Java for loop with char arrays dis-function

Kevin Cruijssen resolves your problem but you can more:

Use HashMap to this problem. For now, your algorithm time complexity is O(n*m) (n-base string length, m - amount of letters in the table) because you must iterate through the whole array of letters for each letter.

Using HashMap you can find the right letter in O(1). A lot faster. So now your algorithm has O(n) time complexity.

Simple example:

Map<Character, Integer> encoding = new HashMap<>();
encoding.put('a', 0);
encoding.put('b', 1);
encoding.put('c', 2);
encoding.put('d', 3);

String toEncode = "abcd";
char[] chars = toEncode.toCharArray();
StringBuilder sb = new StringBuilder();
for(char c : chars){
    int newInt = encoding.getOrDefault(c, -5); //-5 is just a flag that there is no char to encode
    if(newInt == -5){
       continue; //or do something else, e.g throw exception;
    }
    sb.append(newInt);
}

System.out.println(sb.toString());
//Parse double if you want, but remember that what *Nikolas* said in the comments under your post.
//Double.parseDouble(sb.toString());

The problem is in your inner loop: twoD.length is 2, because twoD contains your two inner array of characters.

You should use twoD[0].length instead:

for(int c=0; c<tab.length; c++) {
  for(int z=0; z<twoD[0].length; z++) {
    ...

However, since you are using all ten digits, perhaps better to use that instead:

char twoD[][] = {{'a','b','c','d','e','f','g','h','i','j'}, {'0','1','2','3','4','5','6','7','8','9'}};
int amountOfDigitsUsed = 10; // Equal to `twoD[0].length` or `twoD[1].length`.

for(int c=0; c<tab.length; c++) {
  for(int z=0; z<amountOfDigitsUsed; z++) {
    ...

Regardless whether you use a hard-coded twoD conversion and amountOfDigits used or not. In your current implementation your twoD.length is 2, causing the issues you have right now.

Length of your twoD array is 2. Your second loop should iterate from z = 0 to twoD[0].length.

Try naming your variables meaningfully so it will be easier to find bugs like this. Also check out foreach loops so you don't have to worry about indexes. Java Maps could be better for mapping characters to numbers.

Since it seems as though in your case the characters are incrementing along with their int values, you don't need a map at all. You can cast the character to an int, and then subtract a's int value. This is a slight variation of B_Osipiuk's answer:

String toEncode = "abcd";
char[] chars = toEncode.toCharArray();
StringBuilder sb = new StringBuilder();
for(char c : chars){
    int newInt = c - 'a';
    if (newInt < 0 || newInt > ('j'-'a')) {
        continue; //or do something else, e.g throw exception;
    }
    sb.append(newInt);
}

System.out.println(sb.toString());