Difference in b += 100; and b = b + 100;

Question

On the java tutorial page on assignment operators, http://java.sun.com/docs/books/tutorial/java/nutsandbolts/assignment.html, the following is mentioned:
"Suppose you wanted to add a number to a variable and assign the result back into the variable, like this:
i = i + 2;
You can shorten this statement using the shortcut operator +=, like this:
i += 2;
The two previous lines of code are equivalent. "
But if this is the case then why the following code gives the compile time error
  byte b = 0;
  b = b + 100;Test.java:14: possible loss of precision
found : int
required: byte
b = b + 100;
^
1 error
where as the following compiles without any error
  byte b = 0;
  b += 100;

Answer 1

This behavior is well defined in the JLS (Java Language Specification). The pertinent JLS section (15.25.2 Compound Assignment Operators) can be found at http://java.sun.com/docs/books/jls/first_edition/html/15.doc.html#5304