SwappingObjs.java

class Car {
    int number;

    Car(int number) {
        this.number = number;
    }

    void printNumber() {
        System.out.println(this.number);
    }
}

class WrapperCar {
    Car c;

    WrapperCar(Car c) {
        this.c = c;
    }
}

public class SwappingObjs {

    static void swap(Car c1, Car c2) {
        Car temp = c1;
        c1 = c2;
        c2 = temp;
    }

    static void swapCorrectly(WrapperCar wc1, WrapperCar wc2) {

        Car temp = wc1.c;
        wc1.c = wc2.c;
        wc2.c = temp;

    }

    public static void main(String[] args) {
        Car c1 = new Car(1);
        Car c2 = new Car(2);

        //This works as we are not passing the objects
        // Car tmp = c1;
        // c1 = c2;
        // c2 = tmp;

        //Passing doesn't work because java is pass by value and not reference
        swap(c1, c2);
        System.out.println(c1.number);
        System.out.println(c2.number);

        //Finally this will work
        WrapperCar wc1 = new WrapperCar(c1);
        WrapperCar wc2 = new WrapperCar(c2);

        swapCorrectly(wc1, wc2);
        System.out.println("Car1: " + wc1.c.number);
        System.out.println("Car2: " + wc2.c.number);
    }
}