java list转String,list转map,String数组转list，带逗号String转list

项目中经常会遇到list转String,list转map,String转list等情况，特在这里记录一下。

1.list转String

将List集合转化为用符号隔开的String字符串，只要用StringUtils中的join就行。

String listToString= org.apache.commons.lang3.StringUtils.join(list, ",")

2.list转map

我们会常遇到将一个list按某个字段作为key转为map，如下面，按年龄转map,传统方法是直接做循环：

      Map<Integer, List<Person>> map = new HashMap<>();
        List<Person> list = new ArrayList<>();
        Person person1 = new Person(20, "张三","男");
        Person person2 = new Person(21, "李四","男");
        Person person3 = new Person(32, "王五","男");
        Person person4 = new Person(21, "刘六","男");
        Person person5 = new Person(20, "田七","男");
        Person person6 = new Person(32, "王八","男");
        Person person7 = new Person(21, "阿狗","男");
        Person person8 = new Person(20, "阿十","男");
        list.add(person1);
        list.add(person2);
        list.add(person3);
        list.add(person4);
        list.add(person5);
        list.add(person6);
        list.add(person7);
        list.add(person8);

        for (Person person : list) {
            map.put(person.getAge(), new ArrayList<>());
        }

        Map<Integer, List<Person>> map1 = new HashMap<>();
        for (Map.Entry<Integer, List<Person>> entry : map.entrySet()) {
            List<Person> userList = new ArrayList<>();
            for (Person person : list) {
                if (entry.getKey() == person.getAge()) {
                    userList.add(person);
                }
            }
            map.put(entry.getKey(), userList);
            userList = null;
        }
    }

但我还是推荐用Java8 的Stream来做，特别简单：

Map<Integer,List<Person>> map2=list.parallelStream().collect(Collectors.groupingBy(Person::getAge));

就一句话的事。Java8的Stream对集合的操作都挺好用的，下次抽时间来分享一下。

3.String数组转list

定义一个String数组，用Arrays.asList转：

  String[] string = {"aa","bb","cc"};
  List<String> stringToList= Arrays.asList(string);

4.带逗号String转list

1.转Integer类型list

  String IntegerType="1,2,3";
  List IntegerResult = Arrays.asList(StringType.split(",")).stream().map(s ->Integer.parseInt(s.trim())).collect(Collectors.toList());

2.转String类型的list

  String StringType="a,b,c";
  List StringResult = Arrays.asList(StringType.split(",")).stream().map(s ->s.trim()).collect(Collectors.toList());

未完待续。。。