Android 2.3.3

This is my code...

String[] expression = {""}; //globally declared as empty

somewhere in the code below, I am trying to assign a string to it.

expression[0] = "Hi";

I keep getting the following error...
12-08 22:12:19.063: E/AndroidRuntime(405): java.lang.ArrayIndexOutOfBoundsException

Can someone help me with this.. Can we access the index 0, directly as i am doing?

Actual Code :::

static int x = 0; // global declaration
String[] assembledArray = {""}; // global declaration

assembleArray(strSubString, expression.charAt(i)); //Passing string to the method

//Method Implementation
private void assembleArray(String strSubString, char charAt) {
        // TODO Auto-generated method stub

        assembledArray[x] = strSubString;
        assembledArray[x+1] = String.valueOf(charAt);
        x = x+2;

    }