Android Java substring instructions

SUBSTRING (parameter) is a way to intercept strings in Java
There are two ways to pass the argument
One is public String substring (int beginindex)
Returns a new string that is a substring of this string. The substring starts at the character at the specified index until the end of the string.
The other is public String substring (int beginindex, int endIndex)
Returns a new string that is a substring of this string. The substring starts at the specified beginindex until the character at index EndIndex-1.

For more information, please refer to API  java.lang.String
As far as LZ says,
String subString;
Substring=number.substring (i,i+1);
Define a variable of type string to store the string where number intercepts the first position to the first i+1
In general, we are going to intercept number I to i+1, then assign to a variable of type string substring

Android Java substring instructions