CODEWITHSUNDEEP
javaarraysarraylist
Paul Blundell
Paul Blundell

Reputation: 1857

Java ArrayList and Objects issue. Values resetting

I have 3 classes and a main class. The classes are City, Tour and Population. A population has an arraylist of X amount of tours and tour has an array list of X amount of cities. Where a City is just an X and Y coordinate. When I create new tours the cities within them are randomises and then i add the tour to the population arraylist and this is working fine, the outputs are randomised. However if I then make output the arraylist again they are no longer randomised and all the tours are the same. I think this will be easier to explain by showing some code:

Population.java

public class Population {

private ArrayList<Tour> toursList;

public Population(int size, ArrayList<City> loadedCities)
{
    toursList = new ArrayList<Tour>();
    
    // Fill the population with the number of tours
    for (int i = 0; i < size; i++) {
        Tour newTour = new Tour(loadedCities);
        newTour.setId(i);
        toursList.add(newTour);
    }
    
}

public void output() 
{
    for (Tour tour : toursList)
        System.out.println(tour.toString());
}
}

Tour.java

public class Tour {

private ArrayList<City> tour;
private int id = 0;

public Tour(ArrayList<City> cities)
{
    Collections.shuffle(cities);
    tour = cities;
}

public void setId(int i)
{
    this.id = i;
    System.out.println("Constructor: "+toString());
}

public int getId()
{
    return id;
}

public String toString()
{
    String str = "Tour: "+id+" - ";
    for (City city : tour) {
         str += city.toString()+" | ";
    }
    return str;
}
}

City.java

public class City {

private int code;
private Double y;
private Double x;

public City(int code, Double y, Double x)
{
    this.code = code;
    this.y = y;
    this.x = x;
}

public int getCode() 
{
    return code;
}

public Double getX() 
{
    return x;
}

public Double getY()
{
    return y;
}

public String toString()
{
    return "Code: "+this.code+" - X: "+this.x+" Y: "+this.y;
}
}

And then the main class just makes these calls after loading the cities arraylist:

Population population = new Population(10, cities);
population.output();

The console output for the few println's are as follows (stripped down version):

Constructor: Tour: 0 - Code: 2 - X: 42373.8889 Y: 11108.6111
Constructor: Tour: 1 - Code: 28 - X: 43026.1111 Y: 11973.0556
Tour: 0 - Code: 8 - X: 42983.3333 Y: 11416.6667
Tour: 1 - Code: 8 - X: 42983.3333 Y: 11416.6667

You can see the tours have now all become the same and in the same order.

Upvotes: 0

Views: 451

Answers (2)

iwein
iwein

Reputation: 26161

Java uses mutable collections. Since you're passing in the same collection into all the Tours it gets shuffled a lot of times, but in the end all tours have a reference to the same, much shuffled, collection.

Two things to learn from this:

  1. if you want to modify a collection you get passed in, first make a copy (e.g. new ArrayList(collectionToCopy))
  2. if you pass a collection you own to someone else, first make sure he can't change it (e.g. Collections.unmodifiableList(myCollection))

You'll step into this trap a lot of times in your Java career. And if you don't, someone else will do it for you.

Upvotes: 0

user1766873
user1766873

Reputation:

You use the same ArrayList<City> cities for all tours.

Tour constructor should be like this:

tour = new ArrayList<City>(cities);
Collections.shuffle(tour);

Upvotes: 2

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