在Java中使用ArrayList

柔光的暖阳◎ 2022-12-06 15:37 251阅读 0赞

Standard arrays in Java are fixed in the number of elements they can have. If you want to increase of decrease the elements in an array then you have to make a [new array with the correct number][] of elements from the contents of the original array. An alternative is to use the `ArrayList`class. The `ArrayList`class provides the means to make dynamic arrays (i.e., their length can increase and decrease).

Java中的标准数组的固定数量是固定的。 如果要增加或减少数组中的元素，则必须使用原始数组内容中的元素[数量正确][new array with the correct number]的[新][new array with the correct number]数组。 一种替代方法是使用`ArrayList`类。 `ArrayList`类提供了制作动态数组的方法(即，它们的长度可以增加和减少)。

## 进口声明 **(** Import Statement **)** ##

import java.util.ArrayList;

## 创建一个ArrayList **(** Create an ArrayList **)** ##

An `ArrayList`can be created using the simple [constructor][]:

可以使用简单的[构造函数][constructor]创建`ArrayList` ：

ArrayList dynamicArray = new ArrayList();

This will create an `ArrayList`with an initial capacity for ten elements. If a larger (or smaller) `ArrayList`is required the initial capacity can be passed to the constructor. To make space for twenty elements:

这将创建一个初始容量为10个元素的`ArrayList` 。 如果需要更大(或更小的) `ArrayList`则可以将初始容量传递给构造函数。 为20个元素腾出空间：

ArrayList dynamicArray = new ArrayList(20);

## 填充ArrayList **(** Populating the ArrayList **)** ##

Use the add method to append a value to the `ArrayList`:

使用add方法将一个值附加到`ArrayList` ：

dynamicArray.add(10);
    dynamicArray.add(12);
    dynamicArray.add(20);

**Note:** The `ArrayList`only stores objects so although the above lines appear to add [int][] values to `ArrayList` the are automatically changed to `Integer`objects as they are appended to the `ArrayList`.

**注意：** `ArrayList`仅存储对象，因此，尽管以上几行似乎向`ArrayList`添加了[int][]值，但在将它们附加到`ArrayList` ，它们会自动更改为`Integer`对象。

A standard array can be used to populate an `ArrayList`by converted it to a List collection using the Arrays.asList method and adding it to the `ArrayList`using the `addAll`method:

一个标准的阵列可用于填充一个`ArrayList`通过使用Arrays.asList方法和将其添加到它转换为一个列表集合`ArrayList`使用`addAll`方法：

String[] names = {"Bob", "George", "Henry", "Declan", "Peter", "Steven"};
    ArrayList dynamicStringArray = new ArrayList(20);
    dynamicStringArray.addAll(Arrays.asList(names));

One thing to note about `ArrayList`is the elements don't have to be of the same object type. Even though the `dynamicStringArray`has been populated by [String][] objects, it still can accept number values:

关于`ArrayList`要注意的一件事是元素不必具有相同的对象类型。 即使`dynamicStringArray`已由[String][]对象填充，它仍然可以接受数字值：

dynamicStringArray.add(456);

To minimize the chance of errors it's best to specify the type of objects you want the `ArrayList`to contain. This can be done at the creation stage by using generics:

为了最大程度地减少出错的机会，最好指定要`ArrayList`包含的对象的类型。 这可以通过使用泛型在创建阶段完成：

ArrayList dynamicStringArray = new ArrayList(20);

Now the if we try to add an object that isn't a `String`a compile-time error will be produced.

现在，如果我们尝试添加一个不是`String`的对象，则会产生编译时错误。

## 在ArrayList中显示项目 **(** Displaying the Items in an ArrayList **)** ##

To display the items in an `ArrayList`the `toString`method can be used:

要显示`ArrayList`的项目，可以使用`toString`方法：

System.out.println("Contents of the dynamicStringArray: " + dynamicStringArray.toString());

which results in:

结果是：

Contents of the dynamicStringArray: [Bob, George, Henry, Declan, Peter, Steven]

## 将项目插入ArrayList **(** Inserting an Item into the ArrayList **)** ##

An object can be inserted anywhere into the `ArrayList`index of elements by using the add method and passing the position for the insertion. To add the `String "Max"` to the `dynamicStringArray`at position 3:

通过使用add方法并传递插入位置，可以将对象插入元素的`ArrayList`索引中的任何位置。 要将`String "Max"`添加到`dynamicStringArray`的位置3：

dynamicStringArray.add(3, "Max");

which results in (don't forget the index of an `ArrayList`starts at 0):

结果(不要忘记`ArrayList`的索引从0开始)：

[Bob, George, Henry, Max, Declan, Peter, Steven]

## 从ArrayList中删除项目 **(** Removing an Item from an ArrayList **)** ##

The `remove`method can be used to remove elements from the `ArrayList`. This can be done in two ways. The first is to supply the index position of the element to be removed:

`remove`方法可用于从`ArrayList`删除元素。 这可以通过两种方式完成。 首先是提供要删除的元素的索引位置：

dynamicStringArray.remove(2);

the `String "Henry"` in postion 2 has been removed:

位置2中的`String "Henry"`已被删除：

[Bob, George, Max, Declan, Peter, Steven]

The second is to supply the object to be removed. This will remove the [first instance][] of the object. To remove "Max" from the `dynamicStringArray`:

第二个是提供要除去的物体。 这将删除该对象的[第一个实例][first instance] 。 要从`dynamicStringArray`删除“ Max”：

dynamicStringArray.remove("Max");

The `String "Max"` is no longer in the `ArrayList`:

`String "Max"`不再位于`ArrayList` ：

[Bob, George, Declan, Peter, Steven]

## 替换ArrayList中的项目 **(** Replacing an Item in an ArrayList **)** ##

Rather than removing an element and inserting a new one in its place the `set`method can be used to replace an element in one go. Just pass the index of the element to be replaced and the object to replace it with. To replace "Peter" with "Paul":

`set`方法可用于一次性替换元素，而不是删除元素并在其位置插入新元素。 只需传递要替换的元素和替换它的对象的索引。 将“ Peter”替换为“ Paul”：

dynamicStringArray.set(3,"Paul");

which results in:

结果是：

[Bob, George, Declan, Paul, Steven]

## 其他有用的方法 **(** Other Useful Methods **)** ##

There are a number of useful methods to help navigate the contents of an arraylist:

有许多有用的方法可帮助导航arraylist的内容：

*  The number of elements contained within an `ArrayList`can be found using the `size`method:
    
    可以使用`size`方法找到`ArrayList`包含的元素数：
    
        System.out.println("There are now " + dynamicStringArray.size() + " elements in the ArrayList");
    
    After all our manipulations of `dynamicStringArray`we're down to 5 elements:`dynamicStringArray`进行所有操作之后，我们将归结为5个元素：
    
     *      There are now 5 elements in the ArrayList
    
    The number of elements contained within an `ArrayList`can be found using the `size`method:
    
    可以使用`size`方法找到`ArrayList`包含的元素数：
    
        System.out.println("There are now " + dynamicStringArray.size() + " elements in the ArrayList");
    
    After all our manipulations of `dynamicStringArray`we're down to 5 elements:`dynamicStringArray`进行所有操作之后，我们将归结为5个元素：

*  Use the `indexOf`method to find the index position of a particular element:
    
    使用`indexOf`方法查找特定元素的索引位置：
    
        System.out.println("The index position of George is : " + dynamicStringArray.indexOf("George"));
    
    The `String "George"` is in index position 1:`String "George"`在索引位置1：
    
     *      The index position of George is : 1
    
    Use the `indexOf`method to find the index position of a particular element:
    
    使用`indexOf`方法查找特定元素的索引位置：
    
        System.out.println("The index position of George is : " + dynamicStringArray.indexOf("George"));
    
    The `String "George"` is in index position 1:`String "George"`在索引位置1：
 *  To clear all the elements from an `ArrayList`the clear method is used:
    
    要清除`ArrayList`的所有元素，请使用clear方法：
    
        dynamicStringArray.clear();
 *  Sometimes it can be useful to see if the `ArrayList`has any elements at all. Use the `isEmpty`method:
    
    有时查看`ArrayList`是否有任何元素可能很有用。 使用`isEmpty`方法：
    
        System.out.println("Is the dynamicStringArray empty? " + dynamicStringArray.isEmpty());
    
    which after `clear`method call above is now true:`clear`方法调用后，现在为true：
    
     *      Is the dynamicStringArray empty? true
    
    Sometimes it can be useful to see if the `ArrayList`has any elements at all. Use the `isEmpty`method:
    
    有时查看`ArrayList`是否有任何元素可能很有用。 使用`isEmpty`方法：
    
        System.out.println("Is the dynamicStringArray empty? " + dynamicStringArray.isEmpty());
    
    which after `clear`method call above is now true:`clear`方法调用后，现在为true：

> 翻译自: [https://www.thoughtco.com/using-the-arraylist-2034204][https_www.thoughtco.com_using-the-arraylist-2034204]

[new array with the correct number]: https://www.thoughtco.com/working-with-arrays-2034318
[constructor]: https://www.thoughtco.com/the-constructor-method-2034336
[int]: https://www.thoughtco.com/primitive-data-types-2034320
[String]: https://www.thoughtco.com/java-programming-4133478
[first instance]: https://www.thoughtco.com/instance-variables-2908385
[https_www.thoughtco.com_using-the-arraylist-2034204]: https://www.thoughtco.com/using-the-arraylist-2034204