How to initialize an array object - arrays

Question

Why I am getting the error while runtime:
Exception in thread "main" java.lang.NullPointerException at arrayTest.main(arrayTest.java:5)
Source code is as following:
public class arrayTest{
int b;
public static void main(String[] args){
arrayTest[] a= new arrayTest[2];
a[0].b=10;
System.out.println(a[0].b);
}
}
Thanks
Sunil

Answer 1

You haven't added any arrayTest instances to your array. You've only initialized the array, so arrayTest[0] is null. To add objects to the array:
arrayTest[0] = new arrayTest();
arrayTest[1] = new arrayTest();

Answer 2

When doing Object-Oriented-Programming (OOP) you need to think a bit differently.
Here what you are doing is creating a new object arrayTest from the class arrayTest.
This object need a constructor and fields which are properties so it can be well defined for instance: size, age or eyes colour for a person.
Here what do you want your object to be?
For instance, I want to do an ArrayList of n object of the same type :
First, my field is going to be an ArrayList because that's what I want to create.
I have the following class :
import java.lang.reflect.Type;
import java.util.ArrayList;
class arrayTestType {
private final ArrayList arrayTest;
}
What should my constructor return?
Well, an array of type Type with n elements is ArrayList<Type>(n)
so my constructor should be written like this :
public arrayTestType(Type type, int length) {
arrayTest = new ArrayList<Type>(length);
}
So finally our class is :
import java.lang.reflect.Type;
import java.util.ArrayList;
class arrayTestType {
private final ArrayList arrayTest;
public arrayTestType(Type type, int length){
arrayTest = new ArrayList<Type>(length);
}
}
Now when we are going to call :
arrayTestType stringArrayOfLength5 = new arrayTestType<String>(5);
We are going to get an object well defined.
Now you need to add things in order for it not to be null hence the null pointer !
Good luck !