Question

I’m trying to write a small internal DSL in Java which generates an tree of objects. The DSL code looks like this:

RuleBuilder builder = new RuleBuilder(new Syntax());
Syntax s =
builder.rule("rule1")
            .identifier("foo")
            .choice()
                .terminal("bar")
            .end() // 1) Here it works.
       .end() // 2) Here complains the compiler.
       .rule("rule2")
            .identifier("bar")
       .end()
       .build();

The compiler complains (at 2) that the object java.lang.Object returned by end() does not have a method rule(). It’s obvious to me that Object does not have this method. The builder code is as follows (The code assembling the tree is left for simplicity):

class RuleBuilder {
    private final Syntax syntax;

    public RuleBuilder(Syntax syntax) {
        this.syntax = syntax;
    }

    public GenericBuilder<RuleBuilder> rule(String name) {
        return new GenericBuilder<RuleBuilder>(this);
    }

    public Syntax build() { return syntax; }
}

class GenericBuilder<P> {
    private final P parentBuilder;

    public GenericBuilder(P parentBuilder) {
        this.parentBuilder = parentBuilder;
    }

    public <P> P end() {
        return (P)parentBuilder;
    }

    public GenericBuilder<P> identifier(String value) { return this; }
    public GenericBuilder<P> terminal(String value) { return this; }
    public GenericBuilder<GenericBuilder> choice() { return new GenericBuilder<GenericBuilder>(this); }
    // ... other sub node types
}           

The principal idea behind my implementation: The generated syntax tree consists of a Syntax root node which has some Rule nodes. This rule nodes can have some sub nodes. The sub nodes are of two types: Leafs and nodes (in the example code only choice for simplicity). I will provide a fluent interface where I can add nodes and leaves to the tree. To ‘end’ a tree branch there is the end() method which returns the parent builder.

The issue I try to solve is, that the method end() should return the parent builder object which may be of type RuleBuilder or GenericBuilder. I do not understand why it works at 1) and not at 2) in the example above.

I’ve read through a lot of resources about generics and I understand how List<T> and Map<K,V> and such works. And I’m aware of the “Erasure” thing, that the type information is lost at runtime. So I can understand that at runtime end() returns java.lang.Object, when type information is erased. But I got a compile time error. I’ve also read through the blog of Neal Gafter about Super Type Tokens and Typesafe Heterogenous Containers. But I’m not sure if this solves my problem. I tried several different approaches (during my search in the web), but now I’m stuck.

Answer 1

There are two problems. First there’s this:

public GenericBuilder<GenericBuilder> choice() { 
    return new GenericBuilder<GenericBuilder>(this);
}

That’s not saying what kind of GenericBuilder the “inner” one is. You should be able to change it to:

public GenericBuilder<GenericBuilder<P>> choice() { 
    return new GenericBuilder<GenericBuilder<P>>(this);
}

At that point, the inner end() call will still know it’s got a GenericBuilder<RuleBuilder> instead of a raw GenericBuilder.

The second problem is here:

public <P> P end() {
    return (P)parentBuilder;
}

That’s a generic method when it shouldn’t be – you don’t want to introduce a new type parameter P here; you just want the existing one:

public P end() {
    return parentBuilder;
}