Planet2.java

interface Planet{
    public 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);
    }
}