Java 中的 Comparable and Comparator-白红宇

Java 中的 Comparable and Comparator

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发布时间：2019-06-27

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本篇文章既是作为Java Comparable and Comparator 的说明，也是作为Strategy Design Pattern（策略模式）的一个典型例子。

Java对原生类型和包装类型均提供排序功能，不用自己去编写排序算法了。代码见下：

package dp.compare;import java.util.ArrayList;import java.util.Arrays;import java.util.Collections;import java.util.List;public class SimpleSort {	/**	 * This class shows how to sort primitive arrays, Wrapper classes Object	 * Arrays	 * 	 * @param args	 */	public static void main(String[] args) {		// sort primitives array like int array		int[] intArr = { 5, 9, 1, 10 };		Arrays.sort(intArr);		System.out.println(Arrays.toString(intArr));		// sorting String array		String[] strArr = { "A", "C", "B", "Z", "E" };		Arrays.sort(strArr);		System.out.println(Arrays.toString(strArr));		// sorting list of objects of Wrapper classes		List
    
      strList = new ArrayList
     
      ();		strList.add("A");		strList.add("C");		strList.add("B");		strList.add("Z");		strList.add("E");		Collections.sort(strList);		for (String str : strList) {			System.out.print(" " + str);		}				System.out.println();		// sorting list of objects of Wrapper classes		List
      
        strList2 = new ArrayList
       
        ();		strList2.add("Acc");		strList2.add("Cebs");		strList2.add("B_kkk");		strList2.add("Z76");		strList2.add("E77");		strList2.add("EA");		strList2.add("AcC");		Collections.sort(strList2);		for (String str : strList2) {			System.out.print(" " + str);		}	}}

结果如下：

[1, 5, 9, 10][A, B, C, E, Z] A B C E Z AcC Acc B_kkk Cebs E77 EA Z76

当然，如果是自己定义的复杂的对象，则需要自己定义比较规则，以判断先后顺序，然后利用java.util.Arrays.sort方法对数组进行排序。也可以利用java.util.Collections.sort方法对列表对象进行排序。

而使用这两种排序方式的前提则是要实现 java.lang.Comparable 或者 java.util.Comparator 接口。通过实现这两个接口，才能给上面的两种sort方法提供合适比较算法，以确定排序的规则。

这样，也能够动态的，根据逻辑或者用户的选择，提供不同的排序算法，这恰好就是一个典型的Strategy Design Pattern（策略模式）。

代码如下：

package dp.compare;import java.util.Comparator;public class User implements Comparable
    
      {	private String userName;	private int userId;	private int age;	private String realName;	public String getUserName() {		return userName;	}	public void setUserName(String userName) {		this.userName = userName;	}	public int getUserId() {		return userId;	}	public void setUserId(int userId) {		this.userId = userId;	}	public int getAge() {		return age;	}	public void setAge(int age) {		this.age = age;	}	public String getRealName() {		return realName;	}	public void setRealName(String realName) {		this.realName = realName;	}	public User(int userId, int age, String userName, String realName) {		this.userId = userId;		this.age = age;		this.userName = userName;		this.realName = realName;	}	@Override	public String toString() {		return "[User Id=" + this.userId + ", real name=" + this.realName				+ ", age=" + this.age + ", user name=" + this.userName + "]";	}	/*	 * 默认的比较方法，用userId进行比较，返回负值表示<, 返回0表示=, 返回正值表示>	 */	@Override	public int compareTo(User user) {		return (this.userId - user.userId);	}	/*	 * 用年龄进行比较的算法	 */	public static Comparator
     
       UserComparatorByAge = new Comparator
      
       () {		@Override		public int compare(User user1, User user2) {			return (int) (user1.getAge() - user2.getAge());		}	};}

package dp.compare;import java.util.Comparator;public class UserComparatorByRealName implements Comparator
    
      {	/*	 * 使用用户真实名字进行对比的算法	 */	@Override	public int compare(User user1, User user2) {		return (user1.getRealName().compareTo(user2.getRealName()));	}}

package dp.compare;import java.util.Comparator;public class UserComparatorByUserAgeAndUserName implements Comparator
    
      {	/*	 * 使用用户年龄和真实姓名进行对比的算法	 */	@Override	public int compare(User user1, User user2) {		int flat = (user1.getAge() - user2.getAge());		if (flat != 0) {			return flat;		} else {			return (user1.getUserName().compareTo(user2.getUserName()));		}	}}

package dp.compare;import java.util.Arrays;public class Main {	public static void main(String args[]) {		// 创建模拟数据		User[] user1 = new User[8];		user1[0] = new User(1, 20, "001", "Wu Zhong");		user1[1] = new User(2, 40, "0011", "Lei Feng");		user1[2] = new User(3, 33, "2001", "Zhang Chen Chen");		user1[3] = new User(4, 22, "zhanglei001", "Su Dang Lei");		user1[4] = new User(21, 17, "76", "Jack Meng");		user1[5] = new User(33, 22, "andong1988", "Aady Long");		user1[6] = new User(9, 15, "31", "John Doe");		user1[7] = new User(45, 22, "jun@tao", "Hoo Jun Tao");		System.out.println("默认排序，用User类实现的 compareTo 方法中的算法进行排序");		Arrays.sort(user1);		for (int i = 0; i < 8; i++) {			System.out.println(user1[i].toString());		}				System.out.println("---------------------------");		System.out.println("使用Age字段进行排序");		Arrays.sort(user1, User.UserComparatorByAge);		for (int i = 0; i < 8; i++) {			System.out.println(user1[i].toString());		}		System.out.println("---------------------------");		System.out.println("使用RealName字段进行排序");		Arrays.sort(user1, new UserComparatorByRealName());		for (int i = 0; i < 8; i++) {			System.out.println(user1[i].toString());		}		System.out.println("---------------------------");		System.out.println("使用Age字段+UserName字段联合进行排序");		Arrays.sort(user1, new UserComparatorByUserAgeAndUserName());		for (int i = 0; i < 8; i++) {			System.out.println(user1[i].toString());		}			}}

运行结果：

默认排序，用User类实现的 compareTo 方法中的算法进行排序[User Id=1, real name=Wu Zhong, age=20, user name=001][User Id=2, real name=Lei Feng, age=40, user name=0011][User Id=3, real name=Zhang Chen Chen, age=33, user name=2001][User Id=4, real name=Su Dang Lei, age=22, user name=zhanglei001][User Id=9, real name=John Doe, age=15, user name=31][User Id=21, real name=Jack Meng, age=17, user name=76][User Id=33, real name=Aady Long, age=22, user name=andong1988][User Id=45, real name=Hoo Jun Tao, age=22, user name=jun@tao]---------------------------使用Age字段进行排序[User Id=9, real name=John Doe, age=15, user name=31][User Id=21, real name=Jack Meng, age=17, user name=76][User Id=1, real name=Wu Zhong, age=20, user name=001][User Id=4, real name=Su Dang Lei, age=22, user name=zhanglei001][User Id=33, real name=Aady Long, age=22, user name=andong1988][User Id=45, real name=Hoo Jun Tao, age=22, user name=jun@tao][User Id=3, real name=Zhang Chen Chen, age=33, user name=2001][User Id=2, real name=Lei Feng, age=40, user name=0011]---------------------------使用RealName字段进行排序[User Id=33, real name=Aady Long, age=22, user name=andong1988][User Id=45, real name=Hoo Jun Tao, age=22, user name=jun@tao][User Id=21, real name=Jack Meng, age=17, user name=76][User Id=9, real name=John Doe, age=15, user name=31][User Id=2, real name=Lei Feng, age=40, user name=0011][User Id=4, real name=Su Dang Lei, age=22, user name=zhanglei001][User Id=1, real name=Wu Zhong, age=20, user name=001][User Id=3, real name=Zhang Chen Chen, age=33, user name=2001]---------------------------使用Age字段+UserName字段联合进行排序[User Id=9, real name=John Doe, age=15, user name=31][User Id=21, real name=Jack Meng, age=17, user name=76][User Id=1, real name=Wu Zhong, age=20, user name=001][User Id=33, real name=Aady Long, age=22, user name=andong1988][User Id=45, real name=Hoo Jun Tao, age=22, user name=jun@tao][User Id=4, real name=Su Dang Lei, age=22, user name=zhanglei001][User Id=3, real name=Zhang Chen Chen, age=33, user name=2001][User Id=2, real name=Lei Feng, age=40, user name=0011]

从上面的例子可以看出，通过选择不同的排序算法，可以得到不同的排序结果。这样，开发者可以指定好不同的排序算法，在运行时动态的进行选择，以实现最灵活的排序。

参考资料： http://www.journaldev.com/780/java-comparable-and-comparator-example-to-sort-objects

转载于:https://my.oschina.net/nox/blog/487478

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