If you wanted to convert the entire string into concatenated ASCII values then you can use this -
String str = "abc"; // or anything else
StringBuilder sb = new StringBuilder();
for (char c : str.toCharArray())
sb.append((int)c);
BigInteger mInt = new BigInteger(sb.toString());
System.out.println(mInt);
wherein you will get 979899 as output.
Credit to this.
I just copied it here so that it would be convenient for others.