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