[英]efficient way to add the elements from three array lists to another list in java?
Suppose I have three arrayList list1, list2 and list3. 假设我有三个arrayList list1,list2和list3。 This is what I do: 这是我的工作:
list1.addAll(list2).addAll(list3);
But I got a "boolean can not be dereferenced" error. 但是我遇到了“布尔值无法取消引用”错误。 Does any know why? 不知道为什么吗? Thanks very much. 非常感谢。
Take a look at the method documentation 看看方法文档
public boolean addAll(Collection<? extends E> c)
Which means the addAll() returns a boolean value. 这意味着addAll()返回一个布尔值。
When you concat two addAll() calls together, you get that error. 当您同时合并两个addAll()调用时,会收到该错误。
It could be easily avoided by doing the addAll() separately. 通过单独执行addAll()可以轻松避免这种情况。
list1.addAll(list2);
list1.addAll(list3);
The addAll() method returns a boolean indicating whether the target collection changed as a result of the call. 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。 You're sending the second addAll() to the results of the first addAll(), which is a boolean. 您将第二个addAll()发送到第一个addAll()的结果,这是一个布尔值。 You want: 你要:
list1.addAll(list2);
list1.addAll(list3);
well the below practice could also work for an example.. 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]
*/
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