My 'decrypt' code doesn't work correctly

Question

I am not sure but I think my problem is that my function doesn't compare the char´s right. Am I using the Switch also right?

my input x is a String and when x = "aaaaa" it returns "aaaaa" instead of "zzzzz".

String c = "";
        for (int i = 0; i < x.length(); i++) {
            char getChar = x.charAt(i);

            switch (getChar) {
                case 1: (getChar) = 'a';
                        c += "z";
                        break;
                case 2: (getChar) = 'b';
                        c += "y";
                        break;
                case 3: (getChar) = 'c';
                        c += "x";
                        break;
                case 4: (getChar) = 'd';
                        c += "w";
                        break;
                case 5: (getChar) = 'e';
                        c += "v";
                        break;
                case 6: (getChar) = 'f';
                        c += "u";
                        break;
                case 7: (getChar) = 'g';
                        c += "t";
                        break;
                case 8: (getChar) = 'h';
                        c += "s";
                        break;
                case 9: (getChar) = 'i';
                        c += "r";
                        break;
                case 10:(getChar) = 'j';
                        c += "q";
                        break;
                case 11:(getChar) = 'k';
                        c+= "p";
                        break;
                case 12:(getChar) = 'l';
                        c += "o";
                        break;
                case 13:(getChar) = 'm';
                        c += "n";
                        break;
                default :
                        c += x.charAt(i);
            }
        }

    System.out.println(c);
}

Answer 1

switch (getChar)//<--You are passing charcter in switch case 
//but checking for 1,2 int as case 1,2...

What you need to change is your case

as in switch you will be passing characters like a,b,c...

switch(getchar)
{
 case 'a':
  //yourwork
  break;
 //do this for all letters
}

---

NOTE

Moreover for concacting String you should use StringBuilder (As Maroun Maroun) has already suggested and use stringBuilder.append('char') method to add your character to String builder directly no need to use String (i.e "a","b" etc.).

Answer 2

A switch statement is not the best option if all cases are treated in almost exactly the same way and can be easily converted into a single calculation. You can replace the entire switch statement with this if/else block:

if (getChar >= 'a' && getChar <= 'm') {
    char newChar = (char) ('z' - (getChar - 'a'));
    c += newChar;
} else {
    c += getChar;
}

NOTE: @TAsk and @MarounMaroun are right in advising you to use StringBuilder to build up your result.