Java Collection Class Traversal Delete method test and use scene record

Package Test0;import Java.util.list;import Java.util.map;import java.util.map.entry;import java.util.Set;import Java.util.arraylist;import Java.util.hashmap;import Java.util.hashset;import Java.util.iterator;import java.util.linkedlist;/** * Test Collection Class Traversal Delete method * is currently the basic traversal method, does not contain java8 inside the traversal way */public class Test2 {public static void main (String [] args) {ForEach (); Operatelist (); Operateset (); Operatemap ();} /** * JDK1.5, the application of new features For-each loop, universal traversal read all the methods of the collection class, but cannot delete, * For-each syntax is eventually converted to Iterator.next () by the compiler, so should be similar to iterator performance * Now when testing to a large amount of data, such as tens of thousands of data, the performance gap will come out * if the traversal is to modify the collection class, the Java.util.ConcurrentModificationException exception will be thrown * For example: * Exception in Thread "main" Java.util.ConcurrentModificationException * at Java.util.arraylist$itr.checkforcomodification (Unknown SOURCE) *at Java.util.arraylist$itr.next (Unknown source) *at Test.main (test.java:12) */public static void ForEach ( {//listarraylist<integer> arr = new arraylist<integer> (); Arr.add (1); Arr.add (2); for (Integer I:arr) {Syst Em.out.priNtln ("For each list:" + i);} sethashset<integer> hs = new hashset<integer> () Hs.add (3); Hs.add (4); for (Integer i:hs) { System.out.println ("For each set:" + i);} Maphashmap<integer, integer> HM = new Hashmap<integer, integer> (); Hm.put (5, 5); Hm.put (6, 6); for (Entry <integer, integer> Entry:hm.entrySet ()) {System.out.println ("for each map: <" + Entry.getkey () + "," + ENTRY.G Etvalue () + ">");}} /** * Operation list * LinkedList is the list form, which is used to traverse the list and delete element Method 2 is more efficient * ArrayList is an array form, the two methods are not much difference */public static void Operatelist () {fi nal int size = 10; list<integer> list = new arraylist<integer> (); for (int i = 0; i < size; i++) {list.add (3);} Traverse list and Delete element method 1for (int i = List.size ()-1; I >= 0; i--) {if (List.get (i) = = 3) {list.remove (i);}} for (int i = 0; i < size; i++) {list.add (3);} Traverse list and Delete element method 2for (Iterator<integer> it = List.iterator (); It.hasnext ();) {Integer E = It.next (); if (E.equals (3)) {It.remove ();}}} /** * Operation set */public static void Operateset () {final int size = 10; set<integer> set = new hashset<integer> (); for (int i = 0; i < size; i++) {set.add (i);} Traverse set and Delete element method for (iterator<integer> it = Set.iterator (); It.hasnext ();) {Integer E = It.next (); if (E.equals (3) | | e.equals (4)) {It.remove ();}}} /** * Operation Map, according to the requirements of the choice of traversal method, a small amount of data is not good contrast performance gap, it takes millions of data to easily compare the effect * 1.ENTRYSET traversal: Key value is required: Key value is always needed to remove the use of higher performance * 2. Keyset traversal method: Key value is required: The main use of key, a small amount of use of value when using higher performance * 3.values traversal value, this does not get key, but also can be deleted: Use the main value when using * * NOTE: * Because HashMap uses the hash algorithm to store, so the time complexity is O (1), * and treemap use red black tree storage, time complexity is O (logn), * So if you always need key value, TreeMap use EntrySet traversal method efficiency will be higher, While the HashMap performance difference is not so obvious */public static void Operatemap () {final int size = 10; Map<integer, integer> map = new Hashmap<integer, integer> (), for (int i = 0; i < size; i++) {Map.put (I, i);} 1.entrySet traversal method for (Iterator<entry<integer, integer>> it = Map.entryset (). Iterator (); It.hasnext ();) {Entry<integer, integer> e = It.next (); Integer key = E.getkey (); Integer value = E.getvalue (); if (Key.equals (3) | | key.equals (4)) {It.remove ()}} 2. KeySet traversal method for (iterator<integer> it = Map.keyset (). Iterator (); It.hasnext ();) {Integer key = It.next (); if (Key.equals (5) | | key.equals (6)) {//Integer value = Map.get (key); It.remove ();}} 3.values iterates over value, which does not get a key, but is also a for (iterator<integer> it = map.values (). Iterator (); It.hasnext ();) {Integer value = It.next (); if (Value.equals (7) | | value.equals (8)) {//Integer value = Map.get (key); It.remove ();}} System.out.println ("Operatemap:" + map);}}

　　

Java Collection Class Traversal Delete method test and use scene record