Cheatography
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Main Java types, collections, and methods that are used in programming during white board coding interview
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= {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() ) { } |
|
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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 |
|
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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