Planet2.java

public interface Planet {
    void accept(Explorer explorer);
}

public class Mercury implements Planet {
    @Override
    public void accept(Explorer explorer) {
        explorer.visit(this);
    }
}

public class Mars implements Planet {
    @Override
    public void accept(Explorer explorer) {
        explorer.visit(this);
    }
}

public class Saturn implements Planet {
    @Override
    public void accept(Explorer explorer) {
        explorer.visit(this);
    }
}