guava是什么（java第三方工具类库）

guava是什么

Guava是Java项目广泛第三方库，其中包括：集合 、缓存 、原生类型支持 、并发库 、通用注解 、字符串处理 、I/O 等等。 所有这些工具每天都在被java工程b师应用在业务服务中。

集合

再jdk1.7之前，原生jdk对集合操作并不友好，使用guava的集合很大程度上方便了我们对集合的操作。

不可变集合

新集合类型

Multiset

很方便获取集合中元素的个数

public static void main(String[] args) { //此时也是使用guava 很方便吧，直接可以复制，不用在,泛型使用的也更加简单了 // List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f", "a", "c", "d", "a"); //原始方法求集合中元素的个数 List<String> strings = Lists.newArrayList("a", "b", "c", "d", "e", "f", "a", "c", "d", "a"); Map<String, Integer> countMap = Maps.newHashMap(); for (String string : strings) { if (countMap.containsKey(string)){ Integer count = countMap.get(string); countMap.put(string,++count); }else { countMap.put(string,1); } } System.out.println(new Gson().toJson(countMap)); //使用guava的新集合 Multiset multiset = HashMultiset.create(); //求交集 multiset.addAll(strings); System.out.println("新集合元素："+multiset); System.out.println("a的次数："+ multiset.count("a")); System.out.println("c的次数："+multiset.count("c")); System.out.println("f的次数："+multiset.count("f")); //还可以自定义设置参数 multiset.setCount("a",1000); System.out.println(multiset); System.out.println("a的次数："+ multiset.count("a")); } } 

reslut：

{"a":3,"b":1,"c":2,"d":2,"e":1,"f":1} 新集合元素：[a x 3, b, c x 2, d x 2, e, f] a的次数：3 c的次数：2 f的次数：1 [a x 1000, b, c x 2, d x 2, e, f] a的次数：1000 

ListMultimap

集合分组

 public static void main(String[] args) { //list转map，分别计算出3年级与2年级的学生信息 //传统做法 Student student = new Student(19,"王强" , 1, 3); Student student1 = new Student(18,"张强" , 1, 2); Student student2 = new Student(18,"李静" , 2, 2); Student student3 = new Student(19,"王萍" , 1, 3); List<Student> strings = Lists.newArrayList(student,student1,student2,student3); Map<Integer, List<Student>> studentMap = Maps.newHashMap(); for (Student string : strings) { if (studentMap.containsKey(string.getGrade())){ List<Student> students = studentMap.get(string.getGrade()); students.add(string); }else { List<Student> objects = Lists.newArrayList(); objects.add(string); studentMap.put(string.getGrade(),objects); } } System.out.println(new Gson().toJson(studentMap)); System.out.println("****************************************************guava分割**************************************************************"); //第一个泛型就是分组的key 第二个泛型就是集合的类型 ListMultimap<Integer, Student> build = MultimapBuilder.treeKeys().arrayListValues().build(); for (Student string : strings) { //一行代码就搞定 build.put(string.getGrade(),string); } //转成map同上面相同 System.out.println("转map："+new Gson().toJson(build.asMap())); System.out.println("班级2集合："+new Gson().toJson(build.get(2))); System.out.println("班级3集合："+new Gson().toJson(build.get(3))); //删除班级2的王强 boolean remove = build.remove(2, student1); System.out.println("删除班级2的张强转map："+new Gson().toJson(build.asMap())); //删除班级3的所有人 List<Student> students = build.removeAll(3); System.out.println("删除班级3的所有人转map："+new Gson().toJson(build.asMap())); } 

result

{"2":[{"age":18,"name":"张强","sex":1,"grade":2},{"age":18,"name":"李静","sex":2,"grade":2}],"3":[{"age":19,"name":"王强","sex":1,"grade":3},{"age":19,"name":"王萍","sex":1,"grade":3}]} ****************************************************guava分割************************************************************** 转map：{"2":[{"age":18,"name":"张强","sex":1,"grade":2},{"age":18,"name":"李静","sex":2,"grade":2}],"3":[{"age":19,"name":"王强","sex":1,"grade":3},{"age":19,"name":"王萍","sex":1,"grade":3}]} 班级2集合：[{"age":18,"name":"张强","sex":1,"grade":2},{"age":18,"name":"李静","sex":2,"grade":2}] 班级3集合：[{"age":19,"name":"王强","sex":1,"grade":3},{"age":19,"name":"王萍","sex":1,"grade":3}] 删除班级2的张强转map：{"2":[{"age":18,"name":"李静","sex":2,"grade":2}],"3":[{"age":19,"name":"王强","sex":1,"grade":3},{"age":19,"name":"王萍","sex":1,"grade":3}]} 删除班级3的所有人转map：{"2":[{"age":18,"name":"李静","sex":2,"grade":2}]} 

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