list set array map 排序問題

While analyzing source code of a large number of open source Java projects, I found Java developers frequently sort in two ways. One is using the 

sort()

 method of 

Collections

 or 

Arrays

, and the other is using sorted data structures, such as 

TreeMap

 and 

TreeSet

.

1. Using sort() Method

If it is a collection, use Collections.sort() method.

// Collections.sort List<ObjectName> list = new ArrayList<ObjectName>(); Collections.sort(list, new Comparator<ObjectName>() { public int compare(ObjectName o1, ObjectName o2) { return o1.toString().compareTo(o2.toString()); } });

If it is an array, use 

Arrays.sort()

 method.

// Arrays.sort ObjectName[] arr = new ObjectName[10]; Arrays.sort(arr, new Comparator<ObjectName>() { public int compare(ObjectName o1, ObjectName o2) { return o1.toString().compareTo(o2.toString()); } });

This is very convenient if a collection or an array is already set up.

2. Using Sorted Data Structures

If it is a list or set, use 

TreeSet

 to sort.

// TreeSet Set<ObjectName> sortedSet = new TreeSet<ObjectName>(new Comparator<ObjectName>() { public int compare(ObjectName o1, ObjectName o2) { return o1.toString().compareTo(o2.toString()); } }); sortedSet.addAll(unsortedSet);

If it is a map, use 

TreeMap

 to sort. 

TreeMap

 is sorted by key.

// TreeMap - using String.CASE_INSENSITIVE_ORDER which is a Comparator that orders Strings by compareToIgnoreCase Map<String, Integer> sortedMap = new TreeMap<String, Integer>(String.CASE_INSENSITIVE_ORDER); sortedMap.putAll(unsortedMap);

//TreeMap - In general, defined comparator Map<ObjectName, String> sortedMap = new TreeMap<ObjectName, String>(new Comparator<ObjectName>() { public int compare(ObjectName o1, ObjectName o2) { return o1.toString().compareTo(o2.toString()); } }); sortedMap.putAll(unsortedMap);

This approach is very useful, if you would do a lot of search operations for the collection. The sorted data structure will give time complexity of 

O(logn)

, which is lower than 

O(n)

3. Bad Practices

There are still bad practices, such as using self-defined sorting algorithm. Take the code below for example, not only the algorithm is not efficient, but also it is not readable. This happens a lot in different forms of variations.

double t; for (int i = 0; i < 2; i++) for (int j = i + 1; j < 3; j++) if (r[j] < r[i]) { t = r[i]; r[i] = r[j]; r[j] = t; }

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