How can I resolve this error? (This type must be an array type but is resolved to a string)

I am trying to practice string manipulation skills and I came up with this as a solution to check a string array for every word that ends in 'y' or 'z' (a word being defined as a string of characters separated by a space) but its wrong obviously. What can I do to resolve this error?

static int wordEndYZ(String str) 
{

    int i, j, counter;

    for( i = 0; i < str.length(); i++ ) // search through string array
    {
        j=i;                                // copy position so original position intact
        while ( str[j] == ' ' )             // *First error* space occurs, check last letter
        {
            if ( str[j-1] == 'y' || 'x' )   // 'y' or 'z'? If so counter increments
                counter += 1;               
        }           
    }
    return counter;
}