int[ ], Integer[ ], List<Integer>, List<String> mutual conversion

一、int[ ] 转成 Integer[ ], List< Intege r>, List< String >

(1) int[] into Integer[]

 Integer[] integers = Arrays.stream(ints).boxed().toArray(Integer[]::new);

（2）int[ ] 转成 List< Integer >

List<Integer> integerList = Arrays.stream(ints).boxed().collect(Collectors.toList());

（3）int[ ] 转成 List< String >

List<String> strList = Arrays.stream(ints).boxed() 
// 一定要boxed.map(String::valueOf).collect(Collectors.toList());
// 转成List<Integer> 然后再用map转成String，最后用collect转成List<String>

二、Integer[ ] 转成 int[ ] , List< Integer>, List< String >

(1) Integer[] converted to int[]

int[] ints = Arrays.stream(integers).mapToInt(Integer::valueOf).toArray();

(2) Integer[] is converted to List<Integer>. The list cannot be added or removed

List<Integer> integerList = Arrays.asList(integers);

(3) Integer[] is converted to List<Integer> The list supports all operations

List<Integer> integerList1 = new ArrayList<>(Arrays.asList(integers));

(4)Integer[] 转成 List< String >

 List<String> strList = Arrays.stream(integers).map(String::valueOf) .collect(Collectors.toList());

三、List< Integer > 转成 int[ ], Integer[ ], List< String >

(1)List< Integer > 转成 int[ ]

int[] ints = list.stream().mapToInt(Integer::valueOf).toArray(); // 转成流 再转成Integer[] 再转int

(2)List< Integer > 转成 Integer[ ]

Integer[] integers1 = list.toArray(new Integer[0]);
Integer[] integers2 = list.stream().toArray(Integer[]::new);

(3)List< Integer > 转成 List< String >

 List<String> strList = list.stream().map(String::valueOf).collect(Collectors.toList());

四、List< String > 转成 int[ ], Integer[ ], List< List >

(1)List< String > 转成 int[ ]

int[] ints = strList.stream().mapToInt(Integer::valueOf).toArray(); 
// 和List<Integer>转换一样

(2)List< String > 转成 Integer[ ]

 Integer[] integers =strList.stream().mapToInt(Integer::valueOf).boxed().toArray(Integer[]::new);      
 // 和List<Integer>转换一样

(3)List< String > 转成 List< Integer >

  List<Integer> integerList = strList.stream().mapToInt(Integer::valueOf).boxed().collect(Collectors.toList());

Summarize the points to note:

1. int[] To be converted into a List object, you must use boxed to convert to Integer

Arrays.stream(ints).boxed().map(xxx).collection();

2. List and convert List to each other, string to Integer must be converted to Integer with boxed() after mapToInt

List<String> strList = list.stream().map(String::valueOf).collect(Collectors.toList());
     
 List<Integer> integerList = strList.stream().mapToInt(Integer::valueOf).boxed().collect(Collectors.toList());

3. For other words, use stream.toArray() to return T[], stream.collect() to return Collection objects (list, set, etc.)