Question

public class Graph {

    private Node node;

    public void createGraph()
    {

    }

    private class Node<K>{
        K data;
        List<Node> adjacent;
        boolean visited; 

        Node()
        {
            adjacent = new ArrayList<Node>();
            visited = false;
        }

        Node(K data)
        {
            this.data = data;
            this.Node();

        }

    }



}

Why is the compiler complaining that I can’t call this.Node() ?

Answer 1

Try:

 Node(K data)
    {
        this();
        this.data = data;

    }

The call to the “other” constructor needs to be always first. The call to the other constructors from inside a constructor is always done with this(...) not with Node(...).

JLS section 8.8.7 specify how the constructor body should look like:

ConstructorBody: { ExplicitConstructorInvocation(opt)
BlockStatements(opt)
}

where ExplicitConstructorInvocation(opt) is either an alternate constructor or a constructor from the parent class invoked with super(...).

and JLS section 12.5 specify the object initialization steps when a constructor is called:

Just before a reference to the newly created object is returned as the result, the indicated constructor is processed to initialize the new object using the following procedure:

• Assign the arguments for the constructor to newly created parameter variables for this constructor invocation.

• If this constructor begins with an explicit constructor invocation (§8.8.7.1) of another constructor in the same class (using this), then evaluate the arguments and process that constructor invocation recursively using these same five steps.

…

These JLS rules make sure that the parent class constructor is called first and once.