Arrays and collections

Length() Used to find String The length of the string object ,length Is to seek String Length of array

Array declaration and initialization

String[] a= new int[3] The length of the array is fixed and cannot be modified

String [] a=new String[]{“a”,”b”,”c”} The value of the first element in is a instead of “a”;

Array traversal method ：

1for(var i:a ){

system.out.println(i);

}

2For(int i=0;i<a.lenth;i++){

System.out .prontln(i);}

3Arrays.stream(a).foreach(system.out::println)

Assign random numbers to the elements in the array

Random rand=new Random();

Int[] s=new int[10];

S[i]=rand.nextInt(100)+1;

Sort using Arrays.sort The method needs to int[] Convert to Integer object

Descending Arrays.sort（n,（a,b）->b-a）

Arrays.sort(ss, Comparator.reverseOrder())

// Bubbling

for (int i = 0; i < nums.length - 1; i++) {

 for (int j = 0; j < nums.length - 1 - i; j++) { //> Ascending < Descending

 if (nums[j] < nums[j + 1]) {

 int t = nums[j]; nums[j] = nums[j + 1];

 nums[j + 1] = t; }

Arrays.fill（ Array name , Location ）// Fill the array

Sorts// Ascending order

Arrays.binarySearch(n, 150)) Sort before you can find

aggregate Collection

ArrayList yes list Implementation class of

List.of Added list Fixed size list

List.add() Additive elements

List.remove Remove elements （ The element that first appears from the left ）

List.clear() Empty the set

List.contains Determine whether there is

// Instantiation ArrayList Use generics var list = new ArrayList(); Traversal is the same as array

//array list transformation

 int[] arr = new int[]{1,2,3};

 List lis = new ArrayList(Arrays.asList(arr));

System.out.println(lis.get(0)); // take arr Put the whole as a parameter list in

System.out.println(lis.size())

aggregate Arraylist And Array Array type conversion （

// Array to set （） To put it bluntly, it is to add an array as a set element ）

Integer[] nums = new Integer[] { 10, 20, 30, 40, 50 };

List list = Arrays.asList(nums);

// Set array （list.toArray）

//Arrays.toString Output the array as a string

List Generic sort

List list = new ArrayList<>();

List.sort// Ascending

Collcetions.shuffle(list)// Disorder

Collcetions.sort(list)// Disorder

Collections.sort(ls, Comparator.reverseOrder());// Descending

list.sort((a, b) -> b.getScore() - a.getScore());// According to the ascending order of grades

list.sort(Comparator.comparingInt(User::name));// Sort by name    ava.util.Comparator Interface It can realize multi-level sorting

Add two data at the same time

List.add（S）

Set Set :

HashSet :Set The implementation of the interface has good access and search performance At the bottom is the hash table

disorder , Out of sync , The set value can be empty and cannot contain duplicate elements

Traverse set.foreach(system.out::println)