Run the following test in the debugger. As you step, notice that the Debugger puts "b[x++]" into the Variables panel. Each step will cause "x" to increment quickly causing an out of bounds exception. public static void main(String[] args) throws Exception { byte[] b = new byte[4]; int x = 0; for ( int i = 0; i < b.length; ++i ) { b[x++] = (byte)i; } }