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

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i have made 2-d array store references objects of children classes of parent class student. 

student[][] = new student [5][4];

now want initialize array null? how do it? trick doing @ once? also, wanted know if possible store references of children class in array of base class in java?

i want initialize values null. also, let's values filled , want overwrite values null. how do that?

by default java initialises reference objects null.


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