将三个数组列表中的元素添加到Java中的另一个列表的有效方法？

Question

假设我有三个arrayList list1，list2和list3。 这是我的工作：

list1.addAll(list2).addAll(list3);

但是我遇到了“布尔值无法取消引用”错误。 不知道为什么吗？ 非常感谢。

Answer 1

看看方法文档

public boolean addAll(Collection<? extends E> c)

这意味着addAll（）返回一个布尔值。

当您同时合并两个addAll（）调用时，会收到该错误。

通过单独执行addAll（）可以轻松避免这种情况。

list1.addAll(list2);
list1.addAll(list3);

Answer 2

addAll（）方法返回一个布尔值，该布尔值指示目标集合是否由于调用而改变。 您将第二个addAll（）发送到第一个addAll（）的结果，这是一个布尔值。 你要：

list1.addAll(list2);
list1.addAll(list3);

Answer 3

下面的做法也可以作为示例。

/*
  Copy Elements of One Java ArrayList to Another Java ArrayList Example
  This java example shows how to copy all elements of one Java ArrayList object to
  another Java ArrayList object using copy method of Collections class.
*/

import java.util.ArrayList;
import java.util.Collections;

public class CopyElementsOfArrayListToArrayListExample {

  public static void main(String[] args) {

    //create first ArrayList object
    ArrayList arrayList1 = new ArrayList();

    //Add elements to ArrayList
    arrayList1.add("1");
    arrayList1.add("2");
    arrayList1.add("3");

    //create another ArrayList object
    ArrayList arrayList2 = new ArrayList();

    //Add elements to Arraylist
    arrayList2.add("One");
    arrayList2.add("Two");
    arrayList2.add("Three");
    arrayList2.add("Four");
    arrayList2.add("Five");

    /*
      To copy elements of one Java ArrayList to another use,
      static void copy(List dstList, List sourceList) method of Collections class.

      This method copies all elements of source list to destination list. After copy
      index of the elements in both source and destination lists would be identical.

      The destination list must be long enough to hold all copied elements. If it is
      longer than that, the rest of the destination list's elments would remain
      unaffected.      
    */

    System.out.println("Before copy, Second ArrayList Contains : " + arrayList2);

    //copy all elements of ArrayList to another ArrayList using copy
    //method of Collections class
    Collections.copy(arrayList2,arrayList1);

    /*
      Please note that, If destination ArrayList object is not long
      enough to hold all elements of source ArrayList,
      it throws IndexOutOfBoundsException.
    */

    System.out.println("After copy, Second ArrayList Contains : " + arrayList2);  
  }
}

/*
Output would be
Before copy, Second ArrayList Contains : [One, Two, Three, Four, Five]
After copy, Second ArrayList Contains : [1, 2, 3, Four, Five]
*/