Arraylist.clear clearing all array lists?

[gummby8 26]

I am making a tile entity that will spawn up to a preconfigured number of animals. It uses 2 lists a spawnList and a foundList.

it first clears the foundList

Searches for entities in the area. using AABB

compares the entities found in the AABB list with the spawnList

And entity IDs that match the spawnList are put into the foundList

Then the spawnList = foundList

Based on the new spawnList size it will spawn more or do nothing.

 

@Override
public void update() {
	ticksExisted++;
	//called each tick just like onUpdate
	// TODO Auto-generated method stub
	int time = 10 * 20; // 20 ticks per second times 10 seconds
	if (this.ticksExisted % time == (time - 1) && !this.worldObj.isRemote) { // if you check against 0, it will drop an item immediately every time the world loads
		foundList.clear();
		List list = this.worldObj.getEntitiesWithinAABB(EntityLiving.class, this.getRenderBoundingBox().expand(20, 20, 20));
		Iterator iterator = list.iterator();
        while (iterator.hasNext())
        {
            Entity entity = (Entity)iterator.next();
            	if (spawnList.contains(entity.getEntityId())){
            		foundList.add(entity.getEntityId());
            		System.out.println("found");
            		
            	}
            
        }
        

        System.out.println("spawnList = " + spawnList.size());
        System.out.println("foundList = " + foundList.size());

        


		spawnList = foundList;

		if (spawnList.size() < 5){
			spawnCreature(this.worldObj, message, this.pos.getX(),this.pos.getY() + 2, this.pos.getZ() );

		}


	}
}

 

 

The issue that I am seeing is if I have the foundList.clear(); in the code, the two lists are always 0 when it prints out. THe tile entity spawns a creature, lists the spawnList size as 1, but then the next cycle, the spawnList is zeroed out before it even gets to the spawnList = foundList.

 

Even if I comment the spawnList = foundList out. the spawnList is still cleared when the foundList is cleared. If I comment out the foundList.Clear() section, the lists count up normally. But I need the foundList to be cleared out at the beginning of each cycle.

 

So what the heck is going on?

[gummby8 26]

further testing

 

		System.out.println("spawnList = " + this.spawnList.size());
		this.foundList.clear();
		System.out.println("spawnList = " + this.spawnList.size());

 

this prints out

 

1

0

 

What the hell?

[Draco18s 2098]

Ah.

 

Pointers.

 

You have created a reference to a single list.

[jabelar 591]

you set spawnList = foundList.  So anything you do to one will happen to the other.

 

Java doesn't copy when you use an assignment to an object, rather it makes them both point to the same thing. This is powerful, but obviously also dangerous.

 

You need to use a copy method rather than an = assignment.

 

Note that this applies to objects, but for literals and primitives = operator creates a copy. So for example, if you wanted to copy an Array of int values, you can't use = between the arrays, but you could loop through the array and use = between the int elements. I hope you understand what I mean...

[Draco18s 2098]

(Note that strings are complex objects as far as Java is concerned, which can lead to some really naughty things).

[Enginecrafter 4]

As many, many more people said, this is not java school. Assigning and copying data types is Basic Java.

Just wanted to say to be carefull when posting or your thread will get locked and you may not know why. Just a warning.

[gummby8 26]

you set spawnList = foundList.  So anything you do to one will happen to the other.

 

Java doesn't copy when you use an assignment to an object, rather it makes them both point to the same thing. This is powerful, but obviously also dangerous.

 

You need to use a copy method rather than an = assignment.

 

Note that this applies to objects, but for literals and primitives = operator creates a copy. So for example, if you wanted to copy an Array of int values, you can't use = between the arrays, but you could loop through the array and use = between the int elements. I hope you understand what I mean...

 

Got it

 

Collections.copy(spawnList, foundList);

 

That fixed it

Thanks

[diesieben07 6693]

(Note that strings are complex objects as far as Java is concerned, which can lead to some really naughty things).

This is why setting final fields is evil and should never be done.

[Draco18s 2098]

(Note that strings are complex objects as far as Java is concerned, which can lead to some really naughty things).

This is why setting final fields is evil and should never be done.

Oh, absolutely.  I would never recommend hacking objects like that.  Just that the result is fucking hilarious.

This one fucks about with the IntegerCache for much the same result.

[diesieben07 6693]

The one which changes Boolean.TRUE and Boolean.FALSE is better imho

[Draco18s 2098]

The one which changes Boolean.TRUE and Boolean.FALSE is better imho

 

Hehe.