Question

I’m trying out Effective Java book and enjoying it. I read Builder pattern and I tried to play with it. I have a code like this (in Groovy):

public class Anto {
    public static void main(String[] args) {

            def testing = new Java.Builder(1).author("antoaravinth").build()
            println testing.author
        }
}

class Java {
    int version
    def author
    int release_number

    public static class Builder {
        int version

        def author = ""
        int release_number = 0

        public Builder(int version) {
            this.version = version
        }

        public Builder version(int version)
        {
            version = version
            return this
        }

        public Builder author(def author)
        {
            author = author
            return this
        }

        public Builder release_number(int release_number)
        {
            release_number = release_number
            return this
        }

        public Java build() {
            return new Java(this);
        }


        private Java(Builder builder) 
        {
            version = builder.version
            author = builder.author
            release_number = builder.release_number
        }

    }
}

But I get error like this :

Caught: groovy.lang.GroovyRuntimeException: Could not find matching constructor for: Java(Java$Builder)
groovy.lang.GroovyRuntimeException: Could not find matching constructor for: Java(Java$Builder)
    at Java$Builder.build(Anto.groovy:43)
    at Java$Builder$build.call(Unknown Source)
    at Anto.main(Anto.groovy:4)

I don’t know why this happens! Where I went wrong?

Answer 1

This is your build() method:

public Java build() {
    return new Java(this);
}

Look at your Java class (not the Builder) – it doesn’t have any explicitly-declared constructors, so the only constructor it has is the public parameterless one declared automatically for you.

You need to move this:

private Java(Builder builder) 
{
    version = builder.version
    author = builder.author
    release_number = builder.release_number
}

outside the Builder class. (I’m slightly surprised Groovy didn’t complain earlier, but I’m pretty sure that’s the problem…)

EDIT: Now that it’s not throwing an exception, look at your “setter” methods:

public Builder version(int version)
{
    version = version
    return this
}

The version = version line doesn’t do anything. You need to differentiate between the parameter and the field, e.g.

public Builder version(int version)
{
    this.version = version
    return this
}

… or by giving the parameter a different name, e.g.

public Builder version(int value)
{
    version = value
    return this
}