Uncommonly used Java syntax (JavaParser)?

Question

I'm exploring a Java grammar parser and I came across this strange piece of code that I wouldn't normally use in ordinary code. Taken from https://code.google.com/p/javaparser/source/browse/branches/mavenized/JavaParser/src/main/java/japa/parser/ASTParser.java#1998

It has many functions that contains code such as

final public NameExpr Name() throws ParseException {
    NameExpr ret;
    jj_consume_token(IDENTIFIER);
    ret = new NameExpr(token.beginLine, token.beginColumn, token.endLine, token.endColumn, token.image);
    label_23: while (true) {
        if (jj_2_17(2)) {
            ;
        } else {
            break label_23;
        }
        jj_consume_token(DOT);
        jj_consume_token(IDENTIFIER);
        ret = new QualifiedNameExpr(ret.getBeginLine(), ret.getBeginColumn(), token.endLine, token.endColumn, ret, token.image);
    }
    {
        if (true) {
            return ret;
        }
    }
    throw new Error("Missing return statement in function");
}

At a glance it appears strange but no doubt it's valid as I can compile it. But can someone explain how it works? I have tried to input invalid Java syntax and it does it's job! I'm baffled. How does the few lines throw exception after the return?

Answer 1

This is indeed valid code, without seeing everything, I can see some odditities:

• 'Incorrect' variable and method naming, using PascalCase sometimes.
• Instance variable token
• Static variable IDENTIFIER

Then:

label_23: while (true) {
    if (jj_2_17(2)) {
        ;
    } else {
        break label_23;
    }
    jj_consume_token(DOT);
    jj_consume_token(IDENTIFIER);
    ret = new QualifiedNameExpr(ret.getBeginLine(), ret.getBeginColumn(), token.endLine, token.endColumn, ret, token.image);
}

This is an infinite loop that keeps running as long as jj_2_17(2) returns true, but appears to do nothing upon that result. It breaks out of label_23 when the expression was false. To confuse future readers even more, it then actually does things only if the expression is true (as it breaks on false), namely the last three lines.

For futher information, the label_23 is simply a label that may only be used on while and for loops. You can then break out of that loop when using break labelName;.

Example that breaks out of an outer loop from within an inner loop:

outerLoop: for (int i = 0; i < max; i++) {
    innerLoop: for (int j = 0; j < max2 - i; j++) {
        if (something) {
            break outerLoop;
        }
        //...
    }
}

You can actually also use continue in combination with labels.

Then we see a scoped block without guard that always returns ret:

{
    if (true) {
        return ret;
    }
}

So it's all valid. I think we can also conclude with high chance that this code has been machine-generatd.