Groovy constructors

Question

Hello, I'm having a problem while using constructors with a Groovy class.

I have a class Data in a file called Data.groovy with an inner class ContentEntry. I want to initialize ContentEntry instances from a Data method:

static void initContent(nid, uid)
{
    curContent = new ContentEntry()
    curContent.nid = nid;
    curContent.uid = uid;
}

with ContentEntry defined as:

class ContentEntry
{
    public int nid, uid 
    private tags = [:]

    public ContentEntry()
    {

    }

    ContentEntry(int nid, int uid)
    {
        this.nid = nid
        this.uid = uid
    }

    //omitted rest of the class

But when running the project it gives me the following error:

Exception in thread "main" org.codehaus.groovy.runtime.metaclass.MethodSelectionException:
Could not find which method <init>() to invoke from this list:
public it.softit.Data$ContentEntry#<init>(it.softit.Data, int, int)
public it.softit.Data$ContentEntry#<init>(it.softit.Data)

It's like if constructors implicitly need an instance of the outer class passed as a parameter. I'm wondering why..

Answer 1

Of course the constructor need a instance of the outer class. That's why inner class exist: to live only within an outer class. You cannot instanciate an inner class outside of its outer class.

Answer 2

In Java happens the same, but when you instantiate an inner instance you don't have to include the hidden parameter.

Try one of two things:

1. declaring the inner class static (I hope you can do that in Groovy too)
2. passing this in the creation of the ContentEntry instance... :)

Answer 3

The requirement that all (non-static) inner classes need a reference to their outer class is imposed by Java, rather than Groovy. If you instantiate the inner class from a non-static method, the reference should be set to this. However, there is no this reference within a static method.

To fix the problem either:

• Instantiate the inner class from a non-static method
• Make the inner class static. Then you can instantiate it from anywhere (but it will no longer have a reference to an instance of the outer class).