Source code analysis of java Collection classes: Map (2), javamap

Source code analysis of java Collection classes: Map (2), javamap

This section mainly discusses the differences and usage of Several implementation classes of the Map interface.

1. Thread Security

Hashtable is thread-safe (similar to Stringbuffer and Vector), but not others. As for why, read some previous articles, which have been described in detail earlier.

2. Applicable Conditions

HashMap: applicableQuick SearchAndUnordered dataIn this case, the allowed key is null (only once), and the allowed value is null;

TreeMap: applicableOrdered StorageThe key cannot be empty;

Hashtable: It is applicable to ensure synchronizationThread SecurityThe key and value cannot be empty;

LinkedHashMap: saves ElementsInsert sequenceIt has all the features of HashMap, but the time duration is slower than that of HashMap.

Examples of comparing HashMap and LinkedHashMap:

1 private static void linkedHashMapAnal () {2 // LinkedHashMap has all the features of HashMap, and can retain the sequence of elements inserted. 3 Map <String, Integer> linkedHashMap = new LinkedHashMap <String, integer> (); 4 Map <String, Integer> hashMap = new HashMap <String, Integer> (); 5 6 hashMap. put ("A", 120); 7 hashMap. put ("B", 100); 8 hashMap. put ("C", 105); 9 hashMap. put ("D", 200); 10 for (String key: hashMap. keySet () {11 System. out. print (key + ":" + hashMap. get (key) + ""); 12} // D: 200 A: 120 B: 100 C: 10513 14 System. out. println (); 15 linkedHashMap. put ("A", 120); 16 linkedHashMap. put ("B", 100); 17 linkedHashMap. put ("C", 105); 18 linkedHashMap. put ("D", 200); 19 for (String key: linkedHashMap. keySet () {20 System. out. print (key + ":" + linkedHashMap. get (key) + ""); 21} // A: 120 B: 100 C: 105 D: 200 22}

 

3. sorting problems

Map sets are sorted by key and by value.

• Sort by key

There are two ways to sort Map by key: the comparator in TreeMap and the sort method in the Collections class.

1. Sort TreeMap by key

1 // use the default comparator to sort key values in ascending order and store 2 Map <String, Integer> treeMap = new TreeMap <String, Integer> (); 3 treeMap. put ("B", 120); 4 treeMap. put ("C", 105); 5 treeMap. put ("A", 100); 6 for (String key: treeMap. keySet () {7 System. out. print (key + ":" + treeMap. get (key) + ","); 8} // A: 100, B: 120, C: 105, 9}

1 // custom Comparator for storing 2 Map <String, Integer> treeMap = new TreeMap <String, Integer> (new Comparator <String> () in descending order () {3 @ Override 4 public int compare (String o1, String o2) {5 // return 0; 6 return o2.compareTo (o1) by default ); // in descending order 7} 8}); 9 treeMap. put ("B", 120); 10 treeMap. put ("C", 105); 11 treeMap. put ("A", 100); 12 for (String key: treeMap. keySet () {13 System. out. print (key + ":" + treeMap. get (key) + ","); 14} // C: 105, B: 120, A: 100,

1 // use a subset of SortedMap to initialize the TreeMap object. Call the comparator that comes with the parameter. 2 // Note: The parameter must be an implementation class of the SortedMap interface. Otherwise, the default comparator 3 SortedMap <String, integer> subMap = new TreeMap <String, Integer> (new Comparator <String> () {4 5 @ Override 6 public int compare (String o1, String o2) {7 // TODO Auto-generated method stub 8 return o2.compareTo (o1); 9} 10}); 11 subMap. put ("B", 100); 12 subMap. put ("F", 120); 13 subMap. put ("D", 105); 14 for (String key: subMap. keySet () {15 System. out. print (key + ":" + subMap. get (key) + ","); 16} // F: 120, D: 105, B: 100, 17 18 System. out. println (); 19 20 Map <String, Integer> treeMap = new TreeMap <String, Integer> (subMap); 21 for (String key: treeMap. keySet () {22 System. out. print (key + ":" + treeMap. get (key) + ","); 23} // F: 120, D: 105, B: 100, 24 25 System. out. println (); 26 27 treeMap. put ("A", 200); 28 treeMap. put ("C", 300); 29 treeMap. put ("E", 400); 30 for (String key: treeMap. keySet () {31 System. out. print (key + ":" + treeMap. get (key) + ","); 32} // F: 120, E: 400, D: 105, C: 300, B: 100, A: 200,

2. Sort HashMap by key

1 // use the HashMap subset to initialize the TreeMap object and call the default comparator (ascending). 2 // The parameter here is the implementation class of the Map interface. 3 Map <String, integer> subMap = new HashMap <String, Integer> (); 4 subMap. put ("B", 100); 5 subMap. put ("A", 120); 6 subMap. put ("C", 105); 7 subMap. put ("D", 200); 8 for (String key: subMap. keySet () {9 System. out. print (key + ":" + subMap. get (key) + ","); 10} // D: 200, A: 120, B: 100, C:, 11 12 System. out. println (); 13 14 Map <String, Integer> treeMap = new TreeMap <String, Integer> (subMap); 15 for (String key: treeMap. keySet () {16 System. out. print (key + ":" + treeMap. get (key) + ","); 17} // A: 120, B: 100, C: 105, D: 200,

1 Map <String, Integer> hashMap = new HashMap <String, Integer> (); 2 3 // hashMap is arranged by the hash value of the key by default, which is a type of unordered storage 4 hashMap. put ("A", 100); 5 hashMap. put ("B", 120); 6 hashMap. put ("C", 105); 7 hashMap. put ("D", 100); 8 for (String key: hashMap. keySet () {9 System. out. print (key + ":" + hashMap. get (key) + ""); 10} // D: 100 A: 100 B: 120 C: 105 11 12 System. out. println (); 13 // store the HashMap object to 14 ArrayList in the List set <Map. entry <String, Integer> mapList = new ArrayList <Map. entry <String, Integer> (hashMap. entrySet (); 15 // call the sort method of Collections 16 Collections. sort (mapList, new Comparator <Entry <String, Integer> () {17 18 @ Override19 public int compare (Entry <String, Integer> o1, Entry <String, integer> o2) {20 // TODO Auto-generated method stub21 return o1.getKey (). compareTo (o2.getKey (); // 22 in ascending order // return o2.getKey (). compareTo (o1.getKey (); // 23} 24} in descending order); 25 26 for (int I = 0; I <mapList. size (); I ++) {27 System. out. print (mapList. get (I ). getKey () + ":" + mapList. get (I ). getValue () + ""); 28} // A: 100 B: 120 C: 105 D: 100

• Sort by value

With the above example, we know that we can also sort the values of Map sets through the sort method of Collections, including HashMap and TreeMap.

1 Map <String, Integer> hashMap = new HashMap <String, Integer> (); 2 3 hashMap. put ("A", 100); 4 hashMap. put ("B", 120); 5 hashMap. put ("C", 105); 6 hashMap. put ("D", 110); 7 for (String key: hashMap. keySet () {8 System. out. print (key + ":" + hashMap. get (key) + ""); 9} // D: 110 A: 100 B: 120 C: 105 10 11 System. out. println (); 12 // store the HashMap object in the List set. 13 ArrayList <Map. entry <String, Integer> mapList = new ArrayList <Map. entry <String, Integer> (hashMap. entrySet (); 14 // call Collections's sort method 15 Collections. sort (mapList, new Comparator <Entry <String, Integer> () {16 17 @ Override18 public int compare (Entry <String, Integer> o1, Entry <String, integer> o2) {19 // TODO Auto-generated method stub20 return o1.getValue (). compareTo (o2.getValue (); // sort by value in ascending order. You only need to change getKey to getValue to 21 // return o2.getValue (). compareTo (o1.getValue (); // sort by value in descending order. You only need to replace getKey with getValue to 22} 23}); 24 25 for (int I = 0; I <mapList. size (); I ++) {26 System. out. print (mapList. get (I ). getKey () + ":" + mapList. get (I ). getValue () + ""); 27} // A: 100 C: 105 D: 110 B: 120

1 ArrayList <Map. entry <String, Integer> mapList = new ArrayList <Map. entry <String, Integer> (treeMap. entrySet (); 2 Collections. sort (mapList, new Comparator <Entry <String, Integer> () {3 4 @ Override 5 public int compare (Entry <String, Integer> o1, Entry <String, integer> o2) {6 // TODO Auto-generated method stub 7 return o1.getValue (). compareTo (o2.getValue (); // 8 in ascending order // return o2.getValue (). compareTo (o2.getValue (); // 9} 10} in descending order); 11 for (int I = 0; I <mapList. size (); I ++) {12 System. out. print (mapList. get (I ). getKey () + ":" + mapList. get (I ). getValue () + ""); 13} // B: 100 D: 105 F: 120 C: 300 E: 400 A: 500 14 15}