如何设置嵌套列表的初始容量？

Question

我试图将嵌套的初始容量设置为某个值（例如10），但是当我尝试访问内部列表以添加一些元素时，它给了我。请建议一些好的解决方案!!!ArrayListArrayOutOfBoundException

下面是代码片段

public static void main(String[] args) 

{

    ArrayList<ArrayList<Integer>> bucket = new ArrayList<ArrayList<Integer>>();

    System.out.println(bucket.get(5).add(5));       

    System.out.println(bucket);

}

Answer 1

从 Java 8 开始，您可以使用 生成空列表列表。Stream.generateimport java.util.*;import java.util.stream.*;public class ListOfLists {&nbsp; &nbsp; public static void main(String[] args) {&nbsp; &nbsp; &nbsp; &nbsp; List<List<Integer>> bucket = listOfList(10);&nbsp; &nbsp; &nbsp; &nbsp; bucket.get(5).add(5);&nbsp; &nbsp; &nbsp; &nbsp; System.out.println(bucket);&nbsp; &nbsp; }&nbsp; &nbsp; public static <T> List<List<T>> listOfList(int size) {&nbsp; &nbsp; &nbsp; &nbsp; return Stream.generate(ArrayList<T>::new).limit(size).collect(Collectors.toList());&nbsp; &nbsp; }}输出[[], [], [], [], [], [5], [], [], [], []]

Answer 2

System.out.println(bucket.get(5).add(5));您需要了解这行代码，您正在尝试从数组列表“存储桶”中访问第5个元素，但是您是否在存储桶中添加了任何元素（在这种情况下，该元素是另一个数组列表）ArrayList除非您添加元素，否则无法访问它们，因为它们不存在，因此您会在尝试访问第5个元素时看到ArrayOutOfBoundExceptionbucket.get(5)你可能想通过ArrayList的javadoc&nbsp;https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html

Answer 3

法典：&nbsp;public class Example {&nbsp; &nbsp; public static void main(String[] args) {&nbsp; &nbsp; &nbsp; &nbsp; ArrayList<ArrayList<Integer>> bucket = new ArrayList<ArrayList<Integer>>();&nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; //initial capacity of the nested arraylist to 5.&nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; System.out.println(bucket.add(new ArrayList<>(5)));&nbsp; &nbsp; &nbsp; &nbsp; ArrayList<Integer> element = new ArrayList<>();&nbsp; &nbsp; &nbsp; &nbsp; element.add(5);&nbsp; &nbsp; &nbsp; &nbsp; bucket.add(0, element);&nbsp; &nbsp; &nbsp; &nbsp; System.out.println(bucket);&nbsp; &nbsp; }}输出：true[[5], []]