Implementing methods from a class to a interface as a linked list

Question

I have these folowing methods and I want to implement this in a class called LinkedBag

void replace(T oldEntry, T newEntry){}
boolean isDuplicated(T anEntry){}
void doubleBag(){}

1. void replace(T oldEntry,T newEntry) replaces any entry that is equal to oldEntry with the new entry newEntry. For example,suppose that replace(A,D) is called on this bag {A,B,C,A}. The resulting bag should be {D,B,C,D}
2. the boolean returns true if the entry anEntry appears more than one time in the bag and false otherwise. For example, calling isDuplicated(B) on the resulting bag in part 1 should return false, but isDuplicated(D) should return true.
3. void doubleBag()adds to the bag a copy of each entry that appears in the bag. For example, calling doubleBag() on the resulting bag in part 1 should change the bag contents to {4’D, 2’B, 2’C}.

Now so far I though of getting the frequency of a node and checking if the node is duplicated like:

 public boolean isDuplicated(T anEntry){ 
       Node currentNode =firstNode;
       int counter=0;
       while ((counter <numberOfEntries)&&(currentNode!=null))
           if(anEntry.equals(currentNode)){
               return true;
           }
       return false;
    }

and for the replace method I tried assigning values to another variables then removing them and then reassigning them again but I don't think it's right

   T entry=null;
        T entry1 = null;
        oldEntry=entry;
        newEntry=entry1;
        remove(oldEntry);
        entry=newEntry; 
        entry1=oldEntry;
        
        
        
        add(oldEntry);
        add(newEntry);
       System.out.println( oldEntry+" , "+newEntry );
    }

as for the double bag I still don't know how to do it.

Excuse me for my bad programming I am learning java new, and still learning the basics.

Answer 1

Let's just briefly look at your replace method:

T entry = null;
T entry1 = null;
// entry and entry1 are both null
oldEntry = entry;
newEntry = entry1;
// entry, entry1, oldEntry and newEntry are all null.

remove(oldEntry); // remove null

entry = newEntry;  // overwriting null with null
entry1 = oldEntry; // same as previous statement

add(oldEntry); // add null
add(newEntry); // add null again

// print "null , null"
System.out.println(oldEntry + " , " + newEntry);

It's not clear what you're trying to do, but at least what's here is just a bunch of nothing. What you probably want is a little more like this:

// assuming oldEntry and newEntry are set somewhere and are correct, 
// and that remove and add work as expected:
remove(oldEntry);
add(newEntry);
// And you'll want to repeat this for as many duplicates as there are of oldEntry.

As for the doubleBag method, you'll want some way to walk over entries, and then duplicate them:

Node currentNode = head;
Node newHead = null;
Node newTail = null;
while (currentNode != null) {
  Node first = new Node(currentNode.data);
  Node second = new Node(currentNode.data);
  first.next = second;
  if (newHead != null) {
    newTail.next = first;
    newTail = second;
  } else {
    newHead = first;
    newTail = second;
  }
  currentNode = currentNode.next;
}
return newHead;

This code traverses the input list starting at head, creates two copies of the node (named first and second), links them together and appends them to a new list, which it returns at the end.

That should get you started. Also, notice how the traversal works, and see how it differs from your isDuplicated implementation (which currently has an infinite loop). I'll leave it to you to fix your implementation.