How do I initialize this 2D array with null in Java?

Question

I have made a 2-d array which will store references to objects of the children classes of parent class Student.

Student[][] a = new Student [5][4];

Now I want to initialize this array with null? How do I do it? Any trick to doing it at once? Also, I Wanted to know if it is possible to store references of children class in an array of Base class in Java?

I want to initialize all the values with null. Also, let's say some values are filled and then I want to overwrite the values with null. How do I do that?

can you give an example of what you want to do? Student[][] a = null; ? You have to be more precise. you probably want to initialse all values in the array with null, which would be done with nested for-loops. — HopefullyHelpful
– HopefullyHelpful, Commented May 28, 2016 at 13:05

Priyansh Goel · Accepted Answer · 2016-05-28 13:03:27Z

5

By default JAVA initialises the reference objects with null.

answered May 28, 2016 at 13:03

Priyansh Goel

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Comments

Andrew · Accepted Answer · 2016-05-28 13:29:26Z

5

There are three ways, choose the most priority for you.

Student[][] a = null; // reference to a equals null
Student[][] a = new Student[5][];  // {null, null, null, null, null}
Student[][] a = new Student[5][5];  // {{null, null, null, null, null}, {...}, ...}

For your purpose (from the comment), you could use Arrays.fill(Object[] a, Object val). For example,

for(Student[] array : a) Arrays.fill(array, null);

or

for(int i = 0; i < a.length; ++i)
    for(int j = 0; j < a[i].length; ++j)
        a[i][j] = null;

Also, I Wanted to know if it is possible to store references of children class in an array of Base class in Java?

Yes, that's possible. The process named upcasting (SubStudent is upcasted to Student).

a[0][0] = new SubStudent();
a[0][1] = new Student();

edited May 28, 2016 at 13:29

answered May 28, 2016 at 13:07

Andrew

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2 Comments

Harry Lewis Over a year ago

if I fill the array with some values, and then if I want to overwrite the values in the entire array with NULL, then also I can use the above method?

Andrew Over a year ago

@HarryLewis, I showed to you ways of initialization, if you want to change all values in the array, you may use Arrays,fill or a[i][j] = null in for loops manually

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