二维ArrayList触发java.lang.IndexOutOfBoundsException错误求助

Hey there, let's break down why you're hitting this error and fix it up quickly.

First, here's your code as provided:

import java.util.ArrayList;
public class Main {
    public static void main(String[] args) {
        ArrayList<ArrayList<Integer>> arrayLists = new ArrayList<ArrayList<Integer>>();
        arrayLists.get(0).add(100);
        arrayLists.get(0).add(50);
        arrayLists.get(1).add(67);
        System.out.println(arrayLists.get(0).get(0));
    }
}

And the error you encountered:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0 at java.util.ArrayList.rangeCheck(ArrayList....

What's Causing This? #

The core issue is simple: you initialized the outer ArrayList, but it starts completely empty—there are no inner ArrayList<Integer> objects inside it yet. When you call arrayLists.get(0), you're trying to access the element at index 0, but the outer list has a size of 0. That's exactly why the IndexOutOfBoundsException gets thrown.

How to Fix It #

You need to first add empty inner ArrayList instances to the outer list before you can access and modify them. Here's the corrected working code:

import java.util.ArrayList;
public class Main {
    public static void main(String[] args) {
        ArrayList<ArrayList<Integer>> arrayLists = new ArrayList<ArrayList<Integer>>();
        
        // Add the first inner ArrayList to the outer list first
        arrayLists.add(new ArrayList<>());
        arrayLists.get(0).add(100);
        arrayLists.get(0).add(50);
        
        // Do the same for the second inner ArrayList
        arrayLists.add(new ArrayList<>());
        arrayLists.get(1).add(67);
        
        System.out.println(arrayLists.get(0).get(0)); // This will now print 100
    }
}

Bonus: Dynamic Inner List Creation #

If you're working with a variable number of inner lists (and don't want to manually add() each one upfront), you can add a check to create the inner list on the fly when you need it:

int targetIndex = 0;
// Check if the target index is beyond the current size, or if the element is null
if (targetIndex >= arrayLists.size() || arrayLists.get(targetIndex) == null) {
    arrayLists.add(new ArrayList<>());
}
// Now safely add elements to the inner list
arrayLists.get(targetIndex).add(100);

This way, you won't have to pre-add all inner lists manually—they'll be created as you need them.

内容的提问来源于stack exchange，提问作者Mert Yanık