Cheatography
https://cheatography.com
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This is a draft cheat sheet. It is a work in progress and is not finished yet.
Threads and concurrency
- Change the class name to 'implements runnable' - Implement the method for parallelising (e.g. isPrime) - Implement sync method. For any code executed in sync method, threads are paused until the last call is complete. - Implement the actual parallelising in @override run - Use a while loop to execute: while((n=getNextInteger())<N_INTS){ results[n]=isPrime(n); } |
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Arrays and arraylists
Array to Arraylist |
Arrays.stream(row_array).boxed().collect(Collectors.toList()); |
ArrayList to Array (Int to int) |
int[] new = old.stream().mapToInt(i->i).toArray(); |
Get Max |
cats.values().stream().mapToInt(x->x).max().getAsInt() |
Copy array values |
var included= Arrays.copyOfRange(in,0,endIndex); OR USE FORLOOP. |
Reverse list |
Collections.reverse(names); |
Convert integer to int |
int x.intValue() |
Classes
NoClassDefFoundError |
Make sure when you create the class, it's in the right folder |
Initialise non-static inner class from psvm |
Switch the 'new' position:
Cost cost=new Cost(); Cost.Item
item1=cost.new Item("",50); |
Compare class of objects |
Compare classes animals[x][y].getClass().equals(World.Fox.class) |
Misc imperative programming
Case switch syntax |
return switch(group){
case LEGUME -> Group.BRASSICA;
default -> null; } } |
Deep copy |
List.copyOf, Arrays.copyOf or Object.clone() |
End loop of a scanner |
while(sc.hasNextDouble()){} (ctrl+d to end) |
Ifelse shortened |
var last = used.isEmpty() ? null : used.get(used.size() - 1); |
generate random numbers |
var rand=newRandom();
var list=rand.ints(10,0,500); |
try catch |
try{} catch(Exception E){} |
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Recursion
Keep track of |
full_set, used, last, current, expected_next |
Ending recursion |
if(used.size() == x) - > all_sets.add(List.copyOf(used)); return; |
recursing |
used.add(crop); getFixedRotation(crops,seasons,used,rotations); used.remove(crop); |
Hashing and HashCodes
Integer hash by division |
Integer.toUnsignedLong(value) % nBuckets; |
Integer hash by division |
Math.floor(nBuckets ((Integer.toUnsignedLong(value) 1.618) % 1)); |
String hash |
iterate over each char:
for i, int hashchar = hashfn(value.atchar(i),buckets);
hash = (hash + hash_char) % buckets; |
Sets
Add items to a set |
Set<Food>foods=newHashSet<>(){};
foods.addAll(List.of(cake,Vegetable_Soup,Waffles,Potato_Gratin)); |
Sort a Set |
1. Create arraylist 2.add items of the set to the arraylist 3. use arraylist.sort method.
ArrayList<Item>sorted_items=newArrayList<Item>();
sorted_items.addAll(this.items.values()); sorted_items.sort((a,b)->a.compareTo(b)); |
Generics
class header |
public class Q6DelayedRelease<T> |
generic array |
E[] arr = (E[])new Object[INITIAL_ARRAY_LENGTH]; |
ClassCastExceptions (Add code AFTER declaration of T) |
this.items = (Item[]) Array.newInstance(new Item(delay,value).getClass(), slots); |
inner class T |
try add <T> qualifier to inner and outer class |
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