Java class notes------Map, queue, stack

*Java.util.Map*The map looks like a two-column table with multiple rows.*Each record is divided into two parts: Key,value*one of the keys in a map is not allowed to be duplicated. (Equals comparison)*  *Common Implementation classes: JAVA.UTIL.HASHMAP (hash algorithm implementation)*TreeMap (binary tree implementation) Map<String,Integer>Map=NewHashmap<string,integer>(); *v put (K k,v v)*The given key and value are stored in the map. *because key in map does not allow duplication, there are two scenarios:* 1: The key stored in the map does not already exist, then directly* key-value is stored in map with a return value of NULL* 2: The key stored in the map already exists, then the corresponding*value replaces the value of the original key and will be*the value returned for the replacement. Map.put ("Chinese", 98); Map.put ("Mathematics", 97); Map.put ("English", 95); Map.put ("Physics", 92); Map.put ("Chemistry", 96); *The order of deposit is inconsistent with the internal order Map.put ("Chemistry", 99); *V get (k k)*gets the corresponding value according to the given key*If the current map does not contain the given key, the return value is null Integer num= Map.get ("math")); *V remove (k k)*removes this pair of content from the map for the given key*and returns the corresponding value. Num= Map.Remove ("chemistry")); System.out.println ("Deleted value:" +num); *BooleanContainsKey (k k)*See if the current map contains a given key*The included judgment is the result of comparing the equals of keyBooleanContainsKey = Map.containskey ("Chinese"); *Traverse Map*There are three ways of traversing a map:* 1: Traverse all keys* 2: Iterates through each set of key-value pairs* 3: Traverse all the value (relatively infrequently used)*Linkedhashmap*internal use of a linkedlist maintenance sequence can be done*the traversal is consistent with the order of the put. If no order is required,*Typically, this class is not used. *Traverse all the keys:* set<k>KeySet ()*This method will return all the keys to a set set, so traversing the collection is the equivalent of traversing through all the keys set<String> KeySet =Map.keyset ();  for(String key:keyset) {System.out.println ("Key:" +key); }                         *iterate through each set of key-value pairs* set<entry>EntrySet ()*entry is the inner class of map, and each instance represents a set of key-value pairs*It contains two properties, Key,value. *after each set of key-value pairs (entry instances) is stored in a set set*to return it. Set<Entry<String,Integer>> EntrySet =Map.entryset ();  for(entry<string,integer>E:entryset) {String key=E.getkey (); Integer value=E.getvalue (); SYSTEM.OUT.PRINTLN (Key+":"+value); }                         * collection<v>values ()*returns all value values in the current map to a collection after Collection<Integer> values =map.values ();  for(Integer value:values) {System.out.println ("Value:" +value); }                                                                 *This class is used to practice overriding equals with hashcode considerations*When we use an instance of a custom type as the value of key in HashMap*, the result of this class equals and hashcode directly affects*hashmap efficiency of Queries*to avoid the creation of something:* Hashcode values, but equals does not compare to true!this will*Severe impact on hashmap efficiency*A rewrite of the two methods in the JAVA API documentation is required:*when we rewrite the Equals method of a class, it should be accompanied by a rewrite*Hashcode method. *It should also meet both:*When the equals of two objects is true, Hashcode returns the*values should be equal. In turn not mandatory, but preferably also different, otherwise affect*HashMap performance. *the Hashcode method should return a stable value, meaning that:*when the value of the property that determines the comparison result of the Equals method is not changed*The value returned by the Hashcode method should be the same if multiple calls are made. * Public classCell {Private intRow; Private intCol;  PublicCell (intRowintCol) {        Super();  This. Row =Row;  This. Col =Col; } @Override Public inthashcode () {Final intPrime = 31; intresult = 1; Result= Prime * result +Col; Result= Prime * result +Row; returnresult; } @Override Public Booleanequals (Object obj) {if( This==obj)return true; if(obj = =NULL)            return false; if(GetClass ()! =Obj.getclass ())return false; Cell Other=(Cell) obj; if(Col! =Other.col)return false; if(Row! =Other.row)return false; return true; }        }         *Queue*save a set of elements, but for access requirements, you must follow the FIFO principle Queue<String> queue =NewLinkedlist<string>(); *BooleanOffer (e e)*appends an element to the end of the queue Queue.offer ("One"); Queue.offer ("Both"); Queue.offer ("Three"); Queue.offer ("Four"); *E poll ()*gets the element from the first team. Note that the element is retrieved from the* Removed from the queue!queue Operation String Str=Queue.poll ();         System.out.println (str); *E-Peek ()*You can also get the first element of the team, but unlike poll,*the element will not be removed from the queue. STR=Queue.peek (); System.out.println (str); *Stack: Store a set of elements, access must follow the advanced post-out principle* Typically used to implement a "back"function of the place. *Deque is the sub-interface of the queue. *the characteristic of a double-ended queue is that both ends of the queue can enter and exit elements. *if we only invoke one side of the access method, it forms a stack structure Deque<String> stack =NewLinkedlist<string>(); *voidpush (e E)* Press the element "into"stack, into the stack operation. Elements of the new stack*will be at the top of the stack (the first element in the stack) Stack.push ("One"); Stack.push ("Both"); Stack.push ("Three"); Stack.push ("Four"); *E pop ()*The stack operation, gets the top element of the stack, gets the element after the*the String str is deleted from the stack=Stack.pop ();         System.out.println (str); *You can also use peek to refer to the top element of the stack without making a stack operation str=Stack.peek ();        System.out.println (str); 

Java class notes------Map, queue, stack