1# @ohos.util.Queue (Linear Container Queue) 2 3**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. 4 5Unlike **[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. 6 7**Recommended use case**: Use **Queue** in FIFO scenarios. 8 9This topic uses the following to identify the use of generics: 10- T: Type 11 12> **NOTE** 13> 14> 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. 15 16 17## Modules to Import 18 19```ts 20import Queue from '@ohos.util.Queue'; 21``` 22 23 24## Queue 25 26### Attributes 27 28**System capability**: SystemCapability.Utils.Lang 29 30| Name| Type| Readable| Writable| Description| 31| -------- | -------- | -------- | -------- | -------- | 32| length | number | Yes| No| Number of elements in a queue (called container later).| 33 34 35### constructor 36 37constructor() 38 39A constructor used to create a **Queue** instance. 40 41**System capability**: SystemCapability.Utils.Lang 42 43**Error codes** 44 45For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 46 47| ID| Error Message| 48| -------- | -------- | 49| 10200012 | The Queue's constructor cannot be directly invoked. | 50 51**Example** 52 53```ts 54let queue : Queue<number | string | Object> = new Queue(); 55``` 56 57 58### add 59 60add(element: T): boolean 61 62Adds an element at the end of this container. 63 64**System capability**: SystemCapability.Utils.Lang 65 66**Parameters** 67 68| Name| Type| Mandatory| Description| 69| -------- | -------- | -------- | -------- | 70| element | T | Yes| Target element.| 71 72**Return value** 73 74| Type| Description| 75| -------- | -------- | 76| boolean | Returns **true** if the element is added successfully; returns **false** otherwise.| 77 78**Error codes** 79 80For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 81 82| ID| Error Message| 83| -------- | -------- | 84| 10200011 | The add method cannot be bound. | 85 86**Example** 87 88```ts 89class C1 { 90 name: string = "" 91 age: string = "" 92} 93let queue : Queue<number | string | C1 | number[]> = new Queue(); 94let result = queue.add("a"); 95let result1 = queue.add(1); 96let b = [1, 2, 3]; 97let result2 = queue.add(b); 98let c : C1 = {name : "Dylon", age : "13"}; 99let result3 = queue.add(c); 100``` 101 102### pop 103 104pop(): T 105 106Removes the first element from this container. 107 108**System capability**: SystemCapability.Utils.Lang 109 110**Return value** 111 112| Type| Description| 113| -------- | -------- | 114| T | Element removed.| 115 116**Error codes** 117 118For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 119 120| ID| Error Message| 121| -------- | -------- | 122| 10200011 | The pop method cannot be bound. | 123 124**Example** 125 126```ts 127let queue : Queue<number> = new Queue(); 128queue.add(2); 129queue.add(4); 130queue.add(5); 131queue.add(2); 132queue.add(4); 133let result = queue.pop(); 134``` 135 136### getFirst 137 138getFirst(): T 139 140Obtains the first element of this container. 141 142**System capability**: SystemCapability.Utils.Lang 143 144**Return value** 145 146| Type| Description| 147| -------- | -------- | 148| T | The first element obtained.| 149 150**Error codes** 151 152For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 153 154| ID| Error Message| 155| -------- | -------- | 156| 10200011 | The getFirst method cannot be bound. | 157 158**Example** 159 160```ts 161let queue : Queue<number> = new Queue(); 162queue.add(2); 163queue.add(4); 164queue.add(5); 165queue.add(2); 166let result = queue.getFirst(); 167``` 168 169### forEach 170 171forEach(callbackFn: (value: T, index?: number, Queue?: Queue<T>) => void, 172thisArg?: Object): void 173 174Uses a callback to traverse the elements in this container and obtain their position indexes. 175 176**System capability**: SystemCapability.Utils.Lang 177 178**Parameters** 179 180| Name| Type| Mandatory| Description| 181| -------- | -------- | -------- | -------- | 182| callbackFn | function | Yes| Callback invoked to traverse the elements in the container.| 183| thisArg | Object | No| Value of **this** to use when **callbackFn** is invoked. The default value is this instance.| 184 185callbackFn 186 187| Name| Type| Mandatory| Description| 188| -------- | -------- | -------- | -------- | 189| value | T | Yes| Value of the element that is currently traversed.| 190| index | number | No| Position index of the element that is currently traversed. The default value is **0**.| 191| Queue | Queue<T> | No| Instance that calls the **forEach** API. The default value is this instance.| 192 193**Error codes** 194 195For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 196 197| ID| Error Message| 198| -------- | -------- | 199| 10200011 | The forEach method cannot be bound. | 200 201**Example** 202 203```ts 204let queue : Queue<number> = new Queue(); 205queue.add(2); 206queue.add(4); 207queue.add(5); 208queue.add(4); 209queue.forEach((value : number, index ?: number) : void => { 210 console.log("value:" + value, "index:" + index); 211}); 212``` 213 214### [Symbol.iterator] 215 216[Symbol.iterator]\(): IterableIterator<T> 217 218Obtains an iterator, each item of which is a JavaScript object. 219 220> **NOTE** 221> 222> This API cannot be used in .ets files. 223 224**System capability**: SystemCapability.Utils.Lang 225 226**Return value** 227 228| Type| Description| 229| -------- | -------- | 230| IterableIterator<T> | Iterator obtained.| 231 232**Error codes** 233 234For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 235 236| ID| Error Message| 237| -------- | -------- | 238| 10200011 | The Symbol.iterator method cannot be bound. | 239 240**Example** 241```ts 242let queue : Queue<number> = new Queue(); 243queue.add(2); 244queue.add(4); 245queue.add(5); 246queue.add(4); 247 248// Method 1: 249while(queue.length) { 250 let item = queue.pop() 251 console.log("value:" + item); 252} 253 254// Method 2: 255let iter = queue[Symbol.iterator](); 256let temp: IteratorResult<number> = iter.next().value; 257while(temp != undefined) { 258 console.log("value:" + temp); 259 temp = iter.next().value; 260} 261``` 262