
        map.put("Key1", "Value1");
        map.put("Key2", "Value2");
        String returnedString = Joiner.on("#").withKeyValueSeparator(":").join(map); // #分隔键值，:分隔元素
        System.out.println(returnedString);
    }

    @Test
    public void stringToMap() {
        String str = "Key1:Value1#Key2:Value2";
        Splitter.MapSplitter ms = Splitter.on("#").withKeyValueSeparator(':');
        Map<String, String> map = ms.split(str);
        for (String key : map.keySet()) {
            System.out.println(key + " -> " + map.get(key));
        }
    }
    
    @Test
    public void strings() {
        //字符串是否为null或空字串
        System.out.println(Strings.isNullOrEmpty(""));
        System.out.println(Strings.isNullOrEmpty(null));
        //从null转为空字串
        System.out.println(Strings.nullToEmpty(null));
        //字符串开始处补充到指定长度
        System.out.println(Strings.padStart("hello", 8, 'T'));
        //字符串尾部补充道指定长度
        System.out.println(Strings.padEnd("hello", 8, 'T'));       
    }
    
    
    @Test //字符匹配和替换
    public void charMatcher() {
        String str = "空格    换行\r 换行\r\n  制表符\t";
        str = CharMatcher.breakingWhitespace().replaceFrom(str,'#');
        System.out.println(str);       
    }
}