编译器如何引用自己的输出

Question

"foreach" is a keyword of Java language and when compiler encounters this keyword in the source, it knows that the collection following this keyword must implement "iterable" interface. The "iterable" interface itself is a piece of compiled Java code so how can the compiler reference its own output, how can the compiler be dependent on its own output?

Think of a hypothetical point in past time when there is only Java compiler (probably written with C or C++) and Java framework is not compiled yet. Java.lang package and all the stuff in it is not compiled yet so there is no Iterable interface at that moment in time. The Java compiler will compile the source code for Java.lang package from source code and build the bytecode for iterable interface in the future. At that point in time somehow Java compiler has to know about the iterable interface (since it is part of the language specification) so it can compile source code that uses foreach iterator. How is this made possible?

Answer 1

First of all , " foreach " is not a java keyword. Search for " extended for loop in java ". The second thing , Interfaces in java are just to abstract the real implementation. This is not must to implement Interface when we have the real Implemention Class. But, to follow the good design principles like SOLID principles, we make Interfaces to abstract real implementations first, and also for multiple class hierarchies in java. Keywords do not implement Interfaces.If java keyword implement and refer any java code then its definitely related to bootstrapping means Java Compiler Written in Java Language. Overall answer to your long question is you need to Study about Compiler Construction, Building JVM Language, Bootstrapping etc.