Java Programming

arrays

• [index] - get/set at index
• .length - get the size of array
• int[] arr = new int[size] - create array of size size

ArrayList

• add(value) - add value to end
• add(index, value) - add value at index
• remove(index) - remove value at index
• remove(value) - remove value from list
• clear() - remove all elements
• get(index) - get item at index
• set(index, value) - set value for given index
• size() - size of list
• isEmpty() - check if list is empty
• contains(value) - check if value is present
• indexOf(value) - get the index of value
• lastIndexOf(value)- get the last index of value
• removeIf(lambda: condition) - remove elements which does not match the condition

HashMap (ordering not preserved)

• put(key, value) - insert key value pait in hashmap
• get(key) - get value for given key
• remove(key)- remove key value pair from map
• containsKey(key) - check if key is present in hashmap
• containsValue(value) - check if value exists in hashmap
• size() - give the size of map
• isEmpty() - check if map is empty
• map.keySet() - get list of keys
• map.values() - get list of values
• map.entrySet() - get list of map entries
  • entry.getKey() entry.getValue()
• getOrDefault(key, defaultValue) - get key value, if not present return defaultValue

HashSet

• add(value)
• remove(value)
• contains(value)
• isEmpty()
• size()
• clear()

Stack

• push(value)
• pop() - removes and returns items
• top() - returns top value
• isEmpty()
• size()

Queue, LinkedList, ArrayDeque

• add(item) - add element to tail
• remove() - remove head element and return
• peek() - return head element
• isEmpty()
• size()
• contains()
• clear()

PriorityQueue

• add(item) - add item to priority queue
• poll() - remove and return the highest element
• peek() - return the highest element
• remove(item) - remove item from the priority queue
• contains(item)
• size()
• clear()

• custom comparator

  class PersonAgeComparator implements Comparator<Person> { 
      public int compare(Person p1, Person p2) {
          return Integer.compare(p1.age, p2.age);
      }
  }
  Comparator<Person> byAge = new PersonAgeComparator();
  

Short summary

• HashSet - implemented using hashing
• LinkedHashSet - same as HashSet but it maintains insertion order
• TreeSet - implemented using a binary search tree(Red black trees), so it can be used to access items in order
• ArrayList - like dynamic arrays
• Vector - like array list but synchronize, (legacy)
• LinkedList - a simple double linked list
• PriorityQueue - implemented using heap
• HashMap - implemented using hashing
• LinkedHashMap - same as HashMap but it maintains insertion order
• TreeMap - implemented using binary search tree, so items can be accessed in order