Question

我有一个对象Activity，在初始化时看起来像Activity a = new Activity(String activityName, double calories)。我有另一个对象ActivityPerformed，初始化时看起来像ActivityPerformed ap = new ActivityPerformed(Activity a, double hours）。我还有两个ArrayLists（一组Activity和ActivityPerformed）和ArrayList<ActivityPerformed>依赖于Activity中的信息，这就是为什么它必须通过通过第一个。

因此，理想情况下，信息流基本上等同于此：

Activity a = new Activity(String activityName, double calories);
ActivityPerformed ap = new Activity(String activityName, double calories, 
                                 double hours);

因为添加到ArrayList<ActivityPerformed>需要使用其信息的Activity（这就是ActivityPerformed使用Activity a代替String activityName, double calories的原因）。

如何正确传递信息？当我打印ArrayList<Activity>时，所有输入的活动都在那里，但是当我打印ArrayList<ActivityPerformed>时，它始终会打印double（两者 calories和{ {1}}）为0.0，hours为null但是它会打印正确数量的项目。

这是我添加到activityName

的代码

ArrayList<ActivityPerformed>

这是打印ArrayList的代码（这适用于ArrayList，但也许它缺少我忽略的东西）

/**
 * Searches for the Activity record associated with the given name
 *
 * @param name the name of the Activity record to look for
 * @return     the Activity record, or null if it does not exist
 */
public Activity getActivity(String name)
{
    for(Activity a : activityBase) {
        if(a.getName().equals(name))
            return a;
    }
    return null;
}
/**
 * Track that a given kind of activity has been performed for a given number
 * of hours. If the given name does not exist in this ActivityBase, or a
 * negative number of hours is given, an error message will appear.
 *
 * @param name  the name of the activity performed
 * @param hours the number of hours that the activity has been performed
 */
public void trackActivity(String name, double hours)
{
    Activity a = getActivity(name);        
    activityPerformed.add(new ActivityPerformed(a, hours)); 
}

在ActivityPerformed.class下：

public String getActivityPerformed() {
    String done = "";
    for(ActivityPerformed a : activityPerformed) {
        done += a.getHours();
        done += " hours of ";
        done += a.getName();
        done += ", ";
        done += a.getTotalCalories();
        done += " calories.\n";
    }
    return done;
}

那么如何让public ActivityPerformed(Activity a, double hours){ a = new Activity(name, calories); this.hours = hours; this.name = name; this.calories = calories; } public String getName(){ return this.name; } public double getTotalCalories(){ return this.calories; } public double getTotalCalories(){ return this.calories; }从ArrayList<ActivityPerformed>获取活动并为其添加一个额外字段，并使其不打印ArrayList<Activity>？

Answer 1

Culprit是ActivityPerformed构造函数中的这一行

 a = new Activity(name, calories);

它应该将传递的活动分配给var a

this.a = a;
this.name = a.getName();  // you do not need this as Activity has this information
this.calories = a.getCalories();   // you do not need this as Activity has this information
this.hours = hours;

希望这会有所帮助;

Answer 2

看起来您没有将名称，卡路里等传递给ActivityPerformed构造函数。您传递了整个活动a，然后在构造函数中将其ovverrode。最好这样做：

公共类ActivityPerformed {

Activity a;
double hours;

public ActivityPerformed (Activity a, double hours) {

    this.a = a;
    this.hours = hours;

}

}

Answer 3

很难说出你想要做什么，但这不可能是正确的：

public ActivityPerformed(Activity a, double hours){
    a = new Activity(name, calories);
    this.hours = hours;
    this.name = name;
    this.calories = calories;
}

创建新Activity时，您必须将ActivityPerformed传递给构造函数;要做到这一点，你必须已经创建了new Activity。相反，上面的代码会抛弃Activity，创建另一个 new Activity，然后将新的Activity分配到参数 { {1}}，它也将变得无用（a按值传递;因此分配给它不会影响方法之外的任何内容。）

如果您希望a引用ActivityPerformed，则需要在Activity中对某个字段进行裁减才能引用该字段。然后在构造函数中分配它：

ActivityPerformed

无论如何，有很多信息缺失，我无法弄清楚你想要做什么。但这应该是一个开始。它可能无法解决所有问题。

访问时，ArrayList内容变为“null”？

3 个答案: