使用JAVA进行排序

利用JAVA完成排序

　　当我们在进行数据库进行查询的时候，当需要按某个字段来进行排序的时候，可以使用SQL语句来完成排序，可以升序，也可以降序。JAVA中的Collections类也可以完成这种操作，SQL的用法和这种集合工具类的操作，可以用于不同的情况中，至于怎么用，可以根据业务场景来选择，下面我罗列出具体的代码，大家可以参考。

1）创建工程名为sort的JAVA工程

创建包结构：

创建User类：

 package com.sunyard.bean;

 /**
  * <p>
  *     用户类，实现Comparable接口，并且在实体类中实现需要排序的逻辑代码
  * </p>
  * @author:[e-mail]
  * @date:2017-6-25
  */
 public class User implements Comparable<User>{
     private String id;//编号
     private String name;//姓名
     private double salary;//工资
     private int age;//年龄
     private String city;//城市

     public User(){}

     public User(String id, String name, double salary, int age, String city) {
         this.id = id;
         this.name = name;
         this.salary = salary;
         this.age = age;
         this.city = city;
     }

     public String getId() {
         return id;
     }
     public void setId(String id) {
         this.id = id;
     }
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }
     public double getSalary() {
         return salary;
     }
     public void setSalary(double salary) {
         this.salary = salary;
     }
     public int getAge() {
         return age;
     }
     public void setAge(int age) {
         this.age = age;
     }
     public String getCity() {
         return city;
     }
     public void setCity(String city) {
         this.city = city;
     }

     @Override
     public String toString() {
         return "User [id=" + id + ", name=" + name + ", salary=" + salary
                 + ", age=" + age + ", city=" + city + "]";
     }
     @Override
     public int compareTo(User o) {
         int result = ((this.getAge() - o.getAge()) * 10);//按照年龄的降序排序
         //int result = ((o.getAge() - this.getAge()) * 10);//按照年龄的升序排序
         //int result = o.getName().compareTo(this.getName());//按照英文字典顺序的降序
         //int result = this.getName().compareTo(o.getName());//按照英文字典顺序的升序
         return result;
     }

 }

创建测试类测试：

 package com.sunyard.test;

 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;

 import com.sunyard.bean.User;

 public class Test {
     public static void main(String[] args) {
         List<User> users = new ArrayList<User>();
         User user1 = new User("1", "zhangsan", 10000, 29, "hangzhou");
         User user2 = new User("2", "lisi", 12000, 25, "hangzhou");
         User user3 = new User("3", "wangwu", 11000, 27, "hangzhou");
         User user4 = new User("4", "zhaoliu", 11500, 26, "hangzhou");

         users.add(user1);
         users.add(user2);
         users.add(user3);
         users.add(user4);

         System.out.println("排序前 -------------------- >>>");
         for(int i = 0;i < users.size();i++){
             System.out.println(users.get(i));
         }

         Collections.sort(users);

         System.out.println("排序后 -------------------- >>>");
         for(int i = 0;i < users.size();i++){
             System.out.println(users.get(i));
         }
     }
 }

控制台输出：

 排序前 -------------------- >>>
 User [id=1, name=zhangsan, salary=10000.0, age=29, city=hangzhou]
 User [id=2, name=lisi, salary=12000.0, age=25, city=hangzhou]
 User [id=3, name=wangwu, salary=11000.0, age=27, city=hangzhou]
 User [id=4, name=zhaoliu, salary=11500.0, age=26, city=hangzhou]
 排序后 -------------------- >>>
 User [id=2, name=lisi, salary=12000.0, age=25, city=hangzhou]
 User [id=4, name=zhaoliu, salary=11500.0, age=26, city=hangzhou]
 User [id=3, name=wangwu, salary=11000.0, age=27, city=hangzhou]
 User [id=1, name=zhangsan, salary=10000.0, age=29, city=hangzhou]

第二种方式，实现自定义的比较器：

自定义比较器代码：

 package com.sunyard.util;

 import java.util.Comparator;

 import com.sunyard.bean.User;

 /**
  * <p>实现自定义比较器</p>
  * @author:[e-mail]
  * @date:2017-6-25
  */
 public class UserComparator implements Comparator<User>{

     @Override
     public int compare(User o1, User o2) {
         int result = 0;
         result = ((o1.getAge() - o2.getAge()) * 10);//升序
         result = ((o2.getAge() - o1.getAge()) * 10);//降序
         return result;
     }

 }

测试类：

 package com.sunyard.test;

 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;

 import com.sunyard.bean.User;
 import com.sunyard.util.UserComparator;

 public class Demo {
     public static void main(String[] args) {
         List<User> users = new ArrayList<User>();
         User user1 = new User("1", "zhangsan", 10000, 29, "hangzhou");
         User user2 = new User("2", "lisi", 12000, 25, "hangzhou");
         User user3 = new User("3", "wangwu", 11000, 27, "hangzhou");
         User user4 = new User("4", "zhaoliu", 11500, 26, "hangzhou");

         users.add(user1);
         users.add(user2);
         users.add(user3);
         users.add(user4);

         System.out.println("排序前 -------------------- >>>");
         for(int i = 0;i < users.size();i++){
             System.out.println(users.get(i));
         }

         UserComparator comparator = new UserComparator();
         Collections.sort(users, comparator);

         System.out.println("排序后 -------------------- >>>");
         for(int i = 0;i < users.size();i++){
             System.out.println(users.get(i));
         }
     }
 }

控制台输出：

 排序前 -------------------- >>>
 User [id=1, name=zhangsan, salary=10000.0, age=29, city=hangzhou]
 User [id=2, name=lisi, salary=12000.0, age=25, city=hangzhou]
 User [id=3, name=wangwu, salary=11000.0, age=27, city=hangzhou]
 User [id=4, name=zhaoliu, salary=11500.0, age=26, city=hangzhou]
 排序后 -------------------- >>>
 User [id=1, name=zhangsan, salary=10000.0, age=29, city=hangzhou]
 User [id=3, name=wangwu, salary=11000.0, age=27, city=hangzhou]
 User [id=4, name=zhaoliu, salary=11500.0, age=26, city=hangzhou]
 User [id=2, name=lisi, salary=12000.0, age=25, city=hangzhou]