How to insert an object in an ArrayList at a specific position

问题

Suppose I have an ArrayList of objects of size n. Now I want to insert an another object at specific position, let's say at index position k (is greater than 0 and less than n) and I want other objects at and after index position k to shift one index position ahead. So is there any way to do this directly in Java. Actually i want to keep the list sorted while adding new object.

回答1:

To insert value into ArrayList at particular index, use:

public void add(int index, E element)

This method will shift the subsequent elements of the list. but you can not guarantee the List will remain sorted as the new Object you insert may sit on the wrong position according to the sorting order.

To replace the element at the specified position, use:

public E set(int index, E element)

This method replaces the element at the specified position in the list with the specified element, and returns the element previously at the specified position.

回答2:

Here is the simple arraylist example for insertion at specific index

ArrayList<Integer> str=new ArrayList<Integer>();
    str.add(0);
    str.add(1);
    str.add(2);
    str.add(3); 
    //Result = [0, 1, 2, 3]
    str.add(1, 11);
    str.add(2, 12);
    //Result = [0, 11, 12, 1, 2, 3]

回答3:

Note that when you insert into a List at a position, you are really inserting at a dynamic position within the List's current elements. See here:

http://tpcg.io/0KmArS

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {

      // create an empty array list with an initial capacity
      ArrayList<Integer> arrlist = new ArrayList<Integer>(5);

      // use add() method to add elements in the list
      arrlist.add(15, 15);
      arrlist.add(22, 22);
      arrlist.add(30, 30);
      arrlist.add(40, 40);

      // adding element 25 at third position
      arrlist.add(2, 25);

      // let us print all the elements available in list
      for (Integer number : arrlist) {
         System.out.println("Number = " + number);
      }  
   }
}

$javac com/tutorialspoint/ArrayListDemo.java

$java -Xmx128M -Xms16M com/tutorialspoint/ArrayListDemo

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 15, Size: 0
    at java.util.ArrayList.rangeCheckForAdd(ArrayList.java:661)
    at java.util.ArrayList.add(ArrayList.java:473)
    at com.tutorialspoint.ArrayListDemo.main(ArrayListDemo.java:12)

回答4:

Actually the way to do it on your specific question is arrayList.add(1,"INSERTED ELEMENT"); where 1 is the position

回答5:

For example:

I want to move the element from 23th to 1th(index == 0) in the arrayList, so I put 23th element to a temp value, and removing from list, insert it to 1th in list. It's worked, but not more efficient.

 List<ItemBean> list = JSON.parseArray(channelJsonStr,ItemBean.class);
    for (int index = 0; index < list.size(); index++) {
        if (list.get(index).getId() == 23) { // id 23
            ItemBean bean = list.get(index);
            list.remove(index);
            list.add(0, bean);
        }
    }

来源：https://stackoverflow.com/questions/7074402/how-to-insert-an-object-in-an-arraylist-at-a-specific-position

标签

java

arraylist