# Linear Container LinkedList

> **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.

**LinkedList** is implemented based on the doubly linked list. Each node of the doubly linked list has references pointing to the previous element and the next element. When querying an element, the system traverses the list from the beginning or end. **LinkedList** offers efficient insertion and removal operations but supports low query efficiency. **LinkedList** allows null elements.

Unlike **[List](js-apis-list.md)**, which is a singly linked list, **LinkedList** is a doubly linked list that supports insertion and removal at both ends.

**LinkedList** is less efficient in data access than **[ArrayList](js-apis-arraylist.md)**.

**Recommended use case**: Use **LinkedList** for frequent insertion and removal operations.

## Modules to Import

```ts
import LinkedList from '@ohos.util.LinkedList';  
```




## LinkedList

### Attributes

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

| Name| Type| Readable| Writable| Description|
| -------- | -------- | -------- | -------- | -------- |
| length | number | Yes| No| Number of elements in a linked list (called container later).|


### constructor

constructor()

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

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


**Example**

```ts
let linkedList = new LinkedList();
```


### add

add(element: T): boolean

Adds an element at the end of this container.

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

**Parameters**

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

**Return value**

| Type| Description|
| -------- | -------- |
| boolean | Returns **true** if the element is added successfully; returns **false** otherwise.|

**Example**

```ts
let linkedList = new LinkedList();
let result = linkedList.add("a");
let result1 = linkedList.add(1);
let b = [1, 2, 3];
linkedList.add(b);
let c = {name : "lala", age : "13"};
let result3 = linkedList.add(false);
```

### addFirst

addFirst(element: T): void

Adds an element at the top of this container.

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

**Parameters**

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

**Example**

```ts
let linkedList = new LinkedList();
linkedList.addFirst("a");
linkedList.addFirst(1);
let b = [1, 2, 3];
linkedList.addFirst(b);
let c = {name : "lala", age : "13"};
linkedList.addFirst(false);
```

### insert

insert(index: number, element: T): void

Inserts an element at the specified position in this container.

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

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| element | T | Yes| Target element.|
| index | number | Yes| Index of the position where the element is to be inserted.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.insert(0, "A");
linkedList.insert(1, 0);
linkedList.insert(2, true);
```

### has

has(element: T): boolean

Checks whether this container has the specified element.

**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.|

**Example**

```ts
let linkedList = new LinkedList();
let result1 = linkedList.has("Ahfbrgrbgnutfodgorrogorg");
linkedList.add("Ahfbrgrbgnutfodgorrogorg");
let result = linkedList.has("Ahfbrgrbgnutfodgorrogorg");
```

### get

get(index: number): T

Obtains an element at the specified position in this container.

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

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| index | number | Yes| Position index of the target element.|

**Return value**

| Type| Description|
| -------- | -------- |
| T | Element obtained.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(2);
linkedList.add(1);
linkedList.add(2);
linkedList.add(4);
let result = linkedList.get(2);
```

### getLastIndexOf

getLastIndexOf(element: T): number

Obtains the index of the last occurrence of the specified element in this container.

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

**Parameters**

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

**Return value**

| Type| Description|
| -------- | -------- |
| number | Returns the position index if obtained; returns **-1** otherwise.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(2);
linkedList.add(1);
linkedList.add(2);
linkedList.add(4);
let result = linkedList.getLastIndexOf(2);
```

### getIndexOf

getIndexOf(element: T): number

Obtains the index of the first occurrence of the specified element in this container.

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

**Parameters**

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

**Return value**

| Type| Description|
| -------- | -------- |
| number | Returns the position index if obtained; returns **-1** otherwise.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(2);
linkedList.add(1);
linkedList.add(2);
linkedList.add(4);
let result = linkedList.getIndexOf(2);
```

### removeByIndex

removeByIndex(index: number): T

Removes an element at the specified position from this container.

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

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| index | number | Yes| Position index of the target element.|

**Return value**

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

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(2);
linkedList.add(4);
let result = linkedList.removeByIndex(2);
```

### removeFirst

removeFirst(): T

Removes the first element from this container.

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

**Return value**

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

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(2);
linkedList.add(4);
let result = linkedList.removeFirst();
```

### removeLast

removeLast(): T

Removes the last element from this container.

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

**Return value**

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

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(2);
linkedList.add(4);
let result = linkedList.removeLast();
```

### remove

remove(element: T): boolean

Removes the first occurrence of the specified element from this container.

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

**Parameters**

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

**Return value**

| Type| Description|
| -------- | -------- |
| boolean | Returns **true** if the element is removed successfully; returns **false** otherwise.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
let result = linkedList.remove(2);
```

### removeFirstFound

removeFirstFound(element: T): boolean

Removes the first occurrence of the specified element from this container.

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

**Parameters**

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

**Return value**

| Type| Description|
| -------- | -------- |
| boolean | Returns **true** if the element is removed successfully; returns **false** otherwise.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
let result = linkedList.removeFirstFound(4);
```

### removeLastFound

removeLastFound(element: T): boolean

Removes the last occurrence of the specified element from this container.

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

**Parameters**

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

**Return value**

| Type| Description|
| -------- | -------- |
| boolean | Returns **true** if the element is removed successfully; returns **false** otherwise.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
let result = linkedList.removeLastFound(4);
```

### clone

clone(): LinkedList<T>

Clones this container and returns a copy. The modification to the copy does not affect the original instance.

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

**Return value**

| Type| Description|
| -------- | -------- |
| LinkedList<T> | New **LinkedList** instance obtained.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
let result = linkedList.clone();
```

### forEach

forEach(callbackfn: (value: T, index?: number, LinkedList?: LinkedList<T>) => void,
thisArg?: Object): void

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

**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 to use when the callback is invoked.|

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.|
| LinkedList | LinkedList<T> | No| Instance that invokes the **forEach** API.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
linkedList.forEach((value, index) => {
  console.log("value:" + value, index);
});
```

### clear

clear(): void

Clears this container and sets its length to **0**.

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

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
linkedList.clear();
```

### set

set(index: number, element: T): T

Replaces an element at the specified position in this container with a given element.

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

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| index | number | Yes| Position index of the target element.|
| element | T | Yes| Element to be used for replacement.|

**Return value**

| Type| Description|
| -------- | -------- |
| T | New element.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
let result = linkedList.set(2, "b");
```

### convertToArray

convertToArray(): Array<T>

Converts this container into an array.

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

**Return value**

| Type| Description|
| -------- | -------- |
| Array<T> | Array obtained.|

**Example**
```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
let result = linkedList.convertToArray();
```

### getFirst

getFirst(): T

Obtains the first element in this container.

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

**Return value**

| Type| Description|
| -------- | -------- |
| T | Returns the element if obtained; returns **undefined** otherwise.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
let result = linkedList.getFirst();
```

### getLast

getLast(): T

Obtains the last element in this container.

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

**Return value**

| Type| Description|
| -------- | -------- |
| T | Returns the element if obtained; returns **undefined** otherwise.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);
linkedList.getLast();
```

### [Symbol.iterator]

[Symbol.iterator]\(): IterableIterator<T>

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

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

**Return value**

| Type| Description|
| -------- | -------- |
| IterableIterator<T> | Iterator obtained.|

**Example**

```ts
let linkedList = new LinkedList();
linkedList.add(2);
linkedList.add(4);
linkedList.add(5);
linkedList.add(4);

// Method 1:
for (let item of linkedList) { 
  console.log("value:" + item); 
} 

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