HashMap && heap

• Type argument cannot be primitive type
  HashMap<Integer,boolean> hashMap = new HashMap<Integer, boolean>();
  it should use Boolean instead of boolean;
  HashMap<Integer,Boolean> hashMap = new HashMap<Integer, Boolean>();
  hashMap.setEntry 是一个包装了每一个hashmap key-value的一个entry
• languages.entrySet() - returns the set view of all the entries.
• languages.keySet() - returns the set view of all the keys.
• languages.values() - returns the set view of all the values.
  for里面的内容左边是类型 这个函数的返回值类型为 Map.Entry<Integer,Boolean>

SO:this method return a Set view of the mapping contained in the map(hashmap)

Syntax:Set<Map.Entry<K,V>> entrySet()

``

	HashMap<Integer, Boolean> hashMap = new HashMap<Integer, Boolean>();
	hashMap.put(1,hashMap.containsValue(2));
	hashMap.put(4,hashMap.containsValue(2));
	for(Map.Entry<Integer,Boolean> entry : hashMap.entrySet()){
				System.out.println(entry.getKey()+":"+entry.getValue());
	}

``
map can have linkedhashmap this datastructure can solve the LRU question:

THIS is a usage of get the first element of LinkedHashMap:

• 1: transfor LinkedHashMap to set() （encapsulate the key-value） map.entrySet()
• 2:get the iterator method (I think just to have a pointor)
• 3:get the key=value (1=2). next()
• 4:getKey() or getValue()

``

	  Map<Integer,Integer> map = new LinkedHashMap<>();
		map.put(1,2);
		map.put(3,4);
		map.put(5,6);
		System.out.println(map.entrySet().iterator().next().getKey());
		System.out.println(map.entrySet().iterator().next().getValue());

``

heap

it is a Collection framework's iterface "set"
heap like a queue just have the priority
like min heap and max heap
so if you declare max heap like :
``

    PriorityQueue<Integer> maxPQ = new PriorityQueue<>(Collections.reverseOrder());
    maxPQ.add(1);
    maxPQ.add(2);
    maxPQ.add(0);
    for(Integer i : maxPQ){
        System.out.println(i);
    }

``

and you can also declare min heap:
PriorityQueue<Integer> maxPQ = new PriorityQueue<>();

• Ascending order priority queue: In ascending order priority queue, a lower priority number is given as a higher priority in a priority. For example, we take the numbers from 1 to 5 arranged in an ascending order like 1,2,3,4,5; therefore, the smallest number, i.e., 1 is given as the highest priority in a priority queue.
  Priority Queue.

• Descending order priority queue: In descending order priority queue, a higher priority number is given as a higher priority in a priority. For example, we take the numbers from 1 to 5 arranged in descending order like 5, 4, 3, 2, 1; therefore, the largest number, i.e., 5 is given as the highest priority in a priority queue.
  Priority Queue