What trick does Java use to avoid spaces in >>?

Question

In the Java Generic Book, while contrasting the difference between C++ Templates and Java Generic says:

In C++, a problem arises because >> without the space denotes the right-shift operator. Java fixes the problem by a trick in the grammar.)

What is this trick?

Answer 1

The OpenJDK javac parser, JavacParser, massages the lexer tokens GTGTGTEQ (>>>=), GTGTEQ, GTEQ, GTGTGT (>>>) and GTGT into the token with one less '>' character when parsing type arguments.

Here is a snippet of the magic from JavacParser#typeArguments():

    switch (S.token()) {
    case GTGTGTEQ:
        S.token(GTGTEQ);
        break;
    case GTGTEQ:
        S.token(GTEQ);
        break;
    case GTEQ:
        S.token(EQ);
        break;
    case GTGTGT:
        S.token(GTGT);
        break;
    case GTGT:
        S.token(GT);
        break;
    default:
        accept(GT);
        break;
    }

One can clearly see that it is indeed a trick, and it's in the grammar :)

Answer 2

This is actually being fixed in C++ in the next version. There really isn't much of a trick; if you encounter >> while in the process of parsing a generic or template where instead you expected >, then you already have enough information to generate an error message. And, if you have enough information to generate an error message, you also have enough information to interpret >> as two separate tokens: > followed by >.