1. Convert String to character array
String.toCharArray()
2. Concatenate the string array into a string
String.valueOf(char[])
3. Convert string to number
Integer.valueOf("123")
4. Split the string into multiple substrings with a certain mark
String[] arr = String.split(",")
5. Concatenate characters into strings
StringBuilder sb = new StringBuilder();
sb.append('1');
sb.append('2');
String str = sb.toString();