Collection

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

Collection is the primary collection interface for mutable collections

Assuming you want the ability to clear out your collection, you would implement this (or a child of this) interface. Otherwise, you can implement OutputCollection only. If your collection to be immutable, implement ConstCollection only instead.

Guides

• Introduction
• Interfaces

Interface Synopsis

namespace HH;

interface Collection implements \ConstCollection<Te>, \OutputCollection<Te> {...}

Public Methods

• ->clear()
  Removes all items from the collection

Public Methods (ConstCollection)

• ->count(): int
  Get the number of items in the collection

• ->isEmpty(): bool
  Is the collection empty?

• ->items(): HH\Iterable<Te>
  Get access to the items in the collection

• ->toDArray(): darray

• ->toVArray(): varray

Public Methods (IPureStringishObject)

• ->__toString(): string

Public Methods (OutputCollection)

• ->add(Te $e): this
  Add a value to the collection and return the collection itself
• ->addAll(?Traversable<Te> $traversable): this
  For every element in the provided Traversable, append a value into the current collection