如何在Java中使用null初始化此2D数组？

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?

Answer 1

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();

Answer 2

默认情况下，JAVA使用空值初始化参考对象。