Dividing up every second char in a String into a array to print out in a Matrix?

Im trying to divide up a char String into a char array then print it out into a matrix, I thought I had it figured out but then I realized I got a String mismatch, does anyone know a better way to go about this or where im going wrong in my code? Below are the 3 methods im using to achieve this.

 public void Key(){

    for(int row=0;row<keyMatrix.length;row++){   
        for(int col=0;col<keyMatrix[row].length;col++){
            for (String x : getParts(polyCipher.translation, 2)) {
                keyMatrix[row][col] = x;
            } 
        }
        System.out.println();
    }

}//Key end


public String printKeyMatrix(){
    String keyOut = "  J A V A\n";
    for (int i = 0; i < PHRASE_KEY.length; i++) {
        keyOut += PHRASE_KEY[i] + " ";
        for (int j = 0; j < PHRASE_KEY.length; j++) {
            keyOut += keyMatrix[i][j] + " ";
        }
        keyOut += "\n";
    }
    return keyOut;
}


//Method for dividing up the translation by every nth number(Will alway by the 2nd number for this program)
public static List<String> getParts(String string, int partitionSize) {
    List<String> parts = new ArrayList<String>();
    int len = string.length();
    for (int i=0; i<len; i+=partitionSize)
    {
        parts.add(string.substring(i, Math.min(len, i + partitionSize)));
    }
    return parts;
}