动态将元素添加到arraylist

Question

I have below scenario:

ArrayList<String> list = new ArrayList<String>();

list.add("John");
list.add("is");
list.add("good");
list.add("boy");
int count = 2;
if (list.get(count-1)!= null)
{
    list.set(count+1, "mike");
    list.set(count+1,"Tysosn");
}

expected output: ("john","is","good","mike","Tyson","boy")

But i am getting array out of bond exception.

Can some one please suggest.

Answer 1

Use java.util.List#set(int index, E element) to replace the element at any position

Use java.util.List#add(int index, E element) to add the element to any position.

Answer 2

You can use ArrayList.add(int index, E element) method to achieve desired result like this:

import org.junit.Test;

import java.util.ArrayList;

public class ArrayListInsertTest {
    @Test
    public void testWithArrayList() throws Exception {
        ArrayList<String> list = new ArrayList<String>();
        list.add("John");
        list.add("is");
        list.add("good");
        list.add("boy");

        list.add(3, "mike");
        list.add(4, "Tyson");
        System.out.println(list);
    }
}

Note from documentation of ArrayList:

/**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {