Java Arrays and ArrayLists Cheat Sheet

The core ideas of Java Arrays and ArrayLists distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Array Declaration and Initialization

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).

Array Indexing

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.

Array Traversal

Visiting every element of an array using a loop. Standard for loops use an index variable; enhanced for-each loops iterate directly over elements.

ArrayList

A resizable collection from java.util that stores objects. Provides methods like add(), get(), set(), remove(), and size(). Requires import java.util.ArrayList.

Autoboxing and Unboxing

Automatic conversion between primitive types and their wrapper classes (int <-> Integer, double <-> Double) when using ArrayLists, which cannot store primitives directly.

2D Arrays

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].

Linear Search

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).

Array vs ArrayList

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.

Enhanced For Loop (for-each)

A simplified loop syntax that iterates over every element in an array or collection without using an index variable. Cannot modify the array or skip elements.

Key Terms at a Glance

Array:A fixed-size, ordered collection of elements of the same type, accessed by zero-based integer index.

ArrayList:A resizable collection from java.util that stores objects and provides methods like add(), get(), set(), remove(), and size().

Index:An integer position used to access elements in an array or ArrayList, starting from 0.

ArrayIndexOutOfBoundsException:A runtime exception thrown when code attempts to access an array index that is negative or greater than or equal to the array's length.

Autoboxing:Automatic conversion of a primitive type to its corresponding wrapper class (e.g., int to Integer).

Unboxing:Automatic conversion of a wrapper class object to its corresponding primitive type (e.g., Integer to int).

Enhanced For Loop:A simplified loop syntax (for-each) that iterates over every element in an array or collection without using an index.

2D Array:An array of arrays, used to represent grid or tabular data, accessed with two indices (row and column).

Linear Search:A search algorithm that checks each element sequentially until the target is found or the end of the collection is reached.

Traversal:The process of visiting every element in a data structure, typically using a loop.

Wrapper Class:An object type that wraps a primitive value (Integer for int, Double for double, Boolean for boolean) for use in generic collections.

Ragged Array:A 2D array where rows can have different numbers of columns, since each row is an independent array.

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