Delete specified object in arraylist from another class in java

Question

I want to delete specified data in an arraylist from another class based on what the user input so when the user input Rice, the data with 'Rice' in it will be deleted this is the code so far

the data is stored in ArrayList rm in a subclass named RegularMenu.

ArrayList<RegularMenu> rm = new ArrayList<RegularMenu>();

Quiz1(){
    int input;
        do{
            System.out.println("1. Add Regular Menu");
            System.out.println("2. Add Special Menu");
            System.out.println("3. Show All Menu");
            System.out.println("4. Delete Regular Menu");
            System.out.println("5. Delete Special Menu");
            System.out.println("6. Exit" + "\n"); 

            System.out.print("Choice [1-6] ");
            Scanner s = new Scanner(System.in);
            input = s.nextInt();

            if (input == 1 ){

                String code, name;
                int price;

                System.out.println("Add Regular Menu");
                System.out.print("Input Menu Code [R...]: ");
                s.nextLine();
                code = s.nextLine();

                System.out.print("Input Menu Name [5-20]: ");
                name = s.nextLine();

                System.out.print("Input Menu Price [10000-130000]: ");
                price = s.nextInt();

                rm.add(new RegularMenu(code, name, price));
            }

            else if (input == 2){

            }

            else if (input == 3){

            }

            else if (input == 4){


                System.out.println("Input menu code you want to delete");
                String a = s.nextLine();

                for(int i=0;i<rm.size();i++){
                    if(a == rm.get(i).getCode()){
                        String code = rm.get(i).getCode();
                        a = code;
                        rm.remove(a);
                    }

                }

            }

            else if (input == 5){

            }
        }while(input != 6);
}

i can add the data, but when try to remove it, the error occurred. Let me know if I'm not clear enough.

Answer 1

The problem is that you are using List.remove(Object) wrong instead of List.remove(int).

List.remove(Object)

boolean remove(Object o)

Removes the first occurrence of the specified element from this list, if it is present (optional operation). [...] More formally, removes the element with the lowest index i such that (o==null ? get(i)==null : o.equals(get(i))) [...]

Trying to remove a RegularMenu with a String instance won't work because a String instance can't compare itself with a RegularMenu, so it will never be equivalent. (It will use String.equals(RegularMenu) method to find where is the instance to remove.

If you want to use List.remove(Object), pass the instance itself :

rm.remove(rm.get(i));

Note:

1. that this will remove the first occurrence only, so if you have two "equivalent" instances, only the first one will be removed.
2. List.remove(Object) will search for the index where the instance passed is using Object.equals to remove by index. But in your case you already did the job with if(a == rm.get(i).getCode()) (incorrectly, see "String comparison"),

List.remove(int)

E remove(int index)

Removes the element at the specified position in this list (optional operation). Shifts any subsequent elements to the left (subtracts one from their indices) [...]

Since you know the index, you can use rm.remove(i) (List.remove(int)) to remove the value at the current index.

Careful with the index that way, the right part of the list with shift on the left. See Lajos Arpad's answer for more information

Iterator

Another solution to remove items from an ArrayList is to use the iterator. You get the Iterator and iterate every items in it. Then if one match you call Iterator.remove to remove properly the item. You can even stop the loop if you only want to remove one item

Sample data :

List<String> datas = new ArrayList<>();
datas.add("foo");
datas.add("bar");
datas.add("bar");
datas.add("foo");
datas.add("bar");
datas.add("bar");
    

Code :

Iterator<String> it = datas.iterator();
String s;
while(it.hasNext()){
    s = it.next();
    if("foo".equals(s)){
        it.remove();
    }
}

System.out.println(datas);

[bar, bar, bar, bar]

I precise using ArrayList because some Collection don't implements the method remove for the Iterator giving an Exception.

Predicate - removeIf

Since Java 8, Collection.removeIf exists and allows you to do a quicker solution (using the same sample data) :

final String a = "foo";
datas.removeIf(s -> a.equals(s));

System.out.println(datas);

[bar, bar, bar, bar]

It will iterate and check for each instance of RegularMenu in it if the Predicate passed will be true, if so, the item will be removed.

---

String comparison

Also, note the comparison "foo".equals(s) instead of "foo" == s.
More information in How do I compare strings in Java?

Answer 2

You can remove elements from List by index

for(int i=rm.size()-1; i>=0; i--) {
    // you are deleting elements from the list while iterating,
    // thus it is better to iterate backwards (rm.size()..0):

    if(a.trim().equals(rm.get(i).getCode().trim())) {
        rm.remove(i);
    }
}

Note: when you delete an element under index i, all elements to the right (i+1, ...) will be moved to the left for one position.

Thus, when iterating from left to right and deleting elements, you will be messing with indices.
On the other hand, when you are iterating from right to left and deleting something at position i, all elements to to right will still be moved one position to the left, but It does not matter for you, because you will not iterate on them.

aaaaBcccc    ->  aaaacccc
^   ^            ^   ^ 
0.. i            0.. i

Answer 3

You can iterate your array list with a loop and remove all matches:

for (int i = 0; i < yourArrayList.size(); i ++) {
    if(a.equals(rm.get(i).getCode())){
        yourArrayList.remove(i--);
    }
}

Your mistake was that you tried to remove a from the ArrayList using remove, but remove expects an int, representing the index to be removed. In my code notice that I am decrementing i after removing to be able to remove consequent matches.

Answer 4

for(int i=0;i<rm.size();i++) {
    if(a.trim().equals(rm.get(i).getCode())) {
        rm.remove(i);
        break;
    }
 }

In the list, you can remove elements by using index.

Answer 5

Find below code :

               for(RegularMenu regularMenu:rm){
                     if(a.equalsIgnoreCase(regularMenu.getCode())){
                           rm.remove(regularMenu);
                           break;
                        }