Java

String

StringBuilder s = new StringBuilder();
s.append(String/int/double);
String res = s.toString();

String a, b;
a.equals(b);

// array to String
String[] a = {"a","b","c"};
String b = a.join("", a);

// 
String a = "abdcde";
char b = a.charAt(1);
int c = a.lastIndexOf('d');

Array

// Initial Array
int[] a = new int[4];
int[] a = {1,2,3,4};
int[][] a = {{1,2},{3,4};
int[] a = new int[]{hashmap.keySet()};

Queue

Queue<Integer> q = new ArrayList<>();
Queue<Integer> q = new LinkedList<>();

q.isEmpty(); // return true or false
q.size();
q.offer(); // add element
q.offer(new int[]{1,2});
q.poll(); // remove element
q.peek();  // check element but not remove 

LinkedList

List<Integer> a = new ArrayList<>();
List<Integer> a = new LinkedList<>();
List<Intger> b = new ArrayList<>(a);

int a = b.size(); // get size of list


Set<String> set = new HashSet<>();
List<String> list = new ArrayList(set);