can't use method removeAll in Java list

I have a list of integers 1,2,3,4 in one list, called a1temp and want to use the items in GA to delete the same items in a1temp, using a1temp.removeAll(GA);

Basically, Got a list of items in list A want to remove same items in list B

So I've got my code

I'd appreciate any help possible. thanks in advance

Obviously, importing couple of things, import java.util.ArrayList; import java.util.List;

class R{

List<Integer> GA = new ArrayList<Integer>();
List<Integer> a1temp = List.of(1, 2, 3, 4);
int a1 = 0;
int a2 = 0;
int a3 = 0;
int a4 = 0;

// if a1 is not 0 (has another value) & it's not already in GA will check it's value and add it to GA.

public void checkGA() {
     if (a1 != 0) {
           if (!GA.contains(a1)) {
               GA.add(a1);
               }}}

// remove items from listA with items from listB.

public void placeGA() {

        // Check if GA has some value.
        if (GA.size() > 0) {
            try {
               //Here is where the problem appears.
               // "a1temp list" should be getting the items removed  
               // when "a1temp" has same items as "GA list"
                a1temp.removeAll(GA);
            }catch (Exception e){
                System.out.println("Can't delete");
}}}

public class Main {
    public static void main(String[] args) {
        R r1 = new R();
        r1.a1 = 4;
        r1.a2 = 3;
        r1.a3 = 2;

        // We add items from a1, a2 & a3 to GA list
        r1.checkGA();
        System.out.println("Show GA");
        // Shows current items in GA list
        System.out.println(r1.GA);

console:

Show GA

[4, 3, 2]

System.out.println("show a1temp);
// defined before a1temp = List.of(1, 2, 3, 4);
System.out.println(r1.a1temp);

console:

Show a1temp

[1, 2, 3, 4]

// try to delete items 2, 3, 4.
r1.placeGA();
}

console Can't delete.

error showed: Error:

Exception in thread "main" java.lang.UnsupportedOperationException at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:142) at java.base/java.util.ImmutableCollections$AbstractImmutableCollection.removeAll(ImmutableCollections.java:151) at R.placeGA(Main.java:124) at Main.main(Main.java:255)

So I've tried deleting the items 2, 3, 4 from a1temp, so it would show only the item 1 on the list.

Can anyone tell my why is not working?

Thanks in advance. Its my first post.

Tried to remove items 2, 3, 4 from one list, but getting an error.

Solution

From the JavaDoc for List

Unmodifiable Lists

The List.of and List.copyOf static factory methods provide a convenient way to create unmodifiable lists. The List instances created by these methods have the following characteristics:

They are unmodifiable. Elements cannot be added, removed, or replaced. Calling any mutator method on the List will always cause UnsupportedOperationException to be thrown. However, if the contained elements are themselves mutable, this may cause the List's contents to appear to change.
They disallow null elements. Attempts to create them with null elements result in NullPointerException.
They are serializable if all elements are serializable.
The order of elements in the list is the same as the order of the provided arguments, or of the elements in the provided array.
The lists and their subList views implement the RandomAccess interface.
They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
They are serialized as specified on the Serialized Form page.

So you need to change it as follows:

List<Integer> a1temp = new ArrayList<>(List.of(1, 2, 3, 4));