initialization
//1.
String str = "hahaha";
//2.
char[] helloArray = {
'h', 'e', 'l', 'l', 'o'};
String str = new String(helloArray);
Get the length
int len = str.length();
Connection string
String s = s1 + s1;
String s = s1.concat(s2);
Substring
String sub_string = s.substring(start, end);
Get the character at a certain position
char ss = s.charAt(idx);
Other methods:
https://www.runoob.com/java/java-string.html