Guava工具类使用介绍

最近常用到guava工具的东西，每次都要百度查询使用方法，现统一记录一下方便本身使用。

参考文章：Google guava工具类的介绍和使用、Guava工具类学习

引入依赖

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>28.0-jre</version>
</dependency>

<<最新依赖版本号点击这里获取>>

集合

建立集合

List<String> list = Lists.newArrayList();
Set<String> set = Sets.newHashSet();
Map<String, String> map = Maps.newHashMap();

不可变集合

ImmutableList<String> iList = ImmutableList.of("a", "b", "c");
ImmutableSet<String> iSet = ImmutableSet.of("e1", "e2");
ImmutableMap<String, String> iMap = ImmutableMap.of("k1", "v1", "k2", "v2");

Multimap

Multimap的特色其实就是能够包含有几个重复Key的value，你能够put进入多个不一样value可是相同的key，可是又不是让后面覆盖前面的内容。

它的业务场景：当你须要构造像Map<K, List<V>>或者Map<K, Set<V>>这样比较复杂的集合类型的数据结构，来作相应的业务逻辑处理。那Multimap在合适不过。

传统作法：

//Item就是封装的对象
Map<String,List<Item>> map = new HashMap<>();
for (Item item : list){
  List<Item> tmp = map.get(item.getName());
  if (null == tmp){
      tmp = new ArrayList<>();
      map.put(item.getName(),tmp);
  }
  tmp.add(item);
}

如今：

Multimap<String,Item> multiMap = ArrayListMultimap.create();
for (Item item : list){
    multiMap.put(item.getName(),item);
}

相似的还有：

MultiSet: 无序+可重复   count()方法获取单词的次数  加强了可读性+操做简单
建立方式:  Multiset<String> set = HashMultiset.create();

Multimap: key-value  key能够重复  
建立方式: Multimap<String, String> teachers = ArrayListMultimap.create();

BiMap: 双向Map(Bidirectional Map) 键与值都不能重复
建立方式:  BiMap<String, String> biMap = HashBiMap.create();

Table: 双键的Map Map--> Table-->rowKey+columnKey+value  //和sql中的联合主键有点像
建立方式: Table<String, String, Integer> tables = HashBasedTable.create();

将集合转换为特定规则的字符串

List<String> list = new ArrayList<String>();
list.add("aa");
list.add("bb");
list.add("cc");
String result = Joiner.on("-").join(list);
//result为  aa-bb-cc

把map集合转换为特定规则的字符串

Map<String, Integer> map = Maps.newHashMap();
map.put("xiaoming", 12);
map.put("xiaohong",13);
String result = Joiner.on(",").withKeyValueSeparator("=").join(map);
// result为 xiaoming=12,xiaohong=13

将String转换为特定的集合

//use guava
String str = "1-2-3-4-5-6";
List<String> list = Splitter.on("-").splitToList(str);
//list为  [1, 2, 3, 4, 5, 6]

String str = "1-2-3-4-  5-  6   ";  
List<String> list = Splitter.on("-").omitEmptyStrings().trimResults().splitToList(str);
System.out.println(list);
//[1, 2, 3, 4, 5, 6]  能够忽略中间的空格

将String转换为map

String str = "xiaoming=11,xiaohong=23";
Map<String,String> map = Splitter.on(",").withKeyValueSeparator("=").split(str);

set的交集, 并集, 差集

HashSet setA = newHashSet(1, 2, 3, 4, 5);  
HashSet setB = newHashSet(4, 5, 6, 7, 8);  
   
SetView union = Sets.union(setA, setB);  
System.out.println("union:");  
for (Integer integer : union)  
    System.out.println(integer);           //union:12345867
   
SetView difference = Sets.difference(setA, setB);  
System.out.println("difference:");  
for (Integer integer : difference)  
    System.out.println(integer);        //difference:123
   
SetView intersection = Sets.intersection(setA, setB);  
System.out.println("intersection:");  
for (Integer integer : intersection)  
    System.out.println(integer);  //intersection:45

map的交集，并集，差集

MapDifference differenceMap = Maps.difference(mapA, mapB);  
differenceMap.areEqual();  
Map entriesDiffering = differenceMap.entriesDiffering();  
Map entriesOnlyOnLeft = differenceMap.entriesOnlyOnLeft();  
Map entriesOnlyOnRight = differenceMap.entriesOnlyOnRight();  
Map entriesInCommon = differenceMap.entriesInCommon();

MoreObjects

Person person = new Person("aa",11);
String str = MoreObjects.toStringHelper("Person").add("age", person.getAge()).toString();
System.out.println(str);  
//输出Person{age=11}

字符串链接器Joiner

StringBuilder stringBuilder = new StringBuilder("hello");
// 字符串链接器，以|为分隔符，同时去掉null元素
Joiner joiner1 = Joiner.on("|").skipNulls();
// 构成一个字符串foo|bar|baz并添加到stringBuilder
stringBuilder = joiner1.appendTo(stringBuilder, "foo", "bar", null, "baz");
System.out.println(stringBuilder); // hellofoo|bar|baz