linkedlist.java

JAVA 2入门经典 练习答案

// Chapter 6 Exercise 4

// Modified to support backwards traversal of the list.
// Additions and modifications are marked by ***.

public class LinkedList {
  private ListItem start;                 // First ListIem in the list.
  private ListItem end;                   // Last ListIem in the list.
  private ListItem current;               // The current item for iterating.

  // Constructor to create a list containing one object:
  public LinkedList(Object item) {
    start = new ListItem(item);           // item is the start
    current = end = start;                // as well as the end and current.
  }

  // Construct a linked list from an array of objects:
  public LinkedList(Object[] items) {
    // Create a one item list:
    start = new ListItem(items[0]);       // First item is the start
    end = start;                          // as well as the end.

    // Now add the other items:
    for(int i = 1; i < items.length; i++)
      addItem(items[i]);
  }

  // Add an item object to the list:
  public void addItem(Object item) {
    ListItem newEnd = new ListItem(item); // Create a new ListItem.
    end.setNext(newEnd);                  // Set next variable for old end as new end.
    newEnd.setPrevious(end);              // So previous for new item.          ***
    current = end = newEnd;               // Store new item as end and current. ***
  }

  // Get the first object in the list:
  public Object getFirst() {
    current = start;
    return start.getObject();
  }

  // Additional method to get the last object in the list:     ***
  public Object getLast() {
    current = end;
    return end.getObject();
  }

  // Get the next object in the list:
  public Object getNext() {
    current = current.getNext();    	  // Get the reference to the next item.
    return current == null ? null : current.getObject();
  }

  // Additional method to get the previous object in the list:   ***
  public Object getPrevious() {
    current = current.getPrevious();   // Get the reference to the previous item.
    return current == null ? null : current.getObject();
  }
}