java.lang.IllegalStateException in iterator.remove()

Question

Rocket class contains: canCarry (Item item)>checks if this item can be carried/ carry updates the weight with total weight.

U2 class is child of Rocket contains: currentweight , maxWeight =18 tons Item class contains: name to be shipped & weight.

In the method loadU2 I am trying to access a list of items and adding it into one rocket until maxWeight of that rocket is reached. For example I have 216 tons of items to carry returning a list of 12 ships.

It throws me java.lang.IllegalStateException error in the line iterator.remove() . I do not know how to go about it, but it looks like it is not allowing me to remove the items while iterating.

public ArrayList<Rocket> loadU2(ArrayList<Item> loadItems){
    //list of ships
    ArrayList<Rocket> U2Ships = new ArrayList<Rocket>();
    for(Iterator<Item> iterator = loadItems.iterator(); iterator.hasNext();) {      
        //create a new ship
        Rocket tempShip = new U2();
        Item tempItem = iterator.next();
        //loop over items check if it can be filled then remove the item that was filled.
        while(tempShip.currentWeight<tempShip.weightLimit) {
            if(tempShip.canCarry(tempItem)){
                tempShip.carry(tempItem);
                iterator.remove();
            }           
        }
        U2Ships.add(tempShip);
    }
    return U2Ships;
}   


Exception in thread "main" java.lang.IllegalStateException
    at java.base/java.util.ArrayList$Itr.remove(ArrayList.java:980)
    at Simulation.loadU1(Simulation.java:35)
    at Main.main(Main.java:14)

Simplified example of what the code is doing: Assuming maxWeight for each ship = 11 tons ArrayList loadItems = [3,5,5,8,1,2,3,5] tons

 - Ship[1]=[3,5,1,2]
 - new list to iterate over >> [5,8,3,5]
 - Ship[2]=[5,3]
 - new list to iterate over >> [8,5]
 - Ship[3]=[8]
 - new list to iterate over >> [5]
 - Ship[4]=[5]

Answer 1

Please, rewrite your code by creating new ArrayList, instead of changing the existing list inside its own iterator:

public ArrayList<Rocket> loadU2(ArrayList<Item> loadItems){
    //list of ships
    ArrayList<Rocket> U2Ships = new ArrayList<Rocket>();
    ArrayList<Item> updatedLoadItems = new ArrayList<Item>();
    for(Iterator<Item> iterator = loadItems.iterator(); iterator.hasNext();) {      
        //create a new ship
        Rocket tempShip = new U2();
        Item tempItem = iterator.next();
        //loop over items check if it can be filled then only leave the load item that was not fully filled.
        boolean addLoadItem = true;
        while(tempShip.currentWeight<tempShip.weightLimit) {
            if(tempShip.canCarry(tempItem)){
                tempShip.carry(tempItem);
                addLoadItem = false;
            }         
        }
        if (addLoadItem) {
          updatedLoadItems.add(tempItem);
        };
        U2Ships.add(tempShip);
    }
    loadItems.removeAll();
    loadItems.addAll(updatedLoadItems);
    return U2Ships;
} 

This is not the best solution, but to provide a better solution, you need to change the signature of public ArrayList<Rocket> loadU2(ArrayList<Item> loadItems)

You can try to improve your code by refactoring it.

Hint: right now your loadU2 method tries to do both things at the same time: change loadItems and create U2Ships . This is a direct violation of the single responsibility principle . Try to imagine the soldier who would try to shoot the gun and throw grenade at the same time. One thing at the time.

Answer 2

The problem is here:

while(tempShip.currentWeight<tempShip.weightLimit) {
    if(tempShip.canCarry(tempItem)){
        tempShip.carry(tempItem);
        iterator.remove();
    }           
}

You are calling iterator.remove() within a loop. If the condition tempShip.canCarry(tempItem) holds twice, you call iterator.remove() twice, and this is not allowed (the second time, the item is already removed).

I don't know how the method canCarry is implemented, but note that if it is the case that tempShip.currentWeight<tempShip.weightLimit is true, but tempShip.canCarry(tempItem) is false, your loop will run forever.

Answer 3

use listIterator instead of Iterator.

ListIterator<Book> iter = books.listIterator();
while(iter.hasNext()){
    if(iter.next().getIsbn().equals(isbn)){
        iter.remove();
    }
}

like used here.

Remove elements from collection while iterating

Answer 4

public ArrayList<Rocket> loadU2(ArrayList<Item> loadItems){
    //list of ships
    int shipNum=0;
    int itemsloaded=0;
    ArrayList<Rocket> U2Ships = new ArrayList<Rocket>();
    while(!loadItems.isEmpty()) {      
        System.out.println("number of ships created: "+shipNum++);
        //create a new ship
        Rocket tempShip = new U2();
        
        //loop over items check if it can be filled then only leave the load item that was not fully filled.
     
        while(iterator.hasNext()) {  
            Item tempItem = iterator.next();
            if(tempShip.canCarry(tempItem)){
                System.out.println("number of items loaded: "+(itemsloaded++));
                tempShip.carry(tempItem);
                iterator.remove();                                      
             
           } 
        }
        
        U2Ships.add(tempShip);
    }

    return U2Ships;
} 

Thank you guys for the help, this should fix 2 problems: infinity, and the iterator.remove().