Question

I have blocks performing calculations using a function step(). The blocks can be connected to
each other with connect(Block).

interface Block {
    void connect(Block b);
    void step();
}

However from within a concrete block implementation (e.g. in step) it should be
possible to read from the connected block:

class ABlockImpl implements Block {
    private Block src; // link to the block this block is connected to
    public void connect(Block b) {
        src = b;
    }

    public void step() {
        double x = src.read(); // XXX src is of type Block and there is no read() in Block
        /* ... */
    }

    public double read() {
        return 3.14;
    }
}

Since there is no read() in Block, this won’t compile. For clients the “public” Block interface is sufficient, I need read only internally. I could add read to the Block interface, but to me this feels wrong.

Since there are multiple different implementations of Block, I cannot cast src to ABlockImpl before the call to read.

Is there an alternative way to “hide” read?

Answer 1

You can have a public interface and a package local one

public interface MyPublicInterface {

}

interface MyDirectInterface extends MyPublicInterface {

}

class MyImpl implements MyDirectInterface {

    public void add(MyPublicInterface mpi) {
         MyDirectInterface mdi = (MyDirectInterface) mpi;
         // use mdi
    }

}