How to manipulate text in NiFi processor IDE?

[#1]

I am trying to develop a custom processor to manipulate flowfile contents via Java. However, I am not sure how constructors are used for the flowfile contents. As a simplified example, how can I create a custom processor that replaces the letter A with the letter Z in a flowfile content?

[#2]

@mburgess

I am using nifi-0.4.0. This version does not have the ExecuteScript processor. How can I install this processor and configure the processor to run a Java program?

-KS