Accessing the elements of the reference type array in Java

Question

I'm trying to make a hash table with java. I share a long code only what I need. I use the for loop to fill the table with -1.But I am getting the error.

---Exception in thread "main" java.lang.NullPointerException at datahashtable04.DataHashTable04.main(DataHashTable04.java:68) Java Result: 1---

class Data {

int index, value;

public Data(int index, int value) {
    this.index = index;
    this.value = value;
}

}

public static void main(String[] args) {

    Data a []= new Data[27];

    for (int i = 0; i <a.length; i++) {

        a[i].index=-1;
        a[i].value=-1;


    }
}

Answer 1

Data a []= new Data[27];

Everything in this array is null until you initialize each element. You need to call the constructor in your loop:

for (int i = 0; i <a.length; i++) {
    a[i] = new Data(-1, -1);
}