historylist.java

这是《Java图形用户界面设计与实例》一书配套的源代码

//HistoryList.java
//A doubly linked list, derived from List, holds a list of previously visited URL's

public class HistoryList extends List{

    private WebNode tail;
    private WebNode current;

    public HistoryList() {
        super();
        tail = null;
    }

    public void add(String url) {//将当前的网页URL加入记录
        super.add(url);

        if (size == 1) {
            tail = head;
            current = head;
        }

        else {
            head.setPrev(current);
            current.setNext(head);
            current = head;
            head.setNext(null);
        }
    }


    public String getLast() throws Exception{//得到前一个网页
        if(current.getPrev() != null)//访问过前一个网页,则显示
            current = current.getPrev();
        else
            throw new Exception("无法后退!");//没有前一个网页,就出错
        return current.getURL();
    }

    public String getNext() throws Exception{//前进网页
        if(current.getNext() != null)//曾经后退,有前一个网页,就前进
            current = current.getNext();
        else
            throw new Exception("不能前进!");//否则出错
        return current.getURL();
    }

}