Strange NullPointerException in array

Question

Read bellow. Why this NullPointerException exception?
class YYY{
     public String a1str = new String();
     public String a2str = new String();
     public String a3str = new String();
     public String a4str = new String();
class XXX
     public YYY myNonArray= new YYY();
     public YYY myArray[]= new YYY[20];
     public void SomeFunction(){
          myNonArray.a1str = "Some string"; //works OK
          myArray[0].a1str = "Some string"; //NullPointerException !!! Why?!
}

Answer 1

you have created (and initialized) the array, but you have not "filled" the array (ie created its items) - so all items of the array are null
class XXX
public YYY myNonArray= new YYY();
public YYY myArray[]= new YYY[20];for (int j=0;j<20;j++) {
myArray[j] = new YYY();
>
public void SomeFunction(){