How to use interface as parameter?

Question

I am using EJML and I want to use the class LinearSolver_B64_to_D64, which has the constructor: LinearSolver_B64_to_D64(LinearSolver<BlockMatrix64F> alg) with the interface LinearSolver<BlockMatrix64F> and the class has already implemented the LinearSolver.

What I know: In general you create a interface, than you would implement that interface in a specific class. I read about functions(in the specific class) that take the interface as a parameter, because that way your functions does not need to know something about the class.

My question: I don't know how to initialise the class LinearSolver_B64_to_D64, because I don't know how to pass an interface as a parameter.

Update: I tried the following code:

public class UseMatrixInterface{
    public UseMatrixInterface(){    
    }

    public void do1(){
        DenseMatrix64F a = new DenseMatrix64F(3,3);
        LinearSolver_B64_to_D64 ls = new LinearSolver_B64_to_D64(null); 

//it throws a nullpointer exeption. I assume, it is because i used null
//instead of the requiered parameter.
        ls.invert(a);
        a.print();
    }

    public void do2(){
        LinearSolver<BlockMatrix64F> lsD;
        LinearSolver_B64_to_D64 ls = new LinearSolver_B64_to_D64(lsD);
        //not working, because lsD cannot be initialised;
    }
}

You need to pass an implementation of an interface as the actual value. — Mark Rotteveel, Dec 23 '16 at 09:55
You need an instance of some class that implements the LinearSolver interface. — Maurice Perry, Dec 23 '16 at 09:56

score 1 · Answer 1 · answered Dec 23 '16 at 10:06

1

Just study javadoc; like starting here for the interface LinearSolver.

And guess what: there is a section

All Known Implementing Classes: AdjLinearSolverQr_D64, ...

Go and pick the one that matches your needs; and create an instance of that class.

So the answer to your question is: you can't instantiate interfaces. Instead you look out for classes implementing the interface, and then you create an instance of such a class. Like in:

List<String> strings = new ArrayList<>();

answered Dec 23 '16 at 10:06

GhostCat

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I am very new to programming, so reading the javadoc is also "difficult", but i tried `LinearSolver_B64_to_D64 ls = new LinearSolver_B64_to_D64(argument);` , but where "argument" ist should be the parameter for the interface and now i dont know how to create an instance, because the constructor requieres a interface. – M.Mac Dec 23 '16 at 10:56
Now i am very confused because an implementing class takes the implemented interface as a parameter?? – M.Mac Dec 23 '16 at 10:57

score 1 · Answer 2 · answered Jan 09 '17 at 05:03

Not really sure what's going on with this question and these answers. LinearSolver_B64_to_D64 is a low level class for converting a linear solver for block matrices into the standard DenseMatrix64F. You probably don't want to use that.

The Manual:

LinearSolverFactory is the preferred way to create a new LinearSolver and hides most of the low level details from you.

score 0 · Answer 3 · answered Dec 23 '16 at 10:12

0

create an instance of a class that implements the interface. Most likely the interfaces contract (the the JavaDoc of the interface) is simple enough to do this as an anonymous inner class
```
LinearSolver<BlockMatrix64F> alg = new LinearSolver<BlockMatrix64F>(){
  // implement interface method(s) here
}   
```

then

pass that object to the constructor of LinearSolver_B64_to_D64()
```
LinearSolver_B64_to_D64(alg);
```

answered Dec 23 '16 at 10:12

Timothy Truckle

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I think you are on the wrong track. As I have outlined in my answer there are **plenty** of existing classes in that framework that implement that interface. Do you really think implementing a **Solver** for linear system is a small "here do implement anonymous interface" kind of activity?! – GhostCat Dec 23 '16 at 10:15
*"As I have outlined in my answer there are plenty of existing classes in that framework that implement that interface."* Then I don't get the point of your question. Do you really wan the community to decide which of that "plenty" implementations you should use? – Timothy Truckle Dec 23 '16 at 10:28
1

No. I want **him** to figure what **he** should do. Do you really suggest that he **implements** a linear solver on his own; instead of **first** looking into the *existing* implementations, already available to him at his fingertips? Or in other words: if he would be asking for an implementation of the List interface, would you also suggest to implement that thing himself to him? – GhostCat Dec 23 '16 at 10:41

How to use interface as parameter?

3 Answers3