SPOJ - ADDREV (Adding Reversed Numbers)
I have some suggestion for you.

Use the java.lang.StringBuilder instead of the older java.lang.StringBuffer

The java.lang.StringBuilder is generally faster than the older java.lang.StringBuffer.

Use the same instance of java.util.Scanner

Move the java.util.Scanner of the loop, you can reuse it. you can use the obj instance and use only this instance.

Reuse the same java.lang.StringBuilder instead of creating a new one each time.

You can reuse the same builder by clearing it with the method java.lang.AbstractStringBuilder#setLength and passing zero to the method; this will prevent the object creation to be put on the stack and use less memory than creating lots of instances of the builder.

Extract similar code to methods.

In your code, you can extract some of the code to methods; this will make the code shorter and easier to read. I suggest that you create a method to read the input and revert it as a String.