Android : Split String to Array, add to Array then convert back to String

Working on an android app the makes a HTTP request based on user input.

I want to achieve the following String as part of an HTTP request:

The%20Cat%20Sat

User input to an EditText:

The Cat Sat

I understand the string needs to be split and placed into an array, however it must then accept %20 after each entered word, before being converted back to a string.

Any guidance would be great!