String of Characters

Next, the program assembles a string of all the characters that could go into a password:

String choices = "abcdefghijklmnopqrstuvwxyz" ;
choices = choices + choices.toUpperCase() ;
choices = choices + "1234567890" ;

The first of these three statements creates a string literal containing the lower case alphabet. Then the expression choices.toUpperCase() creates a string that is all upper case, based on the string currently referred to by choices. This string is then concatenated onto the original choices string:

choices = choices + choices.toUpperCase();

Recall that + means "string concatenation". Finally, the string "1234567890" is concatenated onto the string to produce the final string, shown below:

"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"