java的数组转集合,java list里面放什么数据

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, "ylddy","男"); Person person2 = new Person(21, "dddlq","男"); Person person3 = new Person(32, "qrdhb","男"); Person person4 = new Person(21, "ggdhlb","男"); Person person5 = new Person(20, "田七","男"); Person person6 = new Person(32, "王八","男"); Person person7 = new Person(21, "wmdkl","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<Integer> 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<String> StringResult = Arrays.asList(StringType.split(",")).stream().map(s ->s.trim()).collect(Collectors.toList());

未完待续。。。