Null Pointer Exception in object array

Question

List<String> op= new ArrayList<String>();
        op.add("AA");
        op.add("BB");
        op.add("CC"); 

Data[]  aData = new Data[100];
for(int i = 0 ;i<100;i++) {
    
String currentString = op.get(rand.nextInt(3));
System.out.println("> " + currentString );
aData[i].setOp(currentOperator);             // line number 29

}

This above code is giving below mentioned error

Note: setOp() is setter present in class Data

Error:

> AA
Exception in thread "main" java.lang.NullPointerException
    at com.Learn.Calculation.Test.main(Test.java:29)

Why is this error coming when I can see the value of currentString is AA