Using generics for a template pattern

[Eric C Barnhill]

I'm trying to create a template design pattern, starting with an abstract base class containing generic types, and an application class incorporating concrete types. It doesn't compile and I can't understand why.

The generic class is:

package genericsSandbox;

abstract class GenericClass<T1,T2> {

    private T1 item1;
    private T2 item2;

    public GenericClass(T1 item1, T2 item2) {
        this.item1 = item1;
        this.item2 = item2;
    }

}

and the component class is:

package genericsSandbox;

public class ComponentClass extends GenericClass {

    private double[][] item1;
    private double[] item2;

    public ComponentClass(double[][] item1, double[] item2) {
        this.item1 = item1;
        this.item2 = item2;
    }

    public static void main(String[] args) {
        ComponentClass cc = new ComponentClass(new double[][]{{1,2,3},{1,2,3}}, new double[]{1,2,3});
    }
}

But, the compiler doesn't like it:

genericsSandbox/ComponentClass.java:8: error: constructor GenericClass in class GenericClass<T1,T2> cannot be applied to given types;    public ComponentClass(double[][] item1, double[] item2) {                                                            ^  required: Object,Object  found: no arguments  reason: actual and formal argument lists differ in length  where T1,T2 are type-variables:    T1 extends Object declared in class GenericClass    T2 extends Object declared in class GenericClass 1 error

What really throws me is the "found: no arguments". Anyway I am missing something conceptually here. Why this ExtensionClass is not a valid implementation of the GenericClass?

[Henry Wong]

Eric C Barnhill wrote:
What really throws me is the "found: no arguments". Anyway I am missing something conceptually here. Why this ExtensionClass is not a valid implementation of the GenericClass?

First, you are trying to use the GenericClass in its raw form. This should be done for backwards compatibility only. Second, your GenericClass class doesn't have a no-args constructor. A default no args constructor is only provided if no constructor is defined. And third, what is the purpose of having those instance variables in the GenericClass class, if they won't be used?

Isn't something like this better?

	public class ComponentClass extends GenericClass<double[][], double[]> {
	    public ComponentClass(double[][] item1, double[] item2) {
	    	    super(item1, item2);
	    }
	}

And also... you may want to make the GenericClass instance variables protected, or provide getters (also protected), so that the ComponentClass can access the values.

Henry

[Eric Barnhill]

That clarifies matters a great deal, thank you.