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Java Sorted Map Example

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In this example we shall show you how to make use of Java Sorted Map. A SortedMap is a Map that sort its entries in ascending order according to the keys’ natural ordering, or according to a Comparator provided at the time of the SortedMap creation. All keys inserted into a SortedMap must implement the Comparable interface (or be accepted by the specified Comparator). Furthermore, all such elements must be mutually comparable (i.e, Mutually Comparable simply means that two objects accept each other as the argument to their compareTo method), If you try to sort keys which do not implement Comparable or not has a specific Comparator, a ClassCastException will be thrown.

Tip 1

java.lang.Comparable: int compareTo(Object o):

This method compares this object with o object. Returned int value has the following meanings.

  • positive – this object is greater than o
  • zero – this object equals to o
  • negative – this object is less than o

Also, we can use our own Comparator. If you need to know more about the Comparable andComparator, Take a look on Java Comparable and Comparator Example to sort Objects by Byron Kiourtzoglou.

Tip 2

All SortedMap implementation classes should provide four “standard” constructors as the following:

  • A void (no arguments) constructor, which creates an empty SortedMap sorted according to the natural ordering of its keys.
    1 SortedMap sortedMap= new TreeMap();
  • A constructor with a single argument of type Comparator, which creates an empty SortedMap sorted according to the specified Comparator.
    1 Comparator comparator = new MyComparator();
    2 SortedMap sortedMap = new TreeMap(comparator);
  • A constructor with a single argument of type Map, which creates a new Map with the same key-value mappings as its argument, sorted according to the keys’ natural ordering.
    1 Map map = new HashMap();
    2 SortedMap sortedMap = new TreeMap(map);
  • A constructor with a single argument of type SortedMap, which creates a new SortedMap with the same key-value mappings and the same ordering as the input SortedMap.
    1 SortedMap sortedMap= new TreeMap();
    2 SortedMap newSortedMap = new TreeMap(sortedMap);

1. SortedMap Operations:

The SortedMap interface provides operations for normal Map operations and for the following:

  • Range view — performs arbitrary range operations on the SortedMap
    1. subMap(K fromKey, K toKey): Returns a view of the portion of this Map whose keys range from fromKey, inclusive, to toKey, exclusive.
    2. headMap(K toKey): Returns a view of the portion of this Map whose keys are strictly less than toKey.
    3. tailMap(K fromKey): Returns a view of the portion of this Map whose keys are greater than or equal to fromKey.
  • Endpoints — returns the first or the last key in the SortedMap
    1. firstKey(): Returns the first (lowest) key currently in this Map.
    2. lastKey(): Returns the last (highest) key currently in this Map.
  • Comparator access — returns the Comparator, if any, used to sort the map
    1. comparator(): Returns the Comparator used to order the keys in this Map, or null if this Map uses the natural ordering of its keys.

2. Example:

2.1. SortMapExample.java

01 package com.jcg.util.map;
02  
03 import java.util.Comparator;
04 import java.util.HashMap;
05 import java.util.Map;
06 import java.util.TreeMap;
07  
08 /**
09  * @author ashraf
10  *
11  */
12 public class SortMapExample {
13  
14     /**
15      * The main method.
16      *
17      * @param args the arguments
18      */
19     public static void main(String[] args) {
20         //creating unsorted map of employee id as a key and employee name as a value
21         Map unsortMap = new HashMap();
22         unsortMap.put(10, "Ashraf");
23         unsortMap.put(5, "Sara");
24         unsortMap.put(6, "Mohamed");
25         unsortMap.put(20, "Esraa");
26         unsortMap.put(1, "Bahaa");
27         unsortMap.put(7, "Dalia");
28         unsortMap.put(8, "Amira");
29         unsortMap.put(99, "Ahmed");
30         unsortMap.put(50, "Sama");
31         unsortMap.put(2, "Nada");
32         unsortMap.put(9, "Osama");
33  
34         System.out.println("Unsort Map......");
35         printMap(unsortMap);
36  
37         // Using the default natural ordering of sorted map Integer key which implement Comparable interface
38         System.out.println("\nSorted Map in ascending order......");
39         Map ascSortedMap = new TreeMap();
40         ascSortedMap.putAll(unsortMap);
41         printMap(ascSortedMap);
42  
43         // Forcing the descending order by creating our own comparator then passing it to the sorted map at creation time
44         System.out.println("\nSorted Map in descending order......");
45         Map desSortedMap = new TreeMap(
46                 new Comparator() {
47  
48                     @Override
49                     public int compare(Integer o1, Integer o2) {
50                         return o2.compareTo(o1);
51                     }
52  
53                 });
54         desSortedMap.putAll(unsortMap);
55         printMap(desSortedMap);
56  
57     }
58  
59     /**
60      * Prints the map.
61      *
62      * @param map the map
63      */
64     public static void printMap(Map map) {
65         for (Map.Entry entry : map.entrySet()) {
66             System.out.println("Key : " + entry.getKey() + " Value : "
67                     + entry.getValue());
68         }
69     }
70  
71 }

2.2. Explanation:

Let’s suppose that we want to sort a Map which contains a group of employees according to their ids where we use the employee id as a key and the employee name as a value. After we create a SortedMap using this Map and the key of thisMap is an Integer type which implements the Comparable interface, the keys are ordered in their natural ordering. Also, we can apply the descending order by creating our own Comparator then passing it to the SortedMap at creation time.

2.3. Output:

view sourceprint?
01 Unsort Map......
02 Key : 50 Value : Sama
03 Key : 1 Value : Bahaa
04 Key : 2 Value : Nada
05 Key : 99 Value : Ahmed
06 Key : 20 Value : Esraa
07 Key : 5 Value : Sara
08 Key : 6 Value : Mohamed
09 Key : 7 Value : Dalia
10 Key : 8 Value : Amira
11 Key : 9 Value : Osama
12 Key : 10 Value : Ashraf
13  
14 Sorted Map in ascending order......
15 Key : 1 Value : Bahaa
16 Key : 2 Value : Nada
17 Key : 5 Value : Sara
18 Key : 6 Value : Mohamed
19 Key : 7 Value : Dalia
20 Key : 8 Value : Amira
21 Key : 9 Value : Osama
22 Key : 10 Value : Ashraf
23 Key : 20 Value : Esraa
24 Key : 50 Value : Sama
25 Key : 99 Value : Ahmed
26  
27 Sorted Map in descending order......
28 Key : 99 Value : Ahmed
29 Key : 50 Value : Sama
30 Key : 20 Value : Esraa
31 Key : 10 Value : Ashraf
32 Key : 9 Value : Osama
33 Key : 8 Value : Amira
34 Key : 7 Value : Dalia
35 Key : 6 Value : Mohamed
36 Key : 5 Value : Sara
37 Key : 2 Value : Nada
38 Key : 1 Value : Bahaa

Java Sorted Map Example

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原文地址：http://www.cnblogs.com/hephec/p/4559100.html
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