I can't identify the cause of java.lang.ArrayIndexOutOfBoundsException in my program

Question

I'm trying to do a program wherein a user can input a string, which will be converted into a char array. The program has three functions:

1. toString(): returning the char array as a string again
2. isFull(): checks if the array is full
3. append(): allows the user to input another character

I think my code is going the right way for the most part but I can't seem to get the append() to work. Here's my code:

import java.util.Arrays;
import java.util.Scanner;

public class CharSeq {

private static Scanner sc = new Scanner(System.in);
private static final int maxLength = 5;
private static char[] cArray = new char[maxLength + 1];
private static String input;
private static char append, yesNo;
private static int cLength = 0;

public static void main(String[] args) {
    CharSeq cs = new CharSeq();

    System.out.println("Input sequence: ");
    input = sc.next();
    cArray = input.toCharArray();
    cLength = cArray.length;
    System.out.print(cs.toString());
    System.out.print(" (space left: " + (maxLength - cLength) + ")");
    cs.append(yesNo, append);
    System.out.println(cs.toString());
}

public String toString(){
    String s = "";
    for(int i = 0; i < cLength; i++){
        s = s + cArray[i];
    }
    return s;
}

public boolean isFull(){
    if(cLength == maxLength){
        return true;
    }
    else{
        return false;
    }
}

public void append(char a, char c){
    if(isFull()){
        System.out.println("\nCharSeq is full.");
    }
    else{
        System.out.println("\nDo you want to add sequence? [Y/N]");
        a = sc.next().charAt(0);
        if(a == 'Y' || a == 'y'){
            System.out.println("Input sequence: (space left: " + (maxLength - cLength) + ")");
            c = sc.next().charAt(0);
            cArray[++cLength] = c;
        }

    }

}

}

Here's the stacktrace:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
at CharSeq.append(CharSeq.java:53)
at CharSeq.main(CharSeq.java:22)