# @ohos.util.Queue (Linear Container Queue)

Queue follows the principle of First In First Out (FIFO). It supports insertion of elements at the end and removal from the front of the queue. Queue is implemented based on the queue data structure.

Unlike [Deque](js-apis-deque.md), which supports insertion and removal at both the ends, **Queue** supports insertion at one end and removal at the other end.

**Recommended use case**: Use Queue in FIFO scenarios.

This topic uses the following to identify the use of generics:<br>
- T: Type

> **NOTE**
>
> The initial APIs of this module are supported since API version 8. Newly added APIs will be marked with a superscript to indicate their earliest API version.


## Modules to Import

```ts
import { Queue } from '@kit.ArkTS';
```


## Queue

### Properties

**Atomic service API**: This API can be used in atomic services since API version 12.

**System capability**: SystemCapability.Utils.Lang

| Name| Type| Readable| Writable| Description|
| -------- | -------- | -------- | -------- | -------- |
| length | number | Yes| No| Number of elements in a Queue.|


### constructor

constructor()

A constructor used to create a **Queue** instance.

**Atomic service API**: This API can be used in atomic services since API version 12.

**System capability**: SystemCapability.Utils.Lang

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200012 | The Queue's constructor cannot be directly invoked. |

**Example**

```ts
let queue : Queue<number | string | Object> = new Queue();
```


### add

add(element: T): boolean

Adds an element at the end of this Queue.

**Atomic service API**: This API can be used in atomic services since API version 12.

**System capability**: SystemCapability.Utils.Lang

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| element | T | Yes| Target element.|

**Return value**

| Type| Description|
| -------- | -------- |
| boolean | Operation result. The value **true** is returned if the element is added; otherwise, **false** is returned.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The add method cannot be bound. |

**Example**

```ts
class C1 {
  name: string = ""
  age: string = ""
}
let queue : Queue<number | string | C1 | number[]> = new Queue();
let result = queue.add("a");
let result1 = queue.add(1);
let b = [1, 2, 3];
let result2 = queue.add(b);
let c : C1 = {name : "Dylan", age : "13"};
let result3 = queue.add(c);
```

### pop

pop(): T

Removes the first element from this Queue.

**Atomic service API**: This API can be used in atomic services since API version 12.

**System capability**: SystemCapability.Utils.Lang

**Return value**

| Type| Description|
| -------- | -------- |
| T | Element removed.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The pop method cannot be bound. |

**Example**

```ts
let queue : Queue<number> = new Queue();
queue.add(2);
queue.add(4);
queue.add(5);
queue.add(2);
queue.add(4);
let result = queue.pop();
```

### getFirst

getFirst(): T

Obtains the first element of this Queue.

**Atomic service API**: This API can be used in atomic services since API version 12.

**System capability**: SystemCapability.Utils.Lang

**Return value**

| Type| Description|
| -------- | -------- |
| T | The first element obtained.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The getFirst method cannot be bound. |

**Example**

```ts
let queue : Queue<number> = new Queue();
queue.add(2);
queue.add(4);
queue.add(5);
queue.add(2);
let result = queue.getFirst();
```

### forEach

forEach(callbackFn: (value: T, index?: number, Queue?: Queue&lt;T&gt;) => void,
thisArg?: Object): void

Uses a callback to traverse the elements in this Queue and obtain their indexes.

**Atomic service API**: This API can be used in atomic services since API version 12.

**System capability**: SystemCapability.Utils.Lang

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| callbackFn | function | Yes| Callback invoked to traverse the elements in the Queue.|
| thisArg | Object | No| Value of **this** to use when **callbackFn** is invoked. The default value is this instance.|

callbackFn

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| value | T | Yes| Value of the element that is currently traversed.|
| index | number | No| Position index of the element that is currently traversed. The default value is **0**.|
| Queue | Queue&lt;T&gt; | No| Instance that calls the **forEach** API. The default value is this instance.|

**Error codes**

For details about the error codes, see [Universal Error Codes](../errorcode-universal.md) and [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 401      | Parameter error. Possible causes: 1. Mandatory parameters are left unspecified; 2. Incorrect parameter types. |
| 10200011 | The forEach method cannot be bound. |

**Example**

```ts
let queue : Queue<number> = new Queue();
queue.add(2);
queue.add(4);
queue.add(5);
queue.add(4);
queue.forEach((value : number, index ?: number) : void => {
  console.log("value:" + value, "index:" + index);
});
```

### [Symbol.iterator]

[Symbol.iterator]\(): IterableIterator&lt;T&gt;

Returns an iterator, each item of which is a JavaScript object.

**Atomic service API**: This API can be used in atomic services since API version 12.

**System capability**: SystemCapability.Utils.Lang

**Return value**

| Type| Description|
| -------- | -------- |
| IterableIterator&lt;T&gt; | Iterator obtained.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The Symbol.iterator method cannot be bound. |

**Example**
```ts
let queue : Queue<number> = new Queue();
queue.add(2);
queue.add(4);
queue.add(5);
queue.add(4);

// Method 1:
while(queue.length) {
  let item = queue.pop();
  console.log("value:" + item);
}

// Method 2:
let iter = queue[Symbol.iterator]();
let temp: IteratorResult<number> = iter.next().value;
while(temp != undefined) {
  console.log("value:" + temp);
  temp = iter.next().value;
}
```