Java如何在将一个对象添加到另一个ArrayList时将其删除？

Question

This is my first time asking on this site, and got my exam tomorrow so trying to make my code great! I will link the whole class (Even though its 3 others).

This is the method, and I cant manage to remove an object from lagerplass , adding it to hk.addVarer works tho. The idea behind all this is that lagerplass is were the items is stored in a store(butikk), so when people buy an item from the store and put it in handlekurv the items gets removed from the storage(lagerplass) :

public void leggTilHandlekurv(String varenavn)
    {
        for (Varer a : lagerplass)
        {
            if (a.getVarenavn().equals(varenavn))
            {
                hk.addVarer(a);
                lagerplass.remove(varenavn);
            }
        }
    }

Here is the kode for the whole Butikk class

public class Butikk
{
// instance variables - replace the example below with your own
public ArrayList<Varer> lagerplass;
Handlekurv hk = new Handlekurv();

/**
 * Constructor for objects of class Butikk
 */
public Butikk()
{
    // initialise instance variables
    lagerplass = new ArrayList<Varer>();
}

/**
 * An example of a method - replace this comment with your own
 * 
 * @param  y   a sample parameter for a method
 * @return     the sum of x and y 
 */
public void nyHeadset(String lyd, String vare,int pris, String varenavn )
{
    // put your code here
    Headset nyHeadset = new Headset ( lyd, vare, pris, varenavn);
    lagerplass.add(nyHeadset);

}

public void nyMus(String vare, int pris, String varenavn, int dpi)
{
    Mus nyMus = new Mus(vare, pris, varenavn, dpi);
    lagerplass.add(nyMus);
}

public void printLagerplass()
{
    for (Varer vare : lagerplass)
    {
        vare.printDetails();
    }
}

public int lagerplassSize()
{
    return lagerplass.size();
}

public void fjernHeleLagerplass()
{
    lagerplass.clear();
}

public void leggTilHandlekurv(String varenavn) 
{ 
Iterator<Varer> iterator = lagerplass.iterator();
while(iterator.hasNext()) {
     Varer a = iterator.next();
     if (a.getVarenavn().equals(varenavn)) {
         iterator.remove();
         hk.addVarer(a);
     }
}

}

public void printHeleHandlekurven()
{
    hk.printHandlekurv();
}

}enter code here

Here is the kode for the whole Varer class

enter code here public class Varer { // Representerer merke og pris til en vare. private String vare; private String merke; private int pris; private String varenavn;

/**
 * Constructor for klassen Varer
 */
public Varer(int pris, String varenavn, String vare)
{
    merke = "Razor";
    this.pris = pris;
    this.varenavn = varenavn;
    this.vare = vare;

}

public String getMerke()
{
    return merke;
}

public int getPris() 
{
    return pris;
}

public String getVarenavn()
{
    return varenavn;
}

public String getVare()
{
    return vare;
}

public String getDetails()
{
    return vare + ", " + merke +" " + varenavn + ", "+pris + " kr.";
}
public void printDetails()
{
    System.out.println(vare + ", " + merke +" "+ varenavn + ", "+pris + " kr.");
    System.out.println();
}

Answer 1

Removing an item from a collection while iterating over it should yield a ConcurrentModificationException , which I presume is what you are getting and is why you cannot remove from it.

To remove an element while iterating over a collection, you will need to get that collection's iterator and remove it through the collection's iterator.

public void leggTilHandlekurv(String varenavn) { 
    Iterator<Varer> iterator = lagerplass.iterator();
    while(iterator.hasNext()) {
         Varer a = iterator.next();
         if (a.getVarenavn().equals(varenavn)) {
             iterator.remove();
             hk.addVarer(a);
         }
    }
}

Answer 2

Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

for (Iterator<Varer> iterator = lagerplass.iterator(); iterator.hasNext();) {
    Varer a = iterator.next();
    if (a.getVarenavn().equals(varenavn))
    {
        hk.addVarer(a);
        // Remove the current element from the iterator and the list.
        iterator.remove();
    }
}

Source:

http://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html

Answer 3

It is because you cannot modify a collection while iterating over it. Either use a for loop with indeces, or an Iterator over your collection and call the remove method.