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

Double-ended queue (deque) is a sequence container implemented based on the queue data structure that follows the principles of First In First Out (FIFO) and Last In First Out (LIFO). It allows insertion and removal of elements at both the ends. **Deque** can dynamically adjust the capacity based on project requirements. It doubles the capacity each time. **Deque** differs from **[Queue](js-apis-queue.md)** and **[Vector](js-apis-vector.md)** mainly in the following aspects:

**Queue** allows element removal at the front and insertion at the rear.

**Vector** supports insertion and deletion of elements in between, as well as at both the ends. When compared with **Vector**, **Deque** is more efficient in inserting and removing header elements, but less efficient in accessing elements.

**Recommended use case**: Use **Deque** when you need to frequently insert or remove elements at both the ends of a container.

This topic uses the following to identify the use of generics:
- 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 { Deque } from '@kit.ArkTS';
```

## Deque

### Attributes

**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 deque (called container later).|

### constructor

constructor()

A constructor used to create a **Deque** 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 Deque's constructor cannot be directly invoked. |

**Example**

```ts
let deque: Deque<string | number | boolean | Object> = new Deque();
```

### insertFront

insertFront(element: T): void

Inserts an element at the front of this container.

**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.|

**Error codes**

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

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

**Example**

```ts
class C1 {
  name: string = ""
  age: string = ""
}
let deque: Deque<string | number | boolean | Array<number> | C1> = new Deque();
deque.insertFront("a");
deque.insertFront(1);
let b = [1, 2, 3];
deque.insertFront(b);
let c: C1 = {name : "Dylan", age : "13"};
deque.insertFront(c);
deque.insertFront(false);
```

### insertEnd

insertEnd(element: T): void

Inserts an element at the end of this container.

**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.|

**Error codes**

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

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

**Example**

```ts
class C1 {
  name: string = ""
  age: string = ""
}

let deque: Deque<string | number | boolean | Array<number> | C1> = new Deque();
deque.insertEnd("a");
deque.insertEnd(1);
let b = [1, 2, 3];
deque.insertEnd(b);
let c: C1 = {name : "Dylan", age : "13"};
deque.insertEnd(c);
deque.insertEnd(false);
```

### has

has(element: T): boolean

Checks whether this container has the specified element.

**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 | Returns **true** if the specified element is contained; returns **false** otherwise.|

**Error codes**

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

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

**Example**

```ts
let deque: Deque<string> = new Deque();
deque.insertFront("squirrel");
let result = deque.has("squirrel");
```

### popFirst

popFirst(): T

Removes the first element of this container.

**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 | First element removed.|

**Error codes**

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

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

**Example**

```ts
let deque: Deque<number> = new Deque();
deque.insertFront(2);
deque.insertFront(4);
deque.insertEnd(5);
deque.insertFront(2);
deque.insertFront(4);
let result = deque.popFirst();
```

### popLast

popLast(): T

Removes the last element of this container.

**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 | Last element removed.|

**Error codes**

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

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

**Example**

```ts
let deque: Deque<number> = new Deque();
deque.insertFront(2);
deque.insertEnd(4);
deque.insertFront(5);
deque.insertFront(2);
deque.insertFront(4);
let result = deque.popLast();
```

### forEach

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

Uses a callback to traverse the elements in this container and obtain their position 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 container.|
| 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**.|
| deque | Deque&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 deque: Deque<number> = new Deque();
deque.insertFront(2);
deque.insertEnd(4);
deque.insertFront(5);
deque.insertEnd(4);
deque.forEach((value: number, index?: number | undefined, deque?: Deque<number> | undefined):void => {
  console.log("value:" + value, "index:" + index);
});
```

### getFirst

getFirst(): T

Obtains the first element of this container.

**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 | First element of the T type 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 deque: Deque<number> = new Deque();
deque.insertEnd(2);
deque.insertEnd(4);
deque.insertFront(5);
deque.insertFront(4);
let result = deque.getFirst();
```

### getLast

getLast(): T

Obtains the last element of this container.

**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 | Last element of the T type obtained.|

**Error codes**

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

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

**Example**

```ts
let deque: Deque<number> = new Deque();
deque.insertFront(2);
deque.insertFront(4);
deque.insertFront(5);
deque.insertFront(4);
let result = deque.getLast();
```

### [Symbol.iterator]

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

Obtains 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 deque: Deque<number> = new Deque();
deque.insertFront(2);
deque.insertFront(4);
deque.insertFront(5);
deque.insertFront(4);

// Method 1:
let nums: Array<number> = Array.from(deque)
for (let item of nums) {
  console.log("value:" + item);
}

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