Arrays and ArrayLists — Java Cheat Sheets

1 PRIMITIVE TYPES vs. REFERENCE TYPES

Primitive types

int, double, char, boolean …
Variable holds the actual value directly
Fixed, predefined set (8 total)

Reference types

Arrays, Strings, and every object are reference types
Variable holds a reference (address) to the object in memory
Created with new, default value is null

2 DECLARING & CREATING ARRAYS

int[] numbers = new int[5]; // 5 zeros int[] grades = {87, 68, 94, 100, 83}; // array initializer numbers[0] = 10; // assign element System.out.println(numbers.length); // 5 — a field, not a method

870

681

942

1003

834

Indices always start at 0 and go to length−1.

3 WORKING WITH ARRAY ELEMENTS

Totaling elements

int total = 0; for (int i=0; i<grades.length; i++) total += grades[i];

Using elements as counters

An array like int[] freq = new int[7] can tally how many times each face of a die appears — freq[face]++.

Enhanced for (for-each)

for (int g : grades) { total += g; }

4 PASSING ARRAYS TO METHODS & EXCEPTION HANDLING

Pass-by-value vs pass-by-reference

Java is always pass-by-value — but for arrays/objects, the "value" passed is the reference itself. So a method can modify the array's elements, but reassigning the parameter inside the method does not affect the caller's variable.

try { System.out.println(grades[10]); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Invalid index: " + e.getMessage()); }

5 MULTIDIMENSIONAL ARRAYS

int[][] grid = { {1, 2, 3}, {4, 5, 6} }; System.out.println(grid[1][2]); // 6

Notes

An array of arrays — rows can even have different lengths ("ragged" arrays)
Access with array[row][col]
Traverse with nested for loops (outer = rows, inner = columns)

6 VARARGS, COMMAND-LINE ARGS & CLASS Arrays

Variable-length arguments

static int sum(int... nums){ int t=0; for(int n: nums) t+=n; return t; }

Command-line arguments

public static void main(String[] args) — args holds whatever was typed after the program name.

java.util.Arrays

Arrays.sort()
Arrays.toString()
Arrays.binarySearch()
Arrays.fill()

7 OBJECTS-NATURAL CASE STUDY: INTRO TO ArrayList

ArrayList<String> names = new ArrayList<>(); names.add("Amit"); names.add("Priya"); names.remove("Amit"); System.out.println(names.size());

Arrays vs. ArrayList

Arrays have a fixed size; ArrayList grows/shrinks dynamically
ArrayList is part of the Collections Framework (java.util)
Stores only objects — primitives are auto-boxed (e.g. int → Integer)

8 METHOD REFERENCE TABLE

class Arrays (java.util)

Method	Purpose
`sort(arr)`	sorts in ascending order
`toString(arr)`	readable String like [1, 2, 3]
`binarySearch(arr, key)`	index of key in a sorted array
`fill(arr, value)`	sets every element to value
`equals(a1, a2)`	true if same length & elements
`copyOf(arr, n)`	new array of length n, copied from arr

class ArrayList<T>

Method	Purpose
`add(item)`	appends to end
`add(i, item)`	inserts at index i
`get(i)`	element at index i
`set(i, item)`	replaces element at index i
`remove(i)` / `remove(item)`	removes by index / by value
`size()`	current number of elements
`contains(item)`	true if present
`indexOf(item)`	first index of item, or -1
`clear()`	removes all elements

JAVA — CHAPTER 6