问增强for循环上的ArrayIndexOutOfBounds

EN

private String dnaToRNA(String input) {

    StringBuilder b = new StringBuilder();
    char[] arr = input.toCharArray();
    for (char a : arr) {
        if (a == 'T') {
            arr[a] ='U';

        }
    }   
     for (char a : arr) {
        if (a == 'A'){
            b.append ('U');
        }

        else if (a == 'U') {
            b.append('A');
        }

        else if (a == 'C') {
            b.append('G');
        }   

        else if (a == 'G') {
            b.append('C');
        }   

    }   


    return b.reverse().toString();
}

}

   public void transcribe(int pos1) {

    if (pos1 > linkedList.size()) {
        System.out.println("Position selected out of range");
        return;
    }
    if (linkedList.get(pos1) != null && isValidDNA(linkedList.get(pos1))) {
        linkedList.set(pos1, dnaToRNA(linkedList.get(pos1)));
    }
}