java - 为什么下面for循环中的 “getPassenger()”方法可以直接调用而不需要先创建对象？

Question

为什么下面的 for 循环中使用“getPassenger()”方法可以直接调用不先创建对象？

package org.airline.reservations;

import java.util.ArrayList;

public class Database {

//fields
private ArrayList<Seat> seats;
private ArrayList<Flight> flights;
private ArrayList<Ticket> tickets;
private ArrayList<Passenger> passengers;

//constructor
public Database() {
    seats = new ArrayList<Seat>();
    flights = new ArrayList<Flight>();
    tickets = new ArrayList<Ticket>();
    passengers = new ArrayList<Passenger>();
}
//get the seats arraylist
public ArrayList<Seat> getSeats() {
    return seats;
}
//get the flights arraylist
public ArrayList<Flight> getFlight() {
    return flights;
}
//get the tickets arraylist
public ArrayList<Ticket> getTicket() {
    return tickets;
}
//get the passengers arraylist
public ArrayList<Passenger> getPassenger() {
    return passengers;
}
//add an Seat object to the seats arraylist
public void addSeat(int seatNumber) {
    seats.add(new Seat(seatNumber));

}
//get the seats arraylist
public ArrayList<Seat> getSeat() {
    return seats    ;
}

//this method add a Flight object to the flights arraylist
public void addFlight(String departureCity, String arrivalCity, int flightNumber) {

    flights.add(new Flight(departureCity, arrivalCity, flightNumber));

}

//this method add a Passenger object to the passengers arraylist , if the 
//passenger object already exists in the passengers arraylist return true 
//else reture false

我的问题是为什么下面的for循环中的“getPassenger()”方法可以直接调用而无需先创建对象？

public boolean addPassenger(String passengerName) {
        boolean passengerExists = false;
        for (Passenger item : getPassenger()){
            if (passengerName.equals(item.getName())) {
                passengerExists = true;

            } 
        }
        passengers.add(new Passenger(passengerName));
        return passengerExists;
}

Answer 1

addPassenger() 本身是一个实例级方法。因此，您需要使用Database 实例来调用它。同一实例(也称为 this )隐式传递给其他实例方法调用。