How to initialise an arrayList for fixed size and assign it to nil

Question

How do I initialise an arrayList for fixed size and assign it to nil?
I have code as below in Swift/IoS.
What is the equivalent in Java for below code in Java?

var studentsList:[Result?] = [Result?](repeating: nil, count: 5) //assign to nil
studentsList[2] = newStudent //assign at different level Eg:2
studentsList = studentsList.filter { $0 != nil } //filter all the nil objects

I need an array for fixed length.
I am able to add the elements in the order, but I need the ability to add the element at any of the initialised levels.
For example, if I fix the size of array as 5, I should be able to assign object to arrList[2] even though arrList[0] and arrList[1] are still nil

In English, what are you trying to do here, and more importantly -- why? — DontKnowMuchBut Getting Better, Jun 21 '17 at 01:21
What do you mean "assign it to nil"? You want to fill the array with a certain number of `null`s? — Carcigenicate, Jun 21 '17 at 01:21
I think you mean you want an array with a fixed size not an ArrayList — , Jun 21 '17 at 01:22
@CupofJava: I figured as much, but the whole thing still smells of being an XY Problem, which is why the *why* is so important. — DontKnowMuchBut Getting Better, Jun 21 '17 at 01:24
I think this pretty much a dupe of https://stackoverflow.com/questions/5600668/how-can-i-initialize-an-arraylist-with-all-zeroes-in-java but I'm hesitant to call it since it's not clear what the question is. — Carcigenicate, Jun 21 '17 at 01:24
Possible duplicate of [How can I initialize an ArrayList with all zeroes in Java?](https://stackoverflow.com/questions/5600668/how-can-i-initialize-an-arraylist-with-all-zeroes-in-java) — Isaac, Jun 21 '17 at 01:25
I need an array for fixed length. I am able to add the elements in the order but what i need is ability to add the element at any of the initialised levels. For example, if I fix the size of array as 5, I should be able to assign object to arrList[2] even though arrList[0] and arrList[1] are still nil. The code i have posted is exactly how it works in Swift/iOS — dacscan3669, Jun 21 '17 at 01:25
I tried below and it gives me java.util.ArrayList.throwIndexOutOfBoundsException. StudentList.add(1, newStudent); — dacscan3669, Jun 21 '17 at 01:28
@dacscan3669 - You're probably accessing an array element outside the declared index — , Jun 21 '17 at 01:28
Below is the current code which throws OutofBoundsException. private ArrayList StudentList = new ArrayList<>(5); StudentList.add(1, newStudent); — dacscan3669, Jun 21 '17 at 01:29

Yonah Karp · Answer 1 · 2017-06-21T02:14:11.117

1

ArrayList<Student> studentList = new ArrayList<Student>(Collections.nCopies(5, null));
studentList.set(2, newStudent)
//studentList = studentList.stream().filter(e -> e != null)

Since it seems you're using Android, and streams won't work, use the following:

ArrayList<Student> tempList = new ArrayList<>();
for(Student s : studentList) 
    if(student !=null) 
       studentList.add(student)

edited Jun 21 '17 at 02:14

answered Jun 21 '17 at 01:24

Yonah Karp

581
7
22

The last command shows an error. cannot resolve method 'stream' hovering on stream and also 'Lambda expressions are not supported at this language level' for (e -> e != null) – dacscan3669 Jun 21 '17 at 02:08
@dacscan3669 I assume you're programming for Android then. Streams were added in Java 8, so they aren't available in all android versions – Yonah Karp Jun 21 '17 at 02:09
2

@dacscan3669 Instead, you'll have to use a loop `for(Student s : studentList) if(student !=null) tempList.add(student)` – Yonah Karp Jun 21 '17 at 02:12
@dacscan3669 If my answer helped you out, I'd appreciate if you could accept it (by pressing the green check above). Happy Coding :) – Yonah Karp Jun 21 '17 at 02:48

score 1 · Answer 2 · answered Jun 21 '17 at 01:27

1

Give what you said in the comments, you're probably looking for:

StudentList[] studentarray = new StudentList[5];
studentarray[2]= studentObj;

answered Jun 21 '17 at 01:27

score 1 · Answer 3 · answered Jun 21 '17 at 01:30

1

Whenever you make an array of an Object class, it is all null.

Student[] students = new Student[5];

And you can assign an index, which keeps the rest as null.

students[2] = new Student();

An array is fixed size and cannot be added to.

If you want a stream out of an array, you can do that as well

Arrays.stream(students).filter( Objects::nonNull )

answered Jun 21 '17 at 01:30

OneCricketeer

179,855
19
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The last command shows an error. cannot resolve method 'stream' hovering on stream and also 'Method references are not supported at this language level' for Objects::nonNull – dacscan3669 Jun 21 '17 at 02:01
The last command does not work on Android and hence had to go for alternate solution. The first 2 commands are fine. – dacscan3669 Jun 21 '17 at 02:24
Java 8 features are being added. https://developer.android.com/guide/platform/j8-jack.html Your question was not tagged with android. – OneCricketeer Jun 21 '17 at 02:26
You could also use RxJava, which is an interesting library to wrap Android code with – OneCricketeer Jun 21 '17 at 02:30

score 0 · Answer 4 · 2017-06-21T01:45:19.537

0

Try this.

List<Student> students = Arrays.asList(new Student[fixedSize]);

This is a List, not an ArrayList. But you can modify each element. If you add an element to students, exception will be thrown.

edited Jun 21 '17 at 01:45

answered Jun 21 '17 at 01:34

score 0 · Answer 5 · answered Jun 21 '17 at 02:28

I resolved by using the below code based on suggestions posted.

Student[] studentList = new SchoolItem[5];

ArrayList<Student> sortedList = new ArrayList<>();

compareList[2] = new Student();

for( Student s : compareList) {
       if(s !=null)
       sortedList.add(s); }

How to initialise an arrayList for fixed size and assign it to nil

5 Answers5