Answer by Armin Ronacher for Purpose of "let expression" (LetExpr) in the Java compiler?

Generally speaking, why does it exist?

It exists for autoboxing as Google suggests.

If you have code like this:

Integer foo = 0;foo++;

Java internally makes this into this helper expression:

Integer foo = 0;let int foo_helper = foo.intValue() in foo_helper++;

Source: https://bugs.java.com/bugdatabase/view_bug?bug_id=6614974

That expression obviously has no syntax representation, it's just an AST level transformation to simplify compilation.