Is there a way to delete certain elements in a linked list above a certain number?-CodePudding

I have a linked list and I am trying to make a function that removes charms above a certain price. Lets say I have 4 charms and two of them cost $1.25 how would I remove them?

I got my function to traverse the Linked list and remove the charms above the price but I get an error saying it cannot set the node because it is null

class Main {
  public static void main(String[] args) {
        
        CharmList Bracelet = new CharmList();
        
        Bracelet.add(new Charm("YOLO", 1.25));
        Bracelet.add(new Charm("Donald Duck", 1.25));
        Bracelet.add(new Charm("Hello Kitty", 3.25));
        Bracelet.add(new Charm("Drums", 4.25));

        System.out.print(Bracelet);
        System.out.println(Bracelet.total());
    System.out.println(Bracelet.findPrice("YOLO"));

    Bracelet.remove(1.25);
    System.out.println(Bracelet);
    
                
  }
}

void remove(double price) {
        try {
            CharmNode back = null;
            CharmNode current = list;
            boolean found = false;
            
            while (current != null && !found) {
                if(current.getCharm().getPrice() == price) {
                    found = true;
                } else {
                    back = current;
                    current = current.getNextNode();
                }
                
                if (found) {
                    back.setNode(current.getNextNode());
                    current.setNode(null);
                }
            }
        } catch(NullPointerException e) {
            e.printStackTrace();
        }
        
    }

CodePudding user response：

Your issue is that the first node is the one you are trying to remove, which means back is null hence the NullPointerException. You need to account for this with a special condition. So if the node to be removed is the first node then the next node needs to become the new first node:

void remove(double price) {
    CharmNode prev = null;
    CharmNode current = list;

    while (current != null) {
        // found first instance
        if (current.getCharm().getPrice() == price) {
            // current is the first
            if (prev == null) {
                Bracelet.setFirst(current.getNextNode())
                current.setNode(null)
            } else {
                back.setNode(current.getNextNode());
                current.setNode(null);
            }
            break;
        }
        back = current;
        current = current.getNextNode();
    }
}

CodePudding user response：

The first CharmNode price matches 1.25, so it calls back.setNode() while back is null.

CodePudding user response：

Whether it is a linked list or an array list you can do it like this. No need to reinvent the wheel.

record Charm(String getName, double getPrice) {}
 List<Charm> bracelet = new ArrayList<>(List.of( 
 new Charm("YOLO", 1.25),
 new Charm("Donald Duck", 1.25),
 new Charm("Hello Kitty", 3.25),
 new Charm("Drums", 4.25)));
 
 System.out.println("Before removing");
 bracelet.forEach(System.out::println);
 // remove entries where the price is > 1.25
 bracelet.removeIf(charm->charm.getPrice() > 1.25);
 System.out.println("\nAfter removing");
 bracelet.forEach(System.out::println);

prints

Before removing
Charm[getName=YOLO, getPrice=1.25]
Charm[getName=Donald Duck, getPrice=1.25]
Charm[getName=Hello Kitty, getPrice=3.25]
Charm[getName=Drums, getPrice=4.25]

After removing
Charm[getName=YOLO, getPrice=1.25]
Charm[getName=Donald Duck, getPrice=1.25]