Visitor pattern implementation in case of source code un-availability

Question

One of the reasons to consider the Visitor_pattern:

A practical result of this separation is the ability to add new operations to existing object structu

Answer 1

First I had to made a few assumptions about the legacy code, since you didn't provide much details about it. Let's say I need to add a new method to Legacy without reimplementing everything. This is how I'll do it:

public interface LegacyInterface {
    void A();
}

public final class LegacyClass implements LegacyInterface {
    @Override
    public void A() {
        System.out.println("Hello from A");
    }
}

First extends the "contract"

public interface MyInterface extends LegacyInterface {
    void B();
}

And implement it in a "decorated" way

public final class MyClass implements MyInterface {
    private final LegacyInterface origin;

    public MyClass(LegacyInterface origin) {
        this.origin = origin;
    }

    @Override
    public void A() {
        origin.A();
    }

    @Override
    public void B() {
        System.out.println("Hello from B");
    }
}

The key point is MyInterface extends LegacyInterface: this is the guarantee the implementations will benefit from both the services from the legacy code and your personnal addings.

Usage

MyInterface b = new MyClass(new LegacyClass());