当 object 添加到 ArrayList 时，Android 应用程序崩溃<object><div id="text_translate"><p>我是新手，因此对任何错误表示歉意。</p><p> 背景：作为 object 培训的一部分，我正在尝试添加 object 练习的 arraylist。 然而。 每次我尝试将 object 练习添加到训练的 Arraylist 时，应用程序都会崩溃。 欢迎任何帮助。</p><p> 对象训练和锻炼看起来像这样</p><pre>public class Training { public Integer trainingNumber; public Date datum; public Integer nrOfExercises; public ArrayList&lt;Exercise&gt; exercises; } public class Training { public Integer trainingNumber; public Date datum; public Integer nrOfExercises; public ArrayList&lt;Exercise&gt; exercises; }</pre><p> 试图生成所有这些的代码如下：</p><pre> private Training createRandomTraining(int i){ Training randomTraining = new Training(); Random random = new Random(); int d = random.nextInt(30)+1; int m = random.nextInt(12)+1; //int y = random.nextInt(21)+2000; int amountOfExercises = random.nextInt(9-6)+6; randomTraining.trainingNumber = i; randomTraining.datum = new Date(2021,m,d); for (int j = 0; j &lt; amountOfExercises; j ++){ Exercise exercise = new Exercise(); Random r = new Random(); int set = r.nextInt(5)+1; int nameInt = r.nextInt(3); int rep = r.nextInt(12)+1; //int y = random.nextInt(21)+2000; exercise.trainingNumber = i; exercise.exerciseNumber = j+1; exercise.exerciseName = getExerciseName(nameInt); exercise.sets = set; exercise.reps = rep; exercise.weight = random.nextInt(200-50)+50; randomTraining.exercises.add(exercise); //the app crashes here. //The other parts work fine. } randomTraining.nrOfExercises = amountOfExercises; return randomTraining; }</pre></div></object>

Question

我是新手，因此对任何错误表示歉意。

背景：作为 object 培训的一部分，我正在尝试添加 object 练习的 arraylist。 然而。 每次我尝试将 object 练习添加到训练的 Arraylist 时，应用程序都会崩溃。 欢迎任何帮助。

对象训练和锻炼看起来像这样

public class Training {
    public Integer trainingNumber;
    public Date datum;
    public Integer nrOfExercises;
    public ArrayList<Exercise> exercises;
}

public class Training {
    public Integer trainingNumber;
    public Date datum;
    public Integer nrOfExercises;
    public ArrayList<Exercise> exercises;
}

试图生成所有这些的代码如下：

 private Training createRandomTraining(int i){
        Training randomTraining = new Training();
        Random random = new Random();

        int d = random.nextInt(30)+1;
        int m = random.nextInt(12)+1;
        //int y = random.nextInt(21)+2000;
        int amountOfExercises = random.nextInt(9-6)+6;

        randomTraining.trainingNumber = i;
        randomTraining.datum = new Date(2021,m,d);

       for (int j = 0; j < amountOfExercises ; j ++){
            Exercise exercise = new Exercise();
            Random r = new Random();

            int set = r.nextInt(5)+1;
            int nameInt = r.nextInt(3);
            int rep = r.nextInt(12)+1;
            //int y = random.nextInt(21)+2000;

            exercise.trainingNumber = i;
            exercise.exerciseNumber = j+1;
            exercise.exerciseName = getExerciseName(nameInt);
            exercise.sets = set;
            exercise.reps = rep;
            exercise.weight = random.nextInt(200-50)+50;

            randomTraining.exercises.add(exercise); //the app crashes here.   //The other parts work fine.
         }

        randomTraining.nrOfExercises = amountOfExercises;
        return randomTraining;
    }

Answer 1

您的程序崩溃的原因是因为您在没有创建 object 的情况下定义了一个引用（“练习”），然后您尝试使用该引用，尽管没有 object 存在。 在不创建 object 的情况下， exercises的值为 null，并且在 null 引用上调用.add()将引发NullPointerException 。

这是编译器会标记的一个非常简单的示例（不需要运行程序来检测错误）：

String s;
int l = s.length();

如果您编译该代码，您应该会看到如下编译错误：

ReferenceWithoutObject.java:11: error: variable s might not have been initialized
int l = s.length();
        ^
1 error

您的示例做了同样的事情，尽管它涉及一些类，因此编译器不会检测到错误。 这是您正在执行的操作的简单版本，可以很好地编译：

import java.util.List;

public class ReferenceWithoutObject {
    public static void main(String[] args) {
        Training training = new Training();
        training.exercises.add(new Exercise());
    }
}

class Training {
    public List<Exercise> exercises;
}

class Exercise {
}

如果您运行该代码，则会产生以下错误：

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.List.add(Object)" because "training.exercises" is null
    at ReferenceWithoutObject.main(ReferenceWithoutObject.java:6)

有几种方法可以解决这个问题，但很简单：使用new ArrayList<>()创建一个实际列表，并将其分配给您的exercises参考，如下所示：

class Training {
    public List<Exercise> exercises = new ArrayList<>();
}

此外，您会注意到我修改了代码以定义public List<Exercise> exercises而不是public ArrayList<Exercise> exercises ——也就是说，使用接口 ( List ) 而不是具体的 class ( ArrayList ) 来声明exercises 。 使用接口而不是具体类来定义变量是一种很好的做法。