ArrayList.remove（Object o）不遍历内容

Question

So, I've been sitting on this problem for several hours now and I'm out of ideas. I looked through the documentation (ArrayList and Object) and read a lot of Stackoverflow questions regarding ArrayList.remove(Object obj) , but nothing helped.

I try to implement a simulation (game?) where I have a Cell[][] to store the map. Each Cell has inhabitants (class Person) stored in an ArrayList, which move one step every iteration. When a Person moves I add them to the next Cell and remove them from the old one. Or rather try, as the remove part doesn't work yet.

My Question: I need help implementing the ArrayList.remove(Object obj) method.
What I got so far:

class Cell
{
    private final int x;
    private final int y;
    ArrayList<Person> inhabs = new ArrayList<Person>();

    /** this method is called by a Person that wants to move to another Cell **/
    public void moveMe(Person p){
        try{
            Cell northOfHere = grid.grid[y-1][x]//direction will be changed to be random
            northOfHere.add(p);
        }catch(ArrayIndexOutOfBoundsException ex){/*person falls from the edge of the world*/}

        inhabs.remove(p); //this is not working right
    }

    public void add(Person p){
        inhabs.add(p);
    }
}

Class Person:

class Person extends Object
{
    private int health;
    private int strength;
    private int food;
    private final int nation;

    @Override
    public boolean equals(Object obj){
        if(obj == null){return false;}
        if(obj == this){return true;}
        if(!(obj instanceof Person)){return false;}

        Person p = (Person)obj;
        return p.toString().equals(toString());
    }

    @Override
    public int hashCode(){
        return toString().hashCode();
    }

    @Override
    public String toString(){
        return String.format("Health: %03d, Strength: %03d, Food: %03d, Nation: %02d", health, strength, food, nation);
    }
}

Some key points:

• The inhabs.remove(p); is not working right how I want it.
• It does work in the first iteration, but in the second it only returns false .
• The first iteration over the Person.equals(Object obj) by the ArrayList always returns true because of the line if(obj == this){return true;} .
• On the second iteration it never even starts to compare the objects (?!?).
• The ArrayList is not empty.

I understand that this is a very specific question, however I could not find anything that adressed the problem of the ArrayList not even going over the Elements in it.

edit: more Code. Probably irrelevant to the problem, but i'll post it anyway, for context.

class Grid{        
    Cell[][] grid;

    public Grid(Cell[][] in){
        grid = in; //has some cells with people in it.
    }

    //simply iterates over all cells
    public void tick(){
        for(int y = 0; y<grid.length; y++){
            for(int x = 0; x<grid[y].length; x++){
                grid[y][x].tick();
            }
        }
}


class Cell
{
    Grid grid;

    public Cell(/*some constants constants*/ Grid g, ArrayList<Person> inh){
        grid = g;
        if(inh == null){
            inhabs = new ArrayList<Person>();
        }else{
            inhabs = inh;
        }
    }

    //tells every person to make a step
    public void tick(){
        for(Person p : inhabs.toArray(new Person[inhabs.size()])){
            p.tick();
        }
    }

    //and all the other methods from above, of course
}


class Person extends Object
{
    Cell cell;

    public Person(Cell c, int h, int s, int f, int n){
        cell = c;
        health = h;
        strength = s;
        food = f;
        nation = n;
    }

    public void tick(){
        //for testing, later more options
        cell.moveMe(this);
    }

    //and all the other methods from above, of course
}

Answer 1

This can't be reproduced, here is a small test with your logic :

public static void main(String args[]) {
    Person p1 = new Person(10, 2, 2, 1);
    Person p2 = new Person(10, 2, 2, 2);
    Person p3 = new Person(10, 2, 2, 3);

    List<Person> list = new ArrayList<Person>();
    list.add(p1);
    list.add(p2);
    list.add(p3);

    System.out.println(list.remove(p2)); //true
    System.out.println(list.remove(p2)); //false
    System.out.println(list.remove( new Person(10, 2, 2, 3))); //true
}

My guess is that you don't use the correct Person , you either construct a new instance without giving every variables correctly. (Note that you didn't provide the constructor here, with a final value, this can't work...)

---

I have read your code. Your problem is simple.

You tick a Cell that will tick every Person in that Cell . From there you move that Person from the current Cell using Cell.moveMe(Person p); . You do move the instance in the new Cell.inhabs and remove it from the current one. BUT since you have references of all your instance in both way, your Person is never instructed to which Cell it is moved ... so when you call :

public void tick(){
    //for testing
    cell.moveMe(this);
}

cell is still the initial position... but that cell don't hold that person, so it can't remove the instance received.

A simple correction would be to return the cell where you move him into :

public Cell moveMe(Person p){
    Cell northOfHere = null;
    try{
        northOfHere = grid.grid[y-1][x];
        northOfHere.add(p);
    }catch(ArrayIndexOutOfBoundsException ex){
        System.out.println("fell from edge");/*person falls from the edge of the world*/
    }
    System.out.format("Remove from %s person %s%n", this, p);
    System.out.println("removed: " + inhabs.remove(p));
    return northOfHere;
}

And update that cell :

public void tick(){
    //for testing
    cell = cell.moveMe(this);
}

I agreed here to keep cell to null when he "feel off the map", as it remain outside of any cell.

Answer 2

As Andy Turner pointed out, you need to fix the equals() method. In Person class the equals() must be precise.

There is a contract between hashCode() and equals() : if one object equals another, then their hashCode() must be the same. So the easiest way is to piggyback equals() too on toString() :

class Person // automatically extends Object
{

    // ...
    @Override

    public boolean equals(Object obj) {
        return (obj instanceof Person) && // instanceof is null-safe
            this.toString().equals((Person) obj).toString();
    }
}

Honestly, I don't like writing equals() and hashCode() methods, unless absolutely necessary. They are cumbersome and error-prone. In this case, it doesn't look absolutely necessary. There are a few alternatives:

• generate them with your IDE
• use apache EqualsBuilder and HashCodeBuilder . Here is a nice example.

With HashCodeBuilder you'd write code like this:

@Override
public int hashCode() {
   return new HashCodeBuilder()
      .append(this.health)
      .append(this.strength)
      ...
      .toHashCode();
}

With EqualsBuilder you'd write code like this:

@Override
public boolean equals(Object obj) {
      if (obj instanceof Person == false)
      {
        return false;
      }
      if (this == obj)
      {
         return true;
      }

      Person that = (Person) obj;
      return new EqualsBuilder()
        .append(this.health, that.health)
        .append(this.strength, that.strength)
        ...
        .isEquals();
}