concatenate a string and a null character

concatenate a string and a null character

I am completely new to java, and I am trying to join to string together
with a null character between them.
String header = "text 19"
String content = "this is the content"
String wholething = // this is supposed to be "text 19\0this is the content"
End-goal here is to take sha-1 sum of the wholething and write the
wholething to a file. I have tried some google searches, but couldn't
figure out how to do it.