轉自:http://www.wujianrong.com/archives/2006/07/post_280.html
摘要：經常看見有人還在不厭其煩的用冒泡（最常見！）或是交換做排序，實際上用幾行代碼就可以既快又好地實現排序，不論是簡單類型還是類，數組還是Java聚集(Collection)。
簡單類型的排序
簡單類型不外是byte, char, short, int, long, float, double等數據類型， 這些類型不能放在聚集中，只能使用數組。java.util.Arrays方法提供了對這些類型的sort方法（實際上還有很多其他有用的方法），下面是對一個簡單的int數組排序：
int[] arr = {2, 3, 1,10,7,4};
System.out.print("before sort: ");
for (int i = 0; i< arr.length; i++)
System.out.print(arr[i] + " ");
System.out.println();
Arrays.sort(arr);
System.out.print("after sort: ");
for (int i = 0; i< arr.length; i++)
System.out.print(arr[i] + " ");
System.out.println();
輸出結果：
before sort: 2 3 1 10 7 4
after sort: 1 2 3 4 7 10
我們看到排序結果是按照升序排列的，下面的排序都是如此。
對象的排序
對象可以放在數組里，同樣調用Arrays.sort(Object[] arr)即可；也可以放到聚集里，用java.util.Collections的sort(List list)。注意不是list必須實現List接口而不僅僅是Collection接口。
但是這個類必須實現了java.lang.Comparable接口。這個接口只有一個方法：int compartTo(Object o)，當本對象比傳入的對象大時，返回一個正整數。 以類Programmer為例：
class Programmer implements Comparable{
private String name;
private String language;
private double pay;
public Programmer(String name, String language, double pay) {
this.name = name;
this.language = language;
this.pay = pay;
}
public int compareTo(Object o) {
Programmer other = (Programmer)o;
return (int)pay - (int)other.pay;
}
public String toString(){
return "{name: " + name + ", language: " + language + ", money: " + pay + "}";
}
}
對其進行排序：
ArrayList list = new ArrayList();
list.add(new Programmer("張三", "C", 12000));
list.add(new Programmer("李四", "Java", 200));
list.add(new Programmer("王五", "C++", 5000));
list.add(new Programmer("錢六", "VB", 3000));
System.out.println("before sort: " + list);
Collections.sort(list);
System.out.println("after sort: " + list);
輸出：
before sort: [{name: 張三, language: C, money: 12000.0}, {name: 李四, language: Java, money: 200.0}, {name: 王五, language: C++, money: 5000.0}, {name: 錢六, language: VB, money: 3000.0}]
after sort: [{name: 李四, language: Java, money: 200.0}, {name: 錢六, language: VB, money: 3000.0}, {name: 王五, language: C++, money: 5000.0}, {name: 張三, language: C, money: 12000.0}]
夠簡單吧！查查Comparable的javadoc可以知道，有很多類已經實現了該接口，因此對這些類的排序幾行代碼就可以搞定。
最近看C#發現其中用System.Array.sort對數組排序，適用于所有實現了IComparable接口的對象，看來微軟的借鑒能力還真是強啊！
對已有類進行排序
上面的方法有一個問題，就是一個類已經存在了，并且沒有實現Comparable接口，使用一個子類進行封裝？很麻煩（你可以對下面的例子試試）。還有一種情況就是對一個類沒法實現多種排序。以File類為例，它實現了Comparable接口，但是是按照名稱排序的。如果要按照大小排序，或者按修改時間排序呢？對這兩種情況，使用java.util包的Comparator接口：
Arrays.sort(Object[] arr, Comparator com)
Collections.sort(Object[] arr, Comparator com)
Comparator接口的方法：
public int compare(Object o1, Object o2)當o1比o2大時返回一個正整數
public boolean equals(Object obj) 判斷obj與這個Comparator是否同一個對象
下面使用Comparator對文件實現了按文件大小或修改時間排序：
class FileUtils {
static class CompratorByLastModified implements Comparator {
public int compare(Object o1, Object o2) {
File file1 = (File)o1;
File file2 = (File)o2;
long diff = file1.lastModified() - file2.lastModified();
if (diff > 0)
return 1;
else if (diff == 0)
return 0;
else
return -1;
}
public boolean equals(Object obj){
return true; //簡單做法
}
}
static class CompratorBySize implements Comparator {
public int compare(Object o1, Object o2) {
File file1 = (File)o1;
File file2 = (File)o2;
long diff = file1.length() - file2.length();
if (diff > 0)
return 1;
else if (diff == 0)
return 0;
else
return -1;
}
public boolean equals(Object obj){
return true; //簡單做法
}
}
}
調用的示例：
File dir = new File("C:""temp");
File[] files = dir.listFiles();
System.out.print("before sort: ");
for (int i = 0; i< files.length; i++)
System.out.print(files[i] + " ");
System.out.println();
Arrays.sort(files);
System.out.print("sort by name: ");
for (int i = 0; i< files.length; i++)
System.out.print(files[i] + " ");
System.out.println();
Arrays.sort(files, new FileUtils.CompratorBySize());
System.out.print("sort by size: ");
for (int i = 0; i< files.length; i++)
System.out.print(files[i] + " ");
System.out.println();
Arrays.sort(files, new FileUtils.CompratorByLastModified());
System.out.print("sort by last modified: ");
for (int i = 0; i< files.length; i++)
System.out.print(files[i] + " ");
System.out.println();
自己找個目錄試一下吧。用這些Java類庫中的方法，一般情況下應該是不用自己寫排序算法了吧？
最后附上完整代碼占點版面：
TestSort.java
import java.io.*;
import java.util.*;
public class TestSort {
public static void main(String[] args) {
sortSimpleType();
sortComparable();
sortComparator();
}
public static void sortSimpleType() {
int[] arr = {2, 3, 1,10,7,4};
System.out.print("before sort: ");
for (int i = 0; i< arr.length; i++)
System.out.print(arr[i] + " ");
System.out.println();
Arrays.sort(arr);
System.out.print("after sort: ");
for (int i = 0; i< arr.length; i++)
System.out.print(arr[i] + " ");
System.out.println();
}
public static void sortComparable() {
ArrayList list = new ArrayList();
list.add(new Programmer("張三", "C", 12000));
list.add(new Programmer("李四", "Java", 200));
list.add(new Programmer("王五", "C++", 5000));
list.add(new Programmer("錢六", "VB", 3000));
System.out.println("before sort: " + list);
Collections.sort(list);
System.out.println("after sort: " + list);
}
public static void sortComparator() {
File dir = new File("C:""temp");
File[] files = dir.listFiles();
System.out.print("before sort: ");
for (int i = 0; i< files.length; i++)
System.out.print(files[i] + " ");
System.out.println();
Arrays.sort(files);
System.out.print("sort by name: ");
for (int i = 0; i< files.length; i++)
System.out.print(files[i] + " ");
System.out.println();
Arrays.sort(files, new FileUtils.CompratorBySize());
System.out.print("sort by size: ");
for (int i = 0; i< files.length; i++)
System.out.print(files[i] + " ");
System.out.println();
Arrays.sort(files, new FileUtils.CompratorByLastModified());
System.out.print("sort by last modified: ");
for (int i = 0; i< files.length; i++)
System.out.print(files[i] + " ");
System.out.println();
}
}
class Programmer implements Comparable{
private String name;
private String language;
private double pay;
public Programmer(String name, String language, double pay) {
this.name = name;
this.language = language;
this.pay = pay;
}
public int compareTo(Object o) {
Programmer other = (Programmer)o;
return (int)pay - (int)other.pay;
}
public String toString(){
return "{name: " + name + ", language: " + language + ", money: " + pay + "}";
}
}
class FileUtils {
static class CompratorByLastModified implements Comparator {
public int compare(Object o1, Object o2) {
File file1 = (File)o1;
File file2 = (File)o2;
long diff = file1.lastModified() - file2.lastModified();
if (diff > 0)
return 1;
else if (diff == 0)
return 0;
else
return -1;
}
public boolean equals(Object obj){
return true; //簡單做法
}
}
static class CompratorBySize implements Comparator {
public int compare(Object o1, Object o2) {
File file1 = (File)o1;
File file2 = (File)o2;
long diff = file1.length() - file2.length();
if (diff > 0)
return 1;
else if (diff == 0)
return 0;
else
return -1;
}
public boolean equals(Object obj){
return true; //簡單做法
}
}
}