Client2.java

Planet mars = new Mars()
Planet saturn = new Saturn();
Planet mercury = new Mercury()

Explorer explorer = new LifeExplorer();

List<Planet> planetsToBeVisited = new ArrayList<>();
planetsToBeVisited.add(mars);
planetsToBeVisited.add(saturn);
planetsToBeVisited.add(mercury);

for (Planet planet : planetsToBeVisited) {
    planet.accept(explorer);
}