Examples

1. Return a list from a single object

Immutability: An immutable list is a list whose contents cannot be changed after it is created.

• We cannot add, remove, or modify elements.

• Examples: List.of(), Collections.singletonList()

Mutability: A mutable list is a list we can modify.

• We can add, remove, or update elements.

• Examples: new ArrayList<>(), Arrays.asList() (partially mutable)

Option 1: Using Collections.singletonList(product)

Returns an immutable list containing exactly one element.

Product product = new Product();
List<Product> list = Collections.singletonList(product);

Option 2: Using List.of(product) (Java 9+)

Returns an immutable list with the given element(s).

List<Product> list = List.of(product);

Option 3: Using Arrays.asList(product)

Returns a mutable fixed-size list backed by an array.

List<Product> list = Arrays.asList(product);

Option 4: Using new ArrayList<>(List.of(product)) (Java 9+)

Creates a mutable list with one element.

List<Product> list = new ArrayList<>(List.of(product));

Option 5: Using new ArrayList<>(Collections.singletonList(product))

Similar to above, but compatible with Java 8. Useful if we are working in Java 8 but want a mutable list.

List<Product> list = new ArrayList<>(Collections.singletonList(product));

Recommended Approach

• If we want immutability, prefer Collections.singletonList() or List.of().

• If we want a modifiable list, wrap it in new ArrayList<>().

2. UnsupportedOperationException or NullPointerException when adding element

When working with List in Java, especially when calling add() on a list retrieved via a getter method, we might encounter the following issues:

Scenario

productGroup.getCustomProducts().add(userAccounts.getCurrentAccount());

Even though productGroup is not null, this line can throw either:

• NullPointerException

• UnsupportedOperationException

Root Causes

1. List is null

If the list is not initialized, calling add() will throw a NullPointerException.

Example:

private List<Product> customProducts; // not initialized

2. List is immutable

If the list is initialized using one of the following:

• Collections.emptyList()

• List.of(...) (Java 9+)

• Collections.unmodifiableList(...)

Then calling add() will throw an UnsupportedOperationException.

Solutions

Ensure the list is initialized and modifiable

Option 1: Initialize in the class

private List<Product> customProducts = new ArrayList<>();

Option 2: Lazily initialize before use

if (productGroup.getCustomProducts() == null) {
    productGroup.setCustomProducts(new ArrayList<>());
}
productGroup.getCustomProducts().add(userAccounts.getLoyaltyAccount());

Option 3: Create a modifiable copy if the list is unmodifiable

List<Product> modifiableList = new ArrayList<>(productGroup.getCustomProducts());
modifiableList.add(userAccounts.getLoyaltyAccount());
productGroup.setCustomProducts(modifiableList);