Summarize for loop and for loop enhanced traversal arrays, List,set and map

One. For collections

(1) Normal for loop

        int [] arr = {2, 1, 2 };          for (int i=0;i<arr.length;i++) {            System.out.println (arr[i]);        }

(2) For loop enhancement

        int [] arr = {2, 1, 2 };          for (int  i:arr) {            System.out.println (i);        }        System.out.println ("********************");         // two-dimensional arrays        int [][]arr1={{2,1,3},{1,2,5}};          for (int  []i:arr1) {            for(int  j:i) {                System.out.println (j);            }        }

Two. For list,list is an interface, there are many classes that implement it, such as Arraylist,linkedlist,vector.

(1) using normal for loops and iterators

List<string> list=NewArraylist<string>(); List.add ("AAA"); List.add ("BBB"); List.add ("CCC"); //Normal Traversal         for(intI=0;i<list.size (); i++) {System.out.println (List.get (i)); } System.out.println ("****************"); //using iteratorsIterator iterator=List.iterator ();  while(Iterator.hasnext ()) {System.out.println (Iterator.next ()); }

(2) Use for loop enhancement

         for (String s:list) {            System.out.println (s);        }

Three. For the set interface, there are many classes that implement it, HashSet (unordered), Linkedhashset (ordered)

(1) Use iterators. Note that set does not have a get () Method!!!

        set<string> set=New hashset<>();        Set.add ("a");        Set.add ("B");        Set.add ("C");        Iterator Iterator=set.iterator ();          while (Iterator.hasnext ()) {            System.out.println (Iterator.next ());        }

(2) Use for loop enhancement

         for (String s:set) {            System.out.println (s);        }

Four. For the map interface, there are many classes that implement it, hashmap,linkedhashmap,hashtable. Store as a key-value pair

(1) The use of iterators, there are two ways

Map<string,string> map=NewHashmap<>(); Map.put ("1", "AA"); Map.put ("2", "BB"); Map.put ("3", "CC"); //Method 1: Use iterators, Fetch keyset ()Iterator iterator=Map.keyset (). iterator ();  while(Iterator.hasnext ()) {String key=Iterator.next (). toString (); String value=Map.get (key); System.out.println ("Key=" +key+ "value=" +value); }        //Method 2: Use iterators, Fetch entryset ()Iterator iterator2=Map.entryset (). iterator ();  while(Iterator2.hasnext ()) {Map.entry<string, string> entry= (map.entry<string,string>) Iterator2.next (); System.out.println (Entry.getkey ()+"  "+Entry.getvalue ()); }

(2) Enhance for loop, rewrite the above 2 methods

        // Method 1         for (String s:map.keyset ()) {            System.out.println (s+ "  " +Map.get (s));        }         // Method 2 Using the Enhanced For,entryset ()         for (Map.entry me:map.entrySet ()) {            System.out.println (Me.getkey () + "  " +Me.getvalue ());        }

2

Summarize for loop and for loop enhanced traversal arrays, List,set and map