In the Java 8 specification there is the following syntax description of the conditional operator ?: :
ConditionalExpression:
ConditionalOrExpression
ConditionalOrExpression ? Expression : ConditionalExpression
ConditionalOrExpression ? Expression : LambdaExpression
I understand the first two lines. But why is the third line necessary? What is so special about a LamdaExpression that it needs to be declared explicitly, here? I don't see this discussed in the following text.
I think I know the answer, but it's weird.
Here is a quote from chapter 15:
Precedence among operators is managed by a hierarchy of grammar productions. The lowest precedence operator is the arrow of a lambda expression (->), followed by the assignment operators. Thus, all expressions are syntactically included in the LambdaExpression and AssignmentExpression nonterminals:
Expression: LambdaExpression AssignmentExpression
The ConditionalExpression is a part of AssignmentExpression:
AssignmentExpression: ConditionalExpression Assignment
As you quote, the third part of ConditionalExpression can be only a ConditionalExpression, not an Expression.
Making the third part into Expression would include undesirable things like Assignment:
a? b : c=d // bad idea?
But, limiting the third part to ConditionalExpression rather than Expression leaves out LambdaExpression, so now it needs to be added explicitly with
ConditionalOrExpression ? Expression : LambdaExpression
This actually brings the next question: how come Assignmentis OK in the second part of conditional expression but not in the third?