空指针异常，“尝试从空对象引用上的字段读取”

Question

I am making an app in which the user types in a list of tasks, and that list is saved to an array. Each task in the array is an instance of the Assignment class. However, I realized that in java it is not possible to add an element to an array after the array is created. So, what I did was I created an array called tasks which consisted of many null values: Assignment[]tasks = {null, null, null, null, null, null, null, null, null, null, null, null}; . When I want to add a task to the array, I just replace the next null value with the object. However, I also need to have an array of just the tasks, with no null values. So I created an array called full_tasks for all of the elements that aren't null:

for (Assignment task: tasks) {
    if (task != null) {
        realLength += 1;
    }
}

Assignment[] full_tasks = new Assignment[realLength];

for (int i=0; i <= full_tasks.length - 1; i++) {
        full_tasks[i] = new Assignment(tasks[i].name, tasks[i].days_due, tasks[i].time);
}

So now, the full_tasks array should be an array of all the tasks, none of which are null, right? However, when I run the app, it can't launch the activity, an error it says is caused by a null pointer exception:

 Caused by: java.lang.NullPointerException: Attempt to read from field 'java.lang.String com.example.lb.homeworkappv11.Assignment.name' on a null object reference
        at com.example.lb.homeworkappv11.Schedule.sortTasks(Schedule.java:64)

The line that the error points to is:

full_tasks[i] = new Assignment(tasks[i].name, tasks[i].days_due, tasks[i].time);

I'm still not totally sure what a null object reference is, but I think it means that one of the elements in the full_tasks array is null. Would that be correct? And if it is, what can I do to make sure that the full_tasks array is only the non-null elements in the tasks array?

Thank you so much!

Edit: the constructor function for the assignment class is:

public Assignment(String name, int days, int time) {
    this.name = name;
    this.days_due = days;
    this.time = time;
    this.toSortBy = "nothing";
}

Answer 1

A null reference is just that null . In your code it is tasks[i].name where you try to call name on tasks[i] so tasks[i] is null .

There is one scenario I can think of, where your code would definitely throw a NullPointerException . So, I will assume your tasks array can look looks like this:

tasks = [task0, null, task2, task3, null, task5]

Then full_tasks will have a size of 4 but

for (int i=0; i <= full_tasks.length - 1; i++) {
        full_tasks[i] = new Assignment(tasks[i].name, tasks[i].days_due, tasks[i].time);
}

will throw a NPE as soon as i == 1 because tasks[1] is null .

So, if you want to fill full_tasks with only non-null tasks make sure you got the right indexes of tasks .

Answer 2

Here's what I think

You are just finding the number elements that are not null . You don't know where in the array thest null 's are present.

Suppose ONLY the first element is null . So realLength will be 7. The latter for loop runs from i=0 to i=7 . When i=0 , tasks[i].name tries to access the name field of the first element; but your first element happens to be null`. That's where things go wrong.

Solution:

There are a number of solutions. The most efficient one I can think of uses an ArrayList .

To get around using arrays, you should store the indices of all elements that are not null . That's one way.

Here's another:

for (Assignment task: tasks) {
  if (task != null) {
    realLength += 1;
  }
}
Assignment[] full_tasks = new Assignment[realLength];
int count = 0;
for (Assignment task: tasks) {
  if (task != null) {
    full_tasks[count] = new Assignment(task.name, tasks.days_due, task.time);
    count++;
  }
}

Answer 3

Seems like "tasks" list is empty. Make sure its get populated or put a null check like:

if(tasks[i] !=null) {

full_tasks[i] = new Assignment(tasks[i].name, tasks[i].days_due,  tasks[i].time);

 }

Answer 4

You can think of null as something not exists, for example when you try to get task.name , where task is null , you get error, because you try to get name from some something which does not even exists.

Actually what your code is doing now is check how many non-null items in the original array, and try to extract the extra the first n items from the array to a new array.

ie if the list is {null, null, non-null, non-null} , it has 2 non-null item, however your code wrongly extracts the list of the first 2 items which are {null, null}

Edit, to actually do what you want:

ArrayList<Assignment> fullTaskList = new ArrayList<Assignment>();
for (Assignment task: tasks) {
    if (task != null) {
        fullTaskList.add(task);
    }
}
//optional
Assignment[] fullTasks = fullTaskList.toArray(new Assignment[fullTaskList.size()]);

Answer 5

The problem in your code is here:

(tasks[i].name, tasks[i].days_due, tasks[i].time).

Although you're counting the real size, there could be some null value in between that is causing it to be get a null object from the list tasks[i].

---

A good idea to solve this is by using a List instead of an Array. A List can be increased or decreased as you want, you just have to add or remove an item of your object type. For example:

List<Assignment> list = new ArrayList<>();
Assignment assignment1 = new Assignment(etc.);
list.add(assignment1);
list.get(0) //-> returns the Assignment object that you added.

Then you can use list.size() to get the size to know how many items are there, or to remove the last item. And you would have no problems to add new items to the list.

Answer 6

It seems you are trying to copy and compact the tasks array into the full_tasks array (removing the null elements), but you are doing this partially incorrect since you are accessing tasks[i] in the second loop without checking whether its is null.

Instead of:

for (int i=0; i <= full_tasks.length - 1; i++) {
    full_tasks[i] = new Assignment(tasks[i].name, tasks[i].days_due, tasks[i].time);
}

you could write something like this:

for (int i = 0, f = 0; i < tasks.length; i++) {
    if (tasks[i] != null) {
        full_tasks[f] = new Assignment(tasks[i].name, tasks[i].days_due, tasks[i].time);
        f++;
    }
}

But then again to solve your original problem, regarding the fact that you cannot add any elements to an array, I suggest to use anArrayList instead of a simple array. This allows you to add items with ArrayList.add(assignment) and remove them again with ArrayList.remove(index) .