Multiplication formula string - java

I'm trying to write a code that take a string : (7+5)*(6+9) and return the string : 7*6 + 7*9 + 5*6 + 5*9 my algorithm is to create 2 char arrays (for this example : 75,69) and run in O^2 loop in this 2 arrays and copy value from first array , * value from second array, +. i get index of bounds exception and don't understand why.

this is my code :

 String s = "((7+5)*(6+9))";

 char[] ch = s.toCharArray();
 char[] newCh = new char[30];
 int j=0;
 int blocks = 2;
 int newi = 0;

 for (int i=0 ; i<ch.length ; i++) {
    if (ch[i]=='(' && ch[i+1]!='(') {
        j=i+1;
        while (blocks>0) {

            if (ch[j]==')') {
                blocks--;
                newCh[newi]='-';
                newi++;

            }
            if (ch[j]!='+' && ch[j]!='*' && ch[j]!=')' && ch[j]!='(') {
                if (blocks==0) {

                    break;
                }
                else {

                    newCh[newi]=ch[j];
                    newi++;
                }

            }
            j++;
        }

    }
    continue;
 }

 System.out.println("new Char array : ");
 for (int i=0 ; i<newCh.length ; i++) {
     System.out.print(newCh[i]);
 }
 System.out.println();

 Multy(newCh);

and my multy method :

public static char[] Multy(char[] ch) {
 char[] newc = new char[50];
 char[] c1 = new char[30];
 char[] c2 = new char[30];
 int newi = 0;
 int i1 = 0;
 int i2 = 0;
 int flag = 0;

 for (int i=0 ; i<ch.length ; i++) {
     if (ch[i]!='-') {
         if (flag ==0) {
        c1[i1] = ch[i];
        i1++;
         }
         else {
             if (ch[i]!='-')
             c2[i2]= ch[i];
             i2++;
         }
     }
     if (ch[i]=='-')
         flag = 1;

 }
 System.out.println("In func");

 System.out.print("c1 : ");
 for (int i=0 ; i<c1.length ; i++) {
     System.out.print(c1[i]);
 }
 System.out.println();
 System.out.print("c2 : ");
 for (int i=0 ; i<c1.length ; i++) {
     System.out.print(c2[i]);
 }
 ///////////


 for (int i=0 ; i<c1.length ; i++) {
    for (int j=0 ; j<c2.length ; j++) {
        newc[newi]=c1[i];

        newc[newi+1] = '*';

        newc[newi+2] = c2[j];

        newc[newi+3] = '+';

        newi+=4;


    }
 }

 System.out.print("NEWc2 : ");
 for (int i=0 ; i<newc.length ; i++) {
     System.out.print(newc[i]);
 }


 return newc;

}

in te double for loop you iterate to the end of the array (c1.length and c2.length) while you add to a number in the loop newc[newi+X] but because you loop to the end you will run out of places and you will get the IndexOutOfBoundsException...

for (int i=0 ; i<c1.length ; i++) {
    for (int j=0 ; j<c2.length ; j++) {
        newc[newi]=c1[i];
        newc[newi+1] = '*';
        newc[newi+2] = c2[j];
        newc[newi+3] = '+';
        newi+=4;
    }
}

UPDATE:

Extra explanation :-)

If you do something like this:

public class ArrayExample {
    public static void main(String[] args) {
        String[] strings = new String[3];
        System.out.println("strings.length = " + strings.length);
        strings[4] = "foo";
    }
}

The output will be:

strings.length = 3
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 4
    at nl.ivonet.ArrayExample.main(ArrayExample.java:26)

This is because I tried to assign a value at index 4 of the strings array, but the strings array was initialized with new String[3] giving it a fixed size of 3. That's what goes wrong and why your code failed. An Array is not the same as a List. And Array is fixed size and a list not.