# ArrayList {% embed url="" %} An ArrayList in Java is a dynamic array that can hold elements of any data type. Unlike a static array in Java, an **ArrayList can be resized dynamically.** This means that we can add and remove values from an ArrayList. An ArrayList is a class in the `Java Collection` framework that is part of the `java.util` package ## Java ArrayList Requirements 1. **Import ArrayList from** `java.util` ```java import java.util.ArrayList; // import the ArrayList class ``` 2. Create an **ArrayList** object ```java // Some code import java.util.ArrayList; // import the ArrayList class class Main { public static void main(String[] args) { ArrayList cars = new ArrayList(); // Initializing the new arraylist about cars } } ``` {% hint style="info" %} The data type of the ArrayList's items must be **OBJECTS.** Moreover, they should be `Generic/Wrapper Class` **not primitive data types.** {% endhint %} | Primitive Data Type 👎 | Wrapper Class Equivalent 👍 | | ---------------------- | --------------------------- | | `int` | `Integer` | | `double` | `Double` | | `boolean` | `Boolean` | | *does not exit* | `String` | *In Java, whenever we need data to be “objects” we have a Wrapper class to represent built-in data to be “objects”.* ## Common ArrayList Methods 1. **Add an item to an ArrayList** ```java // Add an item to an ArrayList ArrayList nums = new ArrayList(); nums.add(3); nums.add(1); nums.add(4); // nums is now composed of 3, 1, 4 ``` We can use → .add() to add a value to an ArrayList to the END OF THE ARRAYLIST. *What happens if you want to add a value at a certain index?* ```java // Add an item to an ArrayList at a certain index ArrayList nums = new ArrayList(); nums.add(3); nums.add(1); nums.add(4); // nums is currrently: [3, 1, 4] nums.add(3,1); // Adds 1 to the end of the list even though index is out of bounds; // The provided index can only be from [0, current length of arraylist] nums.add(0, 0); // Adds 0 to the front of the list, shifts everything back nums.add(2, 99); // Adds 99 at index 2; shifts everything from 2 onwards back by one. //nums.add(100, 5); // Creates an error since the index is greater than the length. System.out.println(nums); // Outputs: [0, 3, 99, 1, 4, 1] ``` **`.add(int index, value)`** can be also be used to add values to your ArrayList at certain locations following the rules/concepts below: {% hint style="info" %} **Rule A: Valid Index Range** The index provided must be within the range of 0 to the size of the ArrayList (inclusive). If the index is outside this range, it will throw an `IndexOutOfBoundsException`. **Rule B:. Shift Existing Elements** When you add an element at a specific index, all elements at that index or greater are shifted one position to the right. **Rule C. Capacity Check and Resize** If adding an element exceeds the current capacity of the ArrayList, it will automatically increase its capacity to accommodate the new element. This involves creating a new array with a larger capacity, copying the existing elements into the new array, and then adding the new element. [*More reading on ArrayList Sizing*](https://www.geeksforgeeks.org/internal-working-of-arraylist-in-java/) {% endhint %} 2. **Grab a value from an ArrayList** ```java // Grabbing a Value ArrayList nums = new ArrayList(); nums.add(3); nums.add(1); nums.add(4); System.out.println(nums.get(0)); // prints 3 System.out.println(nums.get(2)); // prints 4 ``` We can use → `.get(INDEX)` to grab value at a positive integer index value. (Index starts at zero always) *You can get an error if the index does not exist.* 3. **Change a value at an index** ```java // Change a value at a index ArrayList nums = new ArrayList(); nums.add(3); nums.add(1); nums.add(4); nums.set(0, 9); // index of 0 has a value of 9 now ``` We can use → `.set(INDEX, NEWVALUE)` to change a value at a certain location. 4. **Remove a value at an index** ```java // Remove a Value ArrayList nums = new ArrayList(); nums.add(3); nums.add(1); nums.add(4); nums.remove(2); // nums ArrayLists now is only [3,1] ``` We can use → `.remove(INDEX)` to remove a value at a certain location. 5. **Get the size of the ArrayList** ```java // Get Size of the ArrayList ArrayList nums = new ArrayList(); nums.add(3); nums.add(1); nums.add(4); System.out.println(nums.size()); // outputs: 3 ``` We can use → `.size()` to determine the number of elements in an ArrayList. 6. **Check if a value exists in an ArrayList** ```java // Does a value exist in an arraylist? ArrayList nums = new ArrayList(); nums.add(3); nums.add(1); nums.add(4); System.out.println(nums.contains(1)); // outputs: true; 1 exists in nums System.out.println(nums.contains(10)); // outputs: false; 10 does not exist in nums ``` We can use → `.contains()` to determine if a value exists in an ArrayList. 7. **Determine the index of a value in an ArrayList** ```java // Does a value exist in an arraylist? ArrayList nums = new ArrayList(); nums.add(3); nums.add(1); nums.add(4); nums.add(1); nums.add(5); nums.add(9); System.out.println(nums.indexOf(4)); // Returns 2 (4 is at index 2) System.out.println(nums.indexOf(1)); // Returns 1 (The first location of 1) System.out.println(nums.indexOf(10)); // Returns -1 (10 DNE) System.out.println(nums.lastIndexOf(1)); // Returns 3 (Last occurrence of 1) ``` We can use → `.indexOf()` to determine an index of a value in an ArrayList. Returns -1 if not found. We can use → `.lastIndexOf()` to determine an index of a value in an ArrayList from the right. Returns -1 if not found. ## ArrayLists are printable! ```java // Example ArrayList cars = new ArrayList(); cars.add("Honda"); cars.add("Toyota"); cars.add("Mercedes"); cars.add("Ford"); System.out.println("Our car brands: " + cars); ``` **Output:** ``` Our car brands: [Honda, Toyota, Mercedes, Ford] ``` As long as the ArrayList contains primitive equivalent wrapper class items, we can simply print the ArrayList within **`System.out.println()`**.