Java ArrayList

The ArrayList class is the most commonly used collection in Java. It's a resizable array implementation of the List interface, offering fast random access and dynamic sizing.

Array vs ArrayList

Unlike fixed-size arrays, ArrayList grows and shrinks automatically:

Creating an ArrayList

import java.util.ArrayList;
import java.util.List;

// Standard way - specify type in angle brackets (generics)
ArrayList<String> cars = new ArrayList<String>();

// Diamond operator (Java 7+) - type inferred
ArrayList<String> cars = new ArrayList<>();

// Best practice: program to interface
List<String> cars = new ArrayList<>();

// With initial capacity (optimization)
List<String> bigList = new ArrayList<>(1000);

Essential Operations

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Iterating Through ArrayList

List<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Cherry");

// 1. For-each loop (preferred for reading)
for (String fruit : fruits) {
    System.out.println(fruit);
}

// 2. Traditional for loop (when you need index)
for (int i = 0; i < fruits.size(); i++) {
    System.out.println(i + ": " + fruits.get(i));
}

// 3. Iterator (when removing during iteration)
Iterator<String> it = fruits.iterator();
while (it.hasNext()) {
    if (it.next().startsWith("B")) {
        it.remove();  // Safe removal
    }
}

// 4. Java 8+ forEach with lambda
fruits.forEach(fruit -> System.out.println(fruit));

// 5. Java 8+ Stream API
fruits.stream()
      .filter(f -> f.length() > 5)
      .forEach(System.out::println);

More Useful Methods

List<Integer> numbers = new ArrayList<>();
numbers.addAll(Arrays.asList(5, 2, 8, 1, 9));

// Sorting
Collections.sort(numbers);           // [1, 2, 5, 8, 9]
Collections.reverse(numbers);        // [9, 8, 5, 2, 1]

// Searching (list must be sorted for binarySearch)
Collections.sort(numbers);
int idx = Collections.binarySearch(numbers, 5);

// Min/Max
int min = Collections.min(numbers);  // 1
int max = Collections.max(numbers);  // 9

// Sublist (view, not copy)
List<Integer> sub = numbers.subList(1, 4);

// Convert to array
Integer[] arr = numbers.toArray(new Integer[0]);

// Check if empty
boolean empty = numbers.isEmpty();

Wrapper Classes for Primitives

ArrayList can only store objects. For primitive types, use wrapper classes:

// Autoboxing: Java automatically converts
ArrayList<Integer> nums = new ArrayList<>();
nums.add(42);        // int autoboxed to Integer
int n = nums.get(0); // Integer unboxed to int

Summary