Since I have utterly no idea about the structure of your data it is impossible to give you anything but vague suggestions.
One of the
Java "collection" classes in the java.util package might be suitable, for example an ArrayList will be Serializable and can store a number of objects assuming the client is using Java in the same version as the server.
The subject of moving data around networks in Java is huge and I am not about to try to create a tutorial when there are already many good ones.
Bill