Chandan Singh | Java ref

java ref

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data structures

- LinkedList, ArrayList
	- add(Element e), add(int idx, Element e), get(int idx)
	- remove(int index)
	- remove(Object o)
- Stack
	- push(E item)
	- peek()
	- pop()
- PriorityQueue
	- peek()
	- poll()
	- default is min-heap
	- PriorityQueue(int initialCapacity, Comparator<? super E> comparator)
	- PriorityQueue(Collection<? extends E> c)
- HashSet, TreeSet
	- add, remove
- HashMap
	- put(K key, V value)
	- get(Object key)
	- keySet()
	- if you try to get something that's not there, will return null
  • default init capacities all 10-20
  • clone() has to be cast from Object

useful

iterator

- it.next() - returns value
- it.hasNext() - returns boolean
- it.remove() - removes last returned value

strings

- String.split(" |\\.|\\?") //split on space, ., and ?
- StringBuilder
	- much faster at concatenating strings
	- thread safe, but slower
	- StringBuilder s = new StringBuilder(CharSequence seq)();
	- s.append("cs3v");
	- s.charAt(int x), s.deleteCharAt(int x), substring
	- s.reverse()
	- Since String is immutable it can safely be shared between many threads
- formatting
	String s = String.format("%d", 3);
	"%05d"	//pad to fill 5 spaces
	"%8.3f" //max number of digits
	"%-d"	//left justify
	"%,d" 	//print commas ex. 1,000,000
	| int | double | string |
	|-----|--------|--------|
	| d   | f      | s      |
	new StringBuilder(s).reverse().toString()
	int count = StringUtils.countMatches(s, something);
- integer
	- String toString(int i, int base)
	- int parseInt(String s, int base)
- array
	char[] data = {'a', 'b', 'c'};
	String str = new String(data);

sorting

- Arrays.sort(Array a)
- Collections.sort(Collection c), Collections.sort(Collection l, Comparator c)
	- use mergeSort (with insertion sort if very small)
- Collections.reverseOrder() returns comparator opposite of default
class ComparatorTest implements Comparator<String>
	public int compare(String one, String two) //if negative, one comes first
class Test implements Comparable<Object>
	public int compareTo(Object two)

exceptions

  • ArrayIndexOutOfBoundsException
  • throw new Exception("Chandan type")

higher level

  • primitives - byte, short, char, int, long, float, double
  • java only has primitive and reference types
    • when you assign primitives to each other, it’s fine
    • when you pass in a primitive, its value is copied
    • when you pass in an object, its reference is copied
      • you can modify the object through the reference, but can’t change the object’s address
  • garbage collection
    • once an object no longer referenced, gc removes it and reclaims memory
    • jvm intermittently runs a mark-and-sweep algorithm
      • runs when short-term stuff gets full
      • older stuff moves to different part
      • eventually older stuff is cleared

object-oriented

| declare | instantiate | initialize | | ——- | ———– | ———- | | Robot k | new | Robot() |

  • class method = static
    • called with Foo.DoIt()
    • initialized before constructor
    • class shares one copy, can’t refer to non-static
  • instance method - invoked on specific instance of the class
    • called with f.DoIt()
  • protected member is accessible within its class and subclasses