java ArrayList 对象存储在 List 变量中
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ArrayList object stored in List variable
提问by liv2hak
I am newly learning java.I am trying to understand the various data structures in util package.
我是新学java.我想了解util包中的各种数据结构。
/*create an ArrayList object*/
List<String> arrayList = new ArrayList<String>();
In the decleration of ArrayList class I have seen
在 ArrayList 类的声明中，我看到了
public class ArrayList
extends AbstractList
implements List, RandomAccess, Cloneable, Serializable
My question is if I use an object of type List<> to store the object returned by constructor of ArrayList<>
我的问题是我是否使用 List<> 类型的对象来存储由 ArrayList<> 的构造函数返回的对象
will it work.You may assume that I need to use only the methods specified in the List interface,such as add() or size().
它会起作用吗？您可能假设我只需要使用 List 接口中指定的方法，例如 add() 或 size()。
I have tried this on my machine and it does not work.I found this in the java tutorial below
我在我的机器上试过这个，但它不起作用。我在下面的 java 教程中找到了这个
The complete example code that I tried is as below.
我试过的完整示例代码如下。
package arraylist;
import java.util.ArrayList;
import java.util.List;
public class ArrayListExample {
public static void main(String[] args)
{
/*create an ArrayList object*/
List<String> arrayList = new ArrayList<String>();
/*
Add elements to Arraylist using
boolean add(Object o) method. It returns true as a general behavior
of Collection.add method. The specified object is appended at the end
of the ArrayList.
*/
arrayList.add("1");
arrayList.add("2");
arrayList.add("3");
/*
Use get method of Java ArrayList class to display elements of ArrayList.
Object get(int index) returns and element at the specified index in
the ArrayList
*/
System.out.println("Getting elements of ArrayList");
System.out.println(arrayList.get(0));
System.out.println(arrayList.get(1));
System.out.println(arrayList.get(2));
}
}
I forgot to import List.that is why it did not work.However my next question is we use a variable of type List to store on object of type ArrayList.
我忘记导入 List。这就是它不起作用的原因。但是我的下一个问题是我们使用 List 类型的变量来存储在 ArrayList 类型的对象上。
Comparing the methods of List and ArrayList ArrayList has a method called trimToSize() that is not there in list.
比较List 和ArrayList 的方法 ArrayList 有一个叫做trimToSize() 的方法，它在list 中是没有的。
In the above case will I be able to call that method?
在上述情况下，我可以调用该方法吗？
If yes,what that generally means is that a base class pointer can be used to store a derived class object as the method list of derived class will always be a super-set of base class methods?
如果是，那通常意味着基类指针可用于存储派生类对象，因为派生类的方法列表将始终是基类方法的超集？
Is my above conclusion correct? It my sound stupid question but I am very new to java.
我上面的结论对吗？这听起来很愚蠢的问题，但我对 Java 很陌生。
回答by Kevin Bowersox
Since ArrayListimplements the Listinterface you should be able to use the object declared as a Listthat is initialized to an ArrayList. This is pretty common practice. It allows you to switch the implementation of the list without code changes.
由于ArrayList实现了List接口，您应该能够使用声明为List初始化为 a 的对象ArrayList。这是很常见的做法。它允许您在不更改代码的情况下切换列表的实现。
Make sure your importing the list interface:
确保您导入列表界面：
package arraylist;
import java.util.ArrayList;
import java.util.List;
public class ArrayListExample {
public static void main(String[] args)
{
/*create an ArrayList object*/
List<String> arrayList = new ArrayList<String>();
/*
Add elements to Arraylist using
boolean add(Object o) method. It returns true as a general behavior
of Collection.add method. The specified object is appended at the end
of the ArrayList.
*/
arrayList.add("1");
arrayList.add("2");
arrayList.add("3");
/*
Use get method of Java ArrayList class to display elements of ArrayList.
Object get(int index) returns and element at the specified index in
the ArrayList
*/
System.out.println("Getting elements of ArrayList");
System.out.println(arrayList.get(0));
System.out.println(arrayList.get(1));
System.out.println(arrayList.get(2));
}
}
When referencing the Listyou will not be able to use the trimToSizefunction since it is not on the interface for List
引用时List您将无法使用该trimToSize功能，因为它不在界面上List
回答by Jayamohan
Consider this example,
考虑这个例子，
class A implements B { }
In this case A class may be assigned to a variable of the type of one of its implemented interfaces.
在这种情况下，可以将类分配给其实现的接口之一类型的变量。
C c = new A();
If you do this casting when you use instance of c, only the methods available in Class C will be accessible.
如果在使用 c 的实例时执行此转换，则只能访问 C 类中可用的方法。
Similarly ArrayListis your ClassA and your Listis Class B in example.
同样ArrayList是你的 ClassA 和你List的 Class B 在示例中。
In the below using arraylistinstance you can invoke methods available in List interface,
在下面的使用arraylist实例中，您可以调用 List 接口中可用的方法，
/*create an ArrayList object*/
List<String> arrayList = new ArrayList<String>();
And If you use the above assignment you cannot call ArrayList methods like trimToSize()etc. If you want to use ArrayList methods just use as below.
如果你使用上面的赋值，你不能调用 ArrayList 方法等trimToSize()。如果你想使用 ArrayList 方法，请使用如下。
/*create an ArrayList object*/
ArrayList<String> arrayList = new ArrayList<String>();
Most of the developers, 90% of the time use the methods provided by List and thats the reason all the example out there is following the former method.
大多数开发人员，90% 的时间使用 List 提供的方法，这就是所有示例都遵循前一种方法的原因。
回答by Recurse
Edit the top of the file to look like this:
将文件顶部编辑为如下所示：
package arraylist; import java.util.ArrayList; import java.util.List;
包数组列表；导入 java.util.ArrayList; 导入 java.util.List;
....
....
If you haven't imported List, you can't use it as the type for a variable. This is the problem you are experiencing.
如果您尚未导入 List，则不能将其用作变量的类型。这是您遇到的问题。
回答by Patashu
List<String>arrayList means 'this variable can only store implements of the List<String>interface'
List<String>arrayList 表示“这个变量只能存储List<String>接口的实现”
ArrayList<String>implements List<String>so it can live there.
ArrayList<String>工具，List<String>所以它可以住在那里。
