Please help me to write the insert and remove code.

public class DList2 {

/** * head references the sentinel node. * * DO NOT CHANGE THE FOLLOWING FIELD DECLARATIONS. */

protected DListNode2 head; protected long size;

/* DList2 invariants: * 1) head != null. * 2) For any DListNode2 x in a DList2, x.next != null. * 3) For any DListNode2 x in a DList2, x.prev != null. * 4) For any DListNode2 x in a DList2, if x.next == y, then y.prev == x. * 5) For any DListNode2 x in a DList2, if x.prev == y, then y.next == x. * 6) size is the number of DListNode2s, NOT COUNTING the sentinel * (denoted by "head"), that can be accessed from the sentinel by * a sequence of "next" references. */

/** * DList2() constructor for an empty DList2. */ public DList2() { head = new DListNode2(); head.item = Integer.MIN_VALUE; head.next = head; head.prev = head; size = 0; }

/** * DList2() constructor for a one-node DList2. */ public DList2(int a) { head = new DListNode2(); head.item = Integer.MIN_VALUE; head.next = new DListNode2(); head.next.item = a; head.prev = head.next; head.next.prev = head; head.prev.next = head; size = 1; }

/** * DList2() constructor for a two-node DList2. */ public DList2(int a, int b) { head = new DListNode2(); head.item = Integer.MIN_VALUE; head.next = new DListNode2(); head.next.item = a; head.prev = new DListNode2(); head.prev.item = b; head.next.prev = head; head.next.next = head.prev; head.prev.next = head; head.prev.prev = head.next; size = 2; }

/** * insertFront() inserts an item at the front of a DList2. */ public void insertFront(int i) { // Your solution here. }

/** * removeFront() removes the first item (and first non-sentinel node) from * a DList2. If the list is empty, do nothing. */ public void removeFront() { // Your solution here. }

/** * toString() returns a String representation of this DList. * * DO NOT CHANGE THIS METHOD. * * @return a String representation of this DList. */ public String toString() { String result = "[ "; DListNode2 current = head.next; while (current != head) { result = result + current.item + " "; current = current.next; } return result + "]"; }

public static void main(String[] args) { // DO NOT CHANGE THE FOLLOWING CODE.

DList2 l = new DList2(); System.out.println("### TESTING insertFront ### Empty list is " + l);

l.insertFront(9); System.out.println(" Inserting 9 at front. List with 9 is " + l); if (l.head.next.item != 9) { System.out.println("head.next.item is wrong."); } if (l.head.next.prev != l.head) { System.out.println("head.next.prev is wrong."); } if (l.head.prev.item != 9) { System.out.println("head.prev.item is wrong."); } if (l.head.prev.next != l.head) { System.out.println("head.prev.next is wrong."); } if (l.size != 1) { System.out.println("size is wrong."); }

l.insertFront(8); System.out.println(" Inserting 8 at front. List with 8 and 9 is " + l); if (l.head.next.item != 8) { System.out.println("head.next.item is wrong."); } if (l.head.next.prev != l.head) { System.out.println("head.next.prev is wrong."); } if (l.head.prev.item != 9) { System.out.println("head.prev.item is wrong."); } if (l.head.prev.next != l.head) { System.out.println("head.prev.next is wrong."); } if (l.head.next.next != l.head.prev) { System.out.println("l.head.next.next != l.head.prev."); } if (l.head.prev.prev != l.head.next) { System.out.println("l.head.prev.prev != l.head.next."); } if (l.size != 2) { System.out.println("size is wrong."); }

l = new DList2(1, 2); System.out.println(" ### TESTING removeFront ### List with 1 and 2 is " + l);

l.removeFront(); System.out.println(" List with 2 is " + l); if (l.head.next.item != 2) { System.out.println("head.next.item is wrong."); } if (l.head.next.prev != l.head) { System.out.println("head.next.prev is wrong."); } if (l.head.prev.item != 2) { System.out.println("head.prev.item is wrong."); } if (l.head.prev.next != l.head) { System.out.println("head.prev.next is wrong."); } if (l.size != 1) { System.out.println("size is wrong."); }

l.removeFront(); System.out.println(" Empty list is " + l); if (l.head.next != l.head) { System.out.println("head.next is wrong."); } if (l.head.prev != l.head) { System.out.println("head.prev is wrong."); } if (l.size != 0) { System.out.println("size is wrong."); }

l.removeFront(); System.out.println(" Empty list is " + l); if (l.head.next != l.head) { System.out.println("head.next is wrong."); } if (l.head.prev != l.head) { System.out.println("head.prev is wrong."); } if (l.size != 0) { System.out.println("size is wrong."); } }

}