Java Nullpointerexception When Adding to Arraylist

Java NullPointerException when adding to ArrayList?

based on the parts of the code you provided, it looks like you haven't initialized myPolygon

NullPointerException when adding elements to ArrayList

You never actually instantiate the ArrayList using new. No memory has been allocated for the ArrayList.

Fix it like this:

ArrayList<Fiducial> activList = new ArrayList<Fiducial>();

From the docs:

Instantiation: The new keyword is a Java operator that creates the object.

Initialization: The new operator is followed by a call to a
constructor, which initializes the new object.

Creating Objects Documentation

Only with primitive types (int, double, etc.) will declaring the variable as you did allocate memory for it. Objects need the new keyword to have memory associated with them.

Java - NullPointerException with initialized ArrayList

You don't initialize the availableCars you are making a local variable in the constructor. To fix it simple remove the ArrayList in the constructor.

private ArrayList<Auto> availableCars;

public CarSharing() {
  availableCars = new ArrayList<Car>();
} 

NullPointerException when adding Object to ArrayList

In your code above the only reason why calling myCustomerList.add(...) could throw is that myCustomerList itself is null. This is because the Clients inside it is initialized in the constructor, and never set to null again. The value of src does not matter as well - the call to Clients.add(src) would succeed even if src is null.

You need to make sure that in your main you do initialize your customer list, like this:

CustomerList list = new CustomerList();

Getting NullPointerException while adding element to ArrayList

You haven't initialize the ArrayList.

ArrayList<int[]> grid = new ArrayList<>();

ArrayList.add() method causing NullPointerException

You never instantiate storedStrings. Try changing:

private ArrayList<String> storedStrings;

To:

private ArrayList<String> storedStrings = new ArrayList<String>();

When you have the line:

this.storedStrings.add(0,s);

It is invoking the method add on an instance of ArrayList<String> stored in this.storedStrings. The new operator is how you get new instances of things.

NullPointerException when using .size() in an Arraylist class

The problem is membersList doesn't exist when you call .size() on it

instead of

ArrayList<UniversityMember> membersList;

you need to initialize it

ArrayList<UniversityMember> membersList = new ArrayList<UniversityMember>();

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