1. If I want to send data as a String data, how to send that from the function (say sendData()) in my Java class as examples on the internet have shown sending only one String as a response?
Yes, you can send it (String can have values with some delimiter)
2. Is it possible/OK to send/return POJO as a response?
Yes, you can send POJO.
3. If I want to send data as POJO, how client which is a Android application can know which type of data is this and retrieve data from POJO?
You can use XML to send the data