Calling the object's own methods

A method can call the other methods of its own object. Inside the class, this.isFull() calls isFull on the same object the current method was called on. This bus uses that:

public class Main {
    public static void main(String[] args) {
        Bus bus = new Bus(2);
        bus.board();
        System.out.println(bus);
        bus.board();
        bus.board();
        System.out.println(bus);
    }
}

public class Bus {
    private int seats;
    private int passengers;

    public Bus(int seats) {
        this.seats = seats;
        this.passengers = 0;
    }

    public int seatsLeft() {
        return this.seats - this.passengers;
    }

    public boolean isFull() {
        return this.seatsLeft() == 0;
    }

    public void board() {
        if (this.isFull()) {
            System.out.println("Sorry, the bus is full");
        } else {
            this.passengers++;
        }
    }

    public String toString() {
        return this.passengers + " on board, seats left: " + this.seatsLeft();
    }
}

1 on board, seats left: 1
Sorry, the bus is full
2 on board, seats left: 0

• seatsLeft() works out the free seats. isFull() asks seatsLeft() instead of doing the sum again, and board() asks isFull() before it lets anyone on.
• Each rule is written in one place. If the bus took standing passengers one day, only seatsLeft would change, and isFull, board and toString would all follow.
• this. in front of the call can be left out: inside the class, isFull() means the same. As with instance variables, writing it shows that the method belongs to this object.

A method that only the class itself needs can be private, like the instance variables. If nothing outside the bus used seatsLeft, it could be private int seatsLeft(): isFull and toString could still call it, but main couldn't.