Java8 Stream

一、将[1,2,3,4,5]转为”1->2->3->4->5”

List<Integer> list = Arrays.asList(1,2,3,4,5);
String list2 = list.stream().map(String::valueOf).collect(Collectors.joining("->"));

List<Integer> list = Arrays.asList(1,2,3,4,5);
String list2 = list.stream().map(i -> String.valueOf(i)).reduce((a,b)-> a+"->"+b).get();

二、类型转换

1. int[] 转为 List

   int[] arr = new int[]{1,2,3,4,5};
   List<Integer> list = Arrays.stream(arr).boxed().collect(Collectors.toList());

2. int[] 转为 Integer[]

   int[] arr = new int[]{1,2,3,4,5};
   Integer[] arr2 =  Arrays.stream(arr).boxed().toArray(Integer[]::new);

3. List转为int[]

   List<Integer> list = Arrays.asList(1,2,3,4,5);
   int[] arr = list.stream().mapToInt(Integer::valueOf).toArray();

4. List转为Integer[]

   List<Integer> list = Arrays.asList(1,2,3,4,5);
   Integer[] arr = list.stream().toArray(Integer[]::new);
   //Integer[] arr = list.toArray(new Integer[0]);

5. integer[]转为int[]

   Integer[] arr = new Integer[] {1,2,3,4,5};
   int[] arr2 = Arrays.stream(arr).mapToInt(Integer::valueOf).toArray();

6. integer[]转为List

   Integer[] arr = new Integer[] {1,2,3,4,5};
   List<Integer> list = Arrays.stream(arr).collect(Collectors.toList());

7. String[]转List

   String[] strs={"1","2","3"};
   List<String> list = Arrays.stream(strings).map(Integer::parseInt).collect(Collectors.toList());

8. list转String[]

   List<Integer> list = Arrays.asList(1,2,3,4,5);
   String[] strArray = list.stream().map(String::valueOf).toArray(String[]::new);

   三、map应用

   使用Stream统计List里面相同字符的数量，并返回Map

   List<Character> list = Arrays.asList('a','b','a','c','d','d','d');
   Map<Character,Integer> map = new HashMap<Character,Integer>();
   map =list.stream().collect(Collectors.toMap(x -> x ,x -> 1,(n1,n2)->n1+n2));
   map.forEach((key,value)->{
    System.out.println(key+":"+value);
   });

   四、经典转化

9. 数组字符串转为数组

   String arrayStr = "[10,9,2,5,3,7,101,18]";
   int[] nums = Arrays.stream(arrayStr.replace("[", "").replace("]", "").split(",")).mapToInt(Integer::parseInt).toArray();
   System.out.println(Arrays.toString(nums));