No, you can't put an object there at all. Just a string. Remember what the <%= operator does, it prints out a String. Now Java has a little feature, where if you try and print out an object, it automatically calls the objects toString() function.
So what you have to do if you really want to do this, is convert it to a string, for example using ";" characters to seperate each string in the array. Then you have to read the string back in and create the objects from it again, for example using a StringTokenizer.
Other things you can consider are like said above, putting the objects on the Session object.
Perhaps just putting an 'id' in a hidden field, which refers to an item in a structure which is held at the application level.