java - 如何在java中的列表结构末尾追加一个元素？

Question

我正在尝试编写一个函数来在列表末尾 append 一个新元素。但我不知道我的方法如何总是在列表的第一个索引中 append 新元素。

我有 2 个类，名为 Waypoint 和 TourElement。 Waypoint 包含处理链表中的点的方法。 TourElment 包含航点和下一个。

Waypoint.java

public class Waypoint {
    int x  ;
    int y  ;
    public int getX()
    {
        return this.x;
    }
    public int getY()
    {
        return this.y;
    }
    public void setXY(int x, int y)
    {
        this.x = x;
        this.y = y;
    }
}

TourElement.java

 public class TourElement {
     private Waypoint points;
     private TourElement next;
     public void setWaypoint( Waypoint points)
     {
         this.points = points; 
     }
     public void setTourElement(TourElement next)
     {
         this.next = next;
     }
     Waypoint getWaypoint()
     {
         return this.points;
     }

     TourElement getNext()
     {
         return this.next;
     }

     //+ method below
}

方法在列表中添加新的 TourElement，为什么总是将 TourElement 添加到列表的第一个索引中？

public TourElement append(Waypoint waypoint)
{
    TourElement newTourElement = new TourElement();
    TourElement current = this;
    while(current.next != null)
    {
        current = current.next;
    }
    newTourElement.setWaypoint(waypoint);
    current.next = newTourElement;
    return newTourElement;
}

这是我的测试用例://创建一个元素列表:

private TourElement createElementList(int[][] waypoints){
        assert waypoints.length > 0;
        TourElement elem = new TourElement();
        int lastIndex = waypoints.length-1;
        Waypoint wp = createWaypoint(waypoints[lastIndex][0], waypoints[lastIndex][1]);
        elem.setWaypoint(wp);
        for (int i = lastIndex-1; i >= 0 ; i--) {
            wp = createWaypoint(waypoints[i][0], waypoints[i][1]);
            elem = elem.addStart(wp);
        }
        return elem;
    }

//创建一个路径点:

private Waypoint createWaypoint(int x, int y) {
        Waypoint wp = new Waypoint();
        wp.setXY(x, y);
        return wp;
    }

//添加开始

public   TourElement addStart(Waypoint wp) {
     TourElement newTourElement = new TourElement();
     newTourElement.setWaypoint(wp);
     newTourElement.setTourElement(this);
     return newTourElement;   
    }  



 public void testAppend() {
        TourElement elem = createElementList(new int[][] {{2, 2}});
        elem = elem.append(createWaypoint(3, 3));
        assertArrayEquals(new int[] {2, 2}, elem.getWaypoint().toArray());
        assertArrayEquals(new int[] {3, 3}, elem.getNext().getWaypoint().toArray());
        assertNull(elem.getNext().getNext());
    }

public void testAppend_AfterTwo() {
    TourElement elem = createElementList(new int[][] {{1, 1}, {2, 2}});
    elem = elem.append(createWaypoint(3, 3));
    assertArrayEquals(new int[] {1, 1}, elem.getWaypoint().toArray());
    assertArrayEquals(new int[] {2, 2}, elem.getNext().getWaypoint().toArray());
    assertArrayEquals(new int[] {3, 3}, elem.getNext().getNext().getWaypoint().toArray());
    assertNull(elem.getNext().getNext().getNext());
}

我希望输出如下所示:

测试用例 1:{2,2} => {3,3}

测试用例 2:{1,1} => {2,2} =>{3,3}

但我的实际输出是:

测试用例 1:{3, 3} => {2,2}

测试用例 2:{3,3} =>{1,1} =>{2,2}

Answer 1

您正在更新 elem 行中的“elem = elem.append(createWaypoint(3, 3)); ” .

所以我想知道，您的测试用例如何返回完整列表？我预计“实际输出”仅为“{3,3} ”。

要解决此问题，测试不应更改 elem变量根本没有，在它首先被初始化之后:

 public void testAppend() {
        // add "final" to prevent accidentially changes of references
        final TourElement elem = createElementList(new int[][] {{2, 2}});

        // DO NOT CHANGE "elem"!
        // elem = elem.append(createWaypoint(3, 3));
        // instead keep the reference to the first elem:
        elem.append(createWaypoint(3, 3));

        assertArrayEquals(new int[] {2, 2}, elem.getWaypoint().toArray());
        assertArrayEquals(new int[] {3, 3}, elem.getNext().getWaypoint().toArray());
        assertNull(elem.getNext().getNext());
    }

顺便说一句:如果你给“Waypoint”一个构造函数“Waypoint(x, y)”，你的代码可能会更清晰，使x和y最终字段，删除 setXY()并添加 toString() .

public class Waypoint {
    final int x;
    final int y;
    public WayPoint(int x, int y) {
        this.x = x;
        this.y = y;
    }
    public int getX() { ... }
    public int getY() { ... }

    @Override
    public String toString() {
        return String.format("{%d,%d}", x, y);
    }
}

您的TourElement可能需要 Collection<Waypoint> waypoints()方法，返回后续 Waypoint对象(使测试更容易)。

public Collection<Waypoint> waypoints() {
    Collection<Waypoint> res = new java.util.ArrayList<>();
    TourElement currTE = this;
    while (currTE.next()!=null) {
        res.add(currTE.getWaypoint());
        currTE = currTE.next();
    }
    return res;
}

然后打印 Waypoint 的列表，你只需这样做

    org.apache.commons.lang.StringUtils.join(elem.waypoints(), ", ");