Java - replace() method using values from arrays is changing the array values?

Question

I'm doing something like

public static String[] list = {"a","b","c","d",}  //It gives me a NullPointeException if I didn't use static
public String encrypt(String a){
   a = a.replace(list[0],list[2]);
   a = a.replace(list[4],list[3]);
   return a;
}

and I have another method that just reverses it

public String decrypt(String a){
   a = a.replace(list[2],list[0]);
   a = a.replace(list[3],list[4]);
   return a;
}

Of course this is simplified, the real code I'm using uses the entire alphabet and some numbers. So here's my problem: If I input something like 123 into encrypt() and it outputs ngV then I input ngV into decrypt() it gives me like 1q3. Only some of the letters are correctly switched and some aren't. Is there something with the replace() method using array values that I'm missing? I'm obviously new to Java.

Also I read Java replace() problems but replaceAll() didn't work out.

Strings are immutable, so you need to store the result of a.replace(...); — Tunaki
– Tunaki, Commented Feb 27, 2016 at 13:47
"If I input something like 123 into encrypt() and it outputs ngV" That's never going to happen. — Joni
– Joni, Commented Feb 27, 2016 at 13:52
I'm very sorry guys, I edited my code again. I had a = a.replace(); So Tunaki are you suggesting that I store the result of a.replace. However, the problem is still the same, Do you want a video of me demonstrating it? — Michael Chen
– Michael Chen, Commented Feb 28, 2016 at 6:09
@MichaelChen Use the "@" before the user's name to send him a notification. — Maroun
– Maroun, Commented Mar 3, 2016 at 16:01
Please provide a full example. The code posted above does not work: The array cannot be assigned like that and list[4] does not exist. Break down the problem into your own small sample program and post it here. — f1sh
– f1sh, Commented Mar 3, 2016 at 16:06

Michael Lloyd Lee mlk · Accepted Answer · 2016-03-03 16:18:57Z

1

I suspect your question is "why is chaining .replace acting oddly" and the array is not changing. You can prove that replace does not change the array quite easily:

    System.out.println(Arrays.toString(list));
    encrypt("abc");
    System.out.println(Arrays.toString(list));

So what is going on with your code? Each time you replace a letter you end up with a new string, that again you replace letters on. I don't have your full source code so I'll show with a real simple version:

a = a.replace("a", "b");
a = a.replace("b", "c");
a = a.replace("c", "d");

For "abc" is.... 'ddd'.

The answer to this is to look at each letter at a time and change it. Loop through the string and create a new one.

answered Mar 3, 2016 at 16:18

Michael Lloyd Lee mlk

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