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Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 28
    at CreateKey.CreateKey.WriteKeyToAFile(CreateKey.java:24)
    at CreateKey.CreateKey.main(CreateKey.java:43)

package CreateKey;
 
import java.io.File;
import java.io.PrintWriter;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
 
public class CreateKey {
    public static final int RUSSIAN_ALPHABET_LENGTH = 38;
    public static char[] RUSSIAN_ALPHABET = {'а','б','в','г','д','е','ж','з','и','й','к','л','м','н','о','п','р','с','т','у','ф','х','ц','ч','ш','щ','ъ','ы','ь','э','ю','я',' ','.',',','!',':','?'};
    
    public static void WriteKeyToAFile(String CurrentKeyFileName, char[] key) throws IOException,FileNotFoundException {
        File CurrentKeyFile=new File(CurrentKeyFileName);
        CurrentKeyFile.delete();
        PrintWriter out = new PrintWriter(new OutputStreamWriter(
                new FileOutputStream(CurrentKeyFile, /* append */ true)),/*autoflush*/ true);
        out.flush();
        for(int i = 0; i < RUSSIAN_ALPHABET_LENGTH; i++){
        out.print(key[i]);}
        out.close();
    }
    
    public static char[] CreatorKey() {
        String[] tmp1 = new String[RUSSIAN_ALPHABET_LENGTH];
        for (int i = 0; i < RUSSIAN_ALPHABET_LENGTH; i ++){
            char[] tmp2 = new char[1];
            tmp2[0]=RUSSIAN_ALPHABET[i];
            tmp1[i] = new String(tmp2);
            }
    List<String> lst = Arrays.asList(tmp1);
    Collections.shuffle(lst);
    tmp1 = lst.toArray(tmp1);       
    return tmp1.toString().toCharArray();
    }
    
    public static void main(String[] args) throws IOException { 
        String CurrentKeyFileName = "currentkey.txt";
        WriteKeyToAFile(CurrentKeyFileName, CreatorKey());      
    }
}

package CreateKey;
 
import java.io.File;
import java.io.PrintWriter;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.Random;
 
public class CreateKey {
    public static final int RUSSIAN_ALPHABET_LENGTH = 38;
    public static char[] RUSSIAN_ALPHABET = {'а','б','в','г','д','е','ж','з','и','й','к','л','м','н','о','п','р','с','т','у','ф','х','ц','ч','ш','щ','ъ','ы','ь','э','ю','я',' ','.',',','!',':','?'};
    
    public static void WriteKeyToAFile(String CurrentKeyFileName, char[] key) throws IOException,FileNotFoundException {
        File CurrentKeyFile=new File(CurrentKeyFileName);
        CurrentKeyFile.delete();
        PrintWriter out = new PrintWriter(new OutputStreamWriter(
                new FileOutputStream(CurrentKeyFile, /* append */ true)),/*autoflush*/ true);
        out.flush();
        for(int i = 0; i < RUSSIAN_ALPHABET_LENGTH; i++) 
        out.print(key[i]);
        out.close();
    }
 
    public static void mix(char[] a) {
        Random rnd = new Random();
        for (int i = 1; i < a.length; i++) {            
            int j = rnd.nextInt(i);
            char temp = a[i];
            a[i] = a[j];
            a[j] = temp;
        }
    }
    
    public static char[] CreatorKey() {
        char[] tmp1 = RUSSIAN_ALPHABET;
        mix(tmp1);
    return tmp1;
    }
    
    public static void main(String[] args) throws IOException { 
        String CurrentKeyFileName = "currentkey.txt"; 
        WriteKeyToAFile(CurrentKeyFileName, CreatorKey());      
    }
}