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Session Length
~18 min
Adaptive Checks
16 questions
Transfer Probes
9
Arrays and ArrayLists are the fundamental data structures for storing collections of elements in Java. An array is a fixed-size, ordered collection of elements of the same type, declared with bracket notation (int[] arr = new int[5]) and accessed by zero-based index. Arrays support efficient random access but cannot be resized after creation.
ArrayLists are part of the java.util package and provide a resizable alternative to arrays. They store objects (not primitives), support dynamic sizing with methods like add(), remove(), get(), set(), and size(), and handle autoboxing for primitive wrapper types. Understanding when to use arrays versus ArrayLists is a key design decision in Java programming.
This topic also covers array traversal using standard for loops and enhanced for-each loops, common algorithms (searching, finding min/max, summing, reversing), 2D arrays for tabular data, and the important distinction between passing arrays by reference versus passing primitives by value. These concepts form the foundation of AP Computer Science A Units 6 and 7, comprising 10-15% of the AP exam.
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Arrays are fixed-size collections declared with a type and size. Elements are initialized to default values (0 for int, null for objects, false for boolean).
Example: int[] scores = new int[5]; // creates array of 5 ints, all initialized to 0 String[] names = {"Alice", "Bob", "Carol"}; // initializer list
Array elements are accessed by zero-based integer index. The valid range is 0 to array.length - 1. Accessing an out-of-bounds index throws ArrayIndexOutOfBoundsException.
Example: int[] arr = {10, 20, 30}; arr[0] // returns 10 arr[2] // returns 30 arr[3] // throws ArrayIndexOutOfBoundsException
Visiting every element of an array using a loop. Standard for loops use an index variable; enhanced for-each loops iterate directly over elements.
Example: for (int i = 0; i < arr.length; i++) { ... } // standard for (int val : arr) { ... } // enhanced for-each
A resizable collection from java.util that stores objects. Provides methods like add(), get(), set(), remove(), and size(). Requires import java.util.ArrayList.
Example: ArrayList<String> names = new ArrayList<String>(); names.add("Alice"); names.get(0); // returns "Alice" names.size(); // returns 1
Automatic conversion between primitive types and their wrapper classes (int <-> Integer, double <-> Double) when using ArrayLists, which cannot store primitives directly.
Example: ArrayList<Integer> nums = new ArrayList<Integer>(); nums.add(5); // autoboxing: int 5 -> Integer 5 int x = nums.get(0); // unboxing: Integer -> int
Arrays of arrays used to represent tabular or grid data. Accessed with two indices: row and column. Declared as type[][] name = new type[rows][cols].
Example: int[][] grid = new int[3][4]; // 3 rows, 4 columns grid[1][2] = 7; // sets row 1, column 2 to 7
A sequential search algorithm that checks each element of an array one by one until the target is found or the end is reached. Time complexity: O(n).
Example: for (int i = 0; i < arr.length; i++) { if (arr[i] == target) return i; } return -1; // not found
Arrays are fixed-size and can hold primitives or objects. ArrayLists are resizable but can only hold objects. Arrays use [] syntax; ArrayLists use method calls.
Example: int[] arr = new int[3]; arr[0] = 1; arr.length; ArrayList<Integer> list = new ArrayList<>(); list.add(1); list.size();
More terms are available in the glossary.
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