blocks|key|455164|text|你可能想看看德克。它使您可以直接访问列表中的第一项和最后一项。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|455165|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You may want to look at Deque. it gives you direct access to both the first and last items in the list.

blocks|key|237123|text|我认为实现应该很简单，但是考虑到效率，你应该使用LinkedList而不是ArrayList作为容器。您可以参考以下代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|237124|import+java.util.LinkedList;
import+java.util.List;

public+class+DataContainer+{

++++private+List<Integer>+list;

++++int+length+=+10;
++++public+void+addDataToArrayList(int+data){
++++++++list.add(0,+data);
++++++++if(list.size()>10){
++++++++++++list.remove(length);
++++++++}
++++}

++++public+static+void+main(String[]+args)+{
++++++++DataContainer+comp+=+new+DataContainer();
++++++++comp.list+=+new+LinkedList<Integer>();

++++++++int+cycleCount+=+100000000;

++++++++for(int+i+=+0;+i+<+cycleCount;+i+%2B%2B){
++++++++++++comp.addDataToArrayList(i);
++++++++}
++++}
}|code-block|syntax|javascript|237125|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I think the implement should be easy, but considering about the efficiency, you should use LinkedList but not ArrayList as the container. You can refer to the following code:

<pre><code>import java.util.LinkedList;
import java.util.List;

public class DataContainer {

 private List&lt;Integer&gt; list;

 int length = 10;
 public void addDataToArrayList(int data){
 list.add(0, data);
 if(list.size()&gt;10){
 list.remove(length);
 }
 }

 public static void main(String[] args) {
 DataContainer comp = new DataContainer();
 comp.list = new LinkedList&lt;Integer&gt;();

 int cycleCount = 100000000;

 for(int i = 0; i &lt; cycleCount; i ++){
 comp.addDataToArrayList(i);
 }
 }
}
</code></pre>

blocks|key|455183|text|Java提供了addFirst(E，e)和push(E，e)方法，可以将元素添加到列表的前面。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|455184|https://docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html#addFirst(E)|offset|length|455185|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|27|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@$D|O|E|P|1|Q]]|A|$]]|$1|F|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|C]]]]

Java LinkedList provides both the addFirst(E e) and the push(E e) method that add an element to the front of the list.

<a href="https://docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html#addFirst(E)" rel="nofollow noreferrer">https://docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html#addFirst(E)</a>

blocks|key|455030|text|您可以使用以下代码|type|unstyled|depth|inlineStyleRanges|entityRanges|data|455031|private+List+myList+=+new+ArrayList();
private+void+addItemToList(Object+obj){
++++if(myList.size()<10){
++++++myList.add(0,obj);
++++}else{
++++++myList.add(0,obj);
++++++myList.remove(10);
++++}
}|code-block|syntax|javascript|455032|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

you can use this code

<pre><code>private List myList = new ArrayList();
private void addItemToList(Object obj){
 if(myList.size()&lt;10){
 myList.add(0,obj);
 }else{
 myList.add(0,obj);
 myList.remove(10);
 }
}
</code></pre>

blocks|key|455149|text|您可以使用列表方法、移除和添加|type|unstyled|depth|inlineStyleRanges|entityRanges|data|455150|list.add(lowestIndex,+element);
list.remove(highestIndex,+element);|code-block|syntax|javascript|455151|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can use list methods, remove and add

<pre><code>list.add(lowestIndex, element);
list.remove(highestIndex, element);
</code></pre>

blocks|key|237082|text|您可以使用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|237083|public+List<E>+addToListStart(List<E>+list,+E+obj){
list.add(0,obj);
return+(List<E>)list;

}|code-block|syntax|javascript|237084|使用您的数据类型更改E|237085|如果需要删除最旧的元素，则可以添加：|237086|list.remove(list.size()-1);+|237087|返回语句之前。否则list将在开头添加对象，并保留最旧的元素。|237088|这将删除列表中的最后一个元素。|237089|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|W|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

You can use

<pre><code>public List&lt;E&gt; addToListStart(List&lt;E&gt; list, E obj){
list.add(0,obj);
return (List&lt;E&gt;)list;

}
</code></pre>

Change E with your datatype

If deleting the oldest element is necessary then you can add:

<pre><code>list.remove(list.size()-1); 
</code></pre>

before return statement. Otherwise list will add your object at beginning and also retain oldest element.

This will delete last element in list.

blocks|key|237206|text|import+java.util.*:
public+class+Logic+{
++List<String>+list+=+new+ArrayList<String>();
++public+static+void+main(String...args)+{
++Scanner+input+=+new+Scanner(System.in);
++++Logic+obj+=+new+Logic();
++++++for+(int+i=0;i<=20;i%2B%2B)+{
++++++++String+string+=+input.nextLine();
++++++++obj.myLogic(string);
++++++++obj.printList();
++++++}
+}
+public+void+myLogic(String+strObj)+{
+++if+(this.list.size()>=10)+{
++++++this.list.remove(this.list.size()-1);
+++}+else+{
+++++list.add(strObj);+
+++}
+}
+public+void+printList()+{
+System.out.print(this.list);
+}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|237207|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>import java.util.*:
public class Logic {
 List&lt;String&gt; list = new ArrayList&lt;String&gt;();
 public static void main(String...args) {
 Scanner input = new Scanner(System.in);
 Logic obj = new Logic();
 for (int i=0;i&lt;=20;i++) {
 String string = input.nextLine();
 obj.myLogic(string);
 obj.printList();
 }
 }
 public void myLogic(String strObj) {
 if (this.list.size()&gt;=10) {
 this.list.remove(this.list.size()-1);
 } else {
 list.add(strObj); 
 }
 }
 public void printList() {
 System.out.print(this.list);
 }
}
</code></pre>

blocks|key|455317|text|import+com.google.common.collect.Lists;

import+java.util.List;

/**
+*+@author+Ciccotta+Andrea+on+06/11/2020.
+*/
public+class+CollectionUtils+{

++++/**
+++++*+It+models+the+prepend+O(1),+used+against+the+common+append/add+O(n)
+++++*+@param+head+first+element+of+the+list
+++++*+@param+body+rest+of+the+elements+of+the+list
+++++*+@return+new+list+(with+different+memory-reference)+made+by+[head,+...body]
+++++*/
++++public+static+<E>+List<Object>+prepend(final+E+head,+List<E>+final+body){
++++++++return+Lists.asList(head,+body.toArray());
++++}

++++/**
+++++*+it+models+the+typed+version+of+prepend(E+head,+List<E>+body)
+++++*+@param+type+the+array+into+which+the+elements+of+this+list+are+to+be+stored
+++++*/
++++public+static+<E>+List<E>+prepend(final+E+head,+List<E>+body,+final+E[]+type){
++++++++return+Lists.asList(head,+body.toArray(type));
++++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|455318|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>import com.google.common.collect.Lists;

import java.util.List;

/**
 * @author Ciccotta Andrea on 06/11/2020.
 */
public class CollectionUtils {

 /**
 * It models the prepend O(1), used against the common append/add O(n)
 * @param head first element of the list
 * @param body rest of the elements of the list
 * @return new list (with different memory-reference) made by [head, ...body]
 */
 public static &lt;E&gt; List&lt;Object&gt; prepend(final E head, List&lt;E&gt; final body){
 return Lists.asList(head, body.toArray());
 }

 /**
 * it models the typed version of prepend(E head, List&lt;E&gt; body)
 * @param type the array into which the elements of this list are to be stored
 */
 public static &lt;E&gt; List&lt;E&gt; prepend(final E head, List&lt;E&gt; body, final E[] type){
 return Lists.asList(head, body.toArray(type));
 }
}
</code></pre>

blocks|key|237199|text|我遇到了类似的问题，试图在现有数组的开头添加一个元素，将现有元素向右移动，并丢弃最旧的元素(arraylength-1)。我的解决方案可能性能不是很好，但它可以满足我的目的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|237200|+Method:

+++updateArray+(Element+to+insert)

+++++-+for+all+the+elements+of+the+Array
+++++++-+start+from+the+end+and+replace+with+the+one+on+the+left;+
+++++-+Array+[0]+<-+Element|code-block|syntax|javascript|237201|祝好运|237202|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I had a similar problem, trying to add an element at the beginning of an existing array, shift the existing elements to the right and discard the oldest one (array[length-1]).
My solution might not be very performant but it works for my purposes.

<pre><code> Method:

 updateArray (Element to insert)

 - for all the elements of the Array
 - start from the end and replace with the one on the left; 
 - Array [0] &lt;- Element
</code></pre>

Good luck

blocks|key|237224|text|举个例子：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|237225|List<String>+element1+=+new+ArrayList<>();
element1.add("two");
element1.add("three");
List<String>+element2+=+new+ArrayList<>();
element2.add("one");
element2.addAll(element1);|code-block|syntax|javascript|237226|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Take this example :-

<pre><code>List&lt;String&gt; element1 = new ArrayList&lt;&gt;();
element1.add("two");
element1.add("three");
List&lt;String&gt; element2 = new ArrayList&lt;&gt;();
element2.add("one");
element2.addAll(element1);
</code></pre>

I need to add elements to an <code>ArrayList</code> queue whatever, but when I call the function to add an element, I want it to add the element at the beginning of the array (so it has the lowest index) and if the array has 10 elements adding a new results in deleting the oldest element (the one with the highest index).

Does anyone have any suggestions?

Java ArrayList how to add elements at the beginning

容器

Java

我需要将元素添加到ArrayList队列中，但是当我调用函数来添加元素时，我希望它在数组的开头添加元素(因此它的索引最低)，如果数组有10个元素，则添加一个新的结果来删除最旧的元素(索引最高的那个)。有人有什么建议吗？

问Java ArrayList如何在开头添加元素
EN

回答 10

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