how to add character to string code example

Example 1: add one character to string java

char ch='A';
String str="pple";
str= ch+str;   // str will change to "Apple"

Example 2: insert character into string

x <- "abcde"
stri_sub(x, 3, 2) # from 3 to 2 so... zero ?
# [1] ""
stri_sub(x, 3, 2) <- 1 # substitute from 3 to 2 ... hmm
x
# [1] "ab1cde"