How to Properly Create an ArrayList of ArrayLists in Java?

What is the correct way to create an ArrayList of ArrayLists in Java?

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

Creating an ArrayList of ArrayLists in Java allows you to construct a dynamic two-dimensional data structure. This is particularly useful when you need to store a collection of lists or lists of varying lengths, such as representing a matrix or grid. Below is a step-by-step guide on how to correctly create and manage an ArrayList of ArrayLists.

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

ArrayList<String> firstList = new ArrayList<>();
firstList.add("Item 1");
firstList.add("Item 2");
listOfLists.add(firstList);

ArrayList<String> secondList = new ArrayList<>();
secondList.add("Item A");
secondList.add("Item B");
listOfLists.add(secondList);

// Access an item: String item = listOfLists.get(0).get(1); // Returns "Item 2"

Solutions

• Declare an ArrayList of ArrayLists using the syntax: `ArrayList<ArrayList<Type>> list = new ArrayList<>();`
• To add an inner ArrayList, create it first: `ArrayList<Type> innerList = new ArrayList<>();` and then add it to the outer list: `list.add(innerList);`
• You can also initialize inner ArrayLists directly when adding: `list.add(new ArrayList<Type>());`
• Use loops to populate inner ArrayLists as required.

Mistake: Not initializing inner ArrayLists before adding to the outer ArrayList.

Solution: Always create and initialize the inner ArrayList before adding it to the outer one.

Mistake: Accessing elements without checking if the inner list exists.

Solution: Ensure to check the size of the outer ArrayList and the inner list before accessing elements.