Do not return null:

• Methods that return null in place of empty collections or arrays require additional client handling to avoid exceptions.

Problems with null:

• Clients need to add redundant checks (if to check for null).
• Omissions in these checks may go unnoticed, resulting in bugs.
• It makes it difficult to implement the method that returns the collection or array.

Argument against null:

• Don't worry about the performance of allocating empty collections or arrays unless it is proven to be a bottleneck.

Efficient alternatives:

• Use empty collections or arrays instead of null.
• Immutable collections can be returned repeatedly (e.g.: Collections.emptyList(), Collections.emptySet()).
• Empty arrays can also be returned efficiently.

Optimized performance:

• Use reusable empty immutable collections to avoid unnecessary new allocations.
• Return the same empty array instead of creating a new one each time

Code examples:
Incorrect method that returns null:

// Exemplo incorreto
public List getCheeses() {
    return cheesesInStock.isEmpty() ? null : new ArrayList(cheesesInStock);
}

Inadequate customer treatment:

List cheeses = shop.getCheeses();
if (cheeses != null && !cheeses.isEmpty()) {
    // Lógica para lidar com queijos disponíveis
}

Correct method that returns an empty collection:

// Exemplo correto
public List getCheeses() {
    return cheesesInStock.isEmpty() ? Collections.emptyList() : new ArrayList(cheesesInStock);
}

Using an immutable empty collection:

public List getCheeses() {
    return cheesesInStock.isEmpty() ? Collections.emptyList() : new ArrayList(cheesesInStock);
}

Use with empty arrays:

// Retorno de array vazio corretamente
public Cheese[] getCheeses() {
    return cheesesInStock.toArray(new Cheese[0]);
}

Optimized use of empty array:

private static final Cheese[] EMPTY_CHEESE_ARRAY = new Cheese[0];

public Cheese[] getCheeses() {
    return cheesesInStock.toArray(EMPTY_CHEESE_ARRAY);
}

Conclusion:
Never return null: Always prefer empty collections or arrays. This simplifies the API, prevents errors, and rarely negatively impacts performance.