Java Coding Interview Cheat Sheet

A Programming › Java cheat sheet in English by nafanya

Main Java types, collections, and methods that are used in programming during white board coding interview

Java Interview

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String

Declaration

String s = "Hello world";

String s = new String("Hello world!");

String s = new String(charArray);

Methods

char c = s.charAt(index);

int index = s.indexOf("world");

String subs = s.substring(startIdx, endIdx);

bool hasRed = s.contains("red");

String newStr = s.replace('a', 'b');

String lowerCaseStr = s.toLowerCase();

String upperCaseStr = s.toUpperCase();

Convert

String s = String.valueOf(number);

String[ ] arrayOfStr = s.split(",");

char[ ] charArray = s.toCharArray();

int number = Integer.parseInt("123");

ArrayList

Declaration

ArrayList<String> cars = new ArrayList<>();

ArrayList<Integer> nums= new ArrayList<>(Arrays.asList(1, 2));

Methods

cars.add("Volvo");

cars.get(0);

cars.set(0, "Opel");

cars.remove(0);

cars.clear(); //remove all

cars.size();

int firstIndex = cars.indexOf("Volvo");

Sort & Iterate

Collections.sort(cars);

Collections.reverse(cars);

for ( int i = 0; i < cars.size(); i++ ) { }

for ( String car : cars ) { }

Iterator it = cars.iterator();
while ( it.hasNext() ) { it.next(); };

Convert to array

Integer[] arr = new Integer[arrayList.size()];

arr = arrayList.toArray(arr);

Array

Declaration

int[ ] array= new int[]{1, 2, 3, 4, 5, 6};

int[ ] array = new int[10];

int[ ][ ] twoDArray = new int[n][m];

Methods

int n = array.length;

Sort & Iterate

Arrays.sort(array);

Arrays.sort(array, (a,b) -> Integer.compare(a,b))

Character

Declaration & Methods

char myGrade = 'B';

(char) (1 + 'a') -> 'b'

Convert to other types

int a = ch - '0';

char myVar1 = 65; // myVar1 = 'A'

Character c1 = new Character('r');

String s1 = c1.toString();

String s2 = String.valueOf(c1);

LinkedList

Declaration

LinkedList<String> cars = new LinkedList<String>();

Method

cars.add("Volvo");

cars.addFirst("Mazda");

cars.addLast("Ford");

cars.removeFirst();

cars.removeLast();

cars.getFirst();

cars.getLast();

cars.size();

Iterate

for ( int i = 0; i < cars.size(); i++ ) { }

for ( String car: cars ) { }

HashMap

Declaration

HashMap<String, Integer> map = new HashMap<>();

Methods

map.put("key1", 10);

map.containsKey("key2")

map.containsValue(2));

map.get("key1");

map.getOrDefault("key1", 0);

map.remove("key1");

map.clear(); //remove all items

map.put("key1", 5); //update key1

map.size();

Iterate

for ( String key : map.keySet() ) { }

for ( Integer i : map.values() ) { }

Queue (LinkedList implementation)

Declaration

Queue<String> queue = new LinkedList<>();

Methods

queue.add("apple");

queue.remove(); // remove first element

queue.peek(); //show first element

queue.size();

LinkedList vs PriorityQueue

LinkedList preserves the insertion order, PriorityQueue does not. The elements of the priority queue are ordered according to their natural ordering, or by a Comparator provided at queue construction time.

Stack

Declaration

Stack<Integer> stack = new Stack<Integer>();

stack.push(1);

stack.peek();

stack.pop();

stack.isEmpty()

int index = stack.search(2);

Iterate

for ( Integer item : stack ) { }

Iterator it = stack.iterator();
while ( it.hasNext() ) { it.next(); };

HashSet

Declaration

HashSet<String> cars = new HashSet<String>();

Methods

cars.add("Volvo");

cars.contains("Mazda");

cars.remove("Volvo");

cars.clear();

cars.size();

Iterate & Convert

for ( String car : cars ) { }

Iterator<Integer> it = cars.iterator();
while ( it.hasNext() ) { it.next();};

String[] array = cars.toArray(

new String[cars.size()])

Queue (PriorityQueue implementation)

Declaration

Queue<Integer> pQueue = new PriorityQueue<Integer>();

Methods

pQueue.add(10);

pQueue.peek(); //return top element

pQueue.poll(); //return and remove

pQueue.remove(2); //remove first 2

Iterate

Iterator it = pQueue.iterator();
while ( it.hasNext() ) { it.next(); };

Comparartor

Comparator examples

Bitwise Operators

OR	0101 \| 0111 = 0111
AND	0101 & 0111 = 0101
XOR	0101 ^ 0111 = 0010
NOT	~ 0101 = 1010
Left shift	000110 << 1 = 001100
Right shift	10110101 >> 1 = 11011010
Unsigned Right Shift	10110101 >>> 1 = 01011010

Other

Min integer	Integer.MIN_VALUE
Max integer	Integer.MAX_VALUE

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Java Coding Interview Cheat Sheet

String

ArrayList

Array

Character

LinkedList

HashMap

Queue (Linke­dList implem­ent­ation)

LinkedList vs Priori­tyQueue

Stack

HashSet

Queue (Prior­ity­Queue implem­ent­ation)

Bitwise Operators

Other

Comments

Queue (LinkedList implementation)

LinkedList vs PriorityQueue

Queue (PriorityQueue implementation)