Java---Comparator接口

                                                       一个人如果没有梦想,跟无忧无虑有什么区别呢?                               

    今天来给大家介绍下java.utils包下的Comparator接口。该接口代表一个比较器,java数组工具类和集合工具类中提供对sort方法排序就是使用    Comparator接口来处理排序的。

    Comparator接口中有一个方法int compare(T o1, T o2)。这个方法返回值是int类型,如果返回值小于0,说明比较结果是o1<o2,如果返回值等于0,说明比较结果是o1=o2,如果返回值大于0,则说明比较结果是o1>o2。

    既然是接口,那么我们就可以实现它,来自定义其中对比较规则,即可实现在一个List列表中将元素按照某个属性进行排序。

    下面我们用代码举例说明,如下:

package com.java.utils.Comparator;

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

/**
* 类说明:
*    比较器Comparator接口的应用
*/
public class ComparatorTest {

/**
* 实体对象类 
*/
static class User {
String name;
int age;
public User(String name,int age) {
this.name=name;
this.age=age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}

}

/**
* 自定义比较器
* 根据名称比较
*/
static class MyComparatorName implements Comparator<User>{
@Override
public int compare(User u1, User u2) {
return u1.getName().compareTo(u2.getName()) * -1;
}
}

/**
* 自定义比较器
* 根据年龄比较
*/
static class MyComparatorAge implements Comparator<User>{
@Override
public int compare(User u1, User u2) {
return (u1.getAge() - u2.getAge());
}
}

//测试
public static void main(String[] args) {
List<User> list =new ArrayList<User>();
User u1 = new User("a", 2);
User u2 = new User("d", 0);
User u3 = new User("c", 3);
list.add(u1);
list.add(u2);
list.add(u3);

Collections.sort(list, new MyComparatorName());
for(User u :list){
System.out.println(u.getName());
}

Collections.sort(list, new MyComparatorAge());
for(User u :list){
System.out.println(u.getAge());
}
}

}


    在上面对例子中,我们首先按照名称从大到小进行排序,输出结果是d、c、a然后又按照年龄从小到大进行排序,输出结果是0、2、3。

    需要注意的是,Collections.sort方法默认是按照返回结果从小到大进行排序,如果我们需要从大到小则在实现比较器大时候将返回结果乘以-1即可。

    是不是很简单?如果是我们自己实现比较器,那么我们就可以根据实际的业务需求去实现排序。

 例题:

Java---Comparator接口

import java.lang.ref.WeakReference;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;
import java.util.*;
import java.util.stream.Collectors;

public class test   {

    public static void main(String[] args) {
        List<Employee> list=new ArrayList<>();
        list.add(new Employee(1,10,5500.00));
        list.add(new Employee(2,20,4800.00));
        list.add(new Employee(3,40,14500.00));
        list.add(new Employee(4,40,44500.00));
        list.add(new Employee(5,50,6500.00));
        list.add(new Employee(6,50,7500.00));
        list.add(new Employee(7,10,4500.00));
        list.add(new Employee(8,40,6500.00));
        list.add(new Employee(9,20,1900.00));

        Comparator<Employee> groupComparator = new Comparator<Employee>() {
            @Override
            public int compare(Employee o1, Employee o2) {
                return o1.getDeptid()-o2.getDeptid();
            }
        };

        Collections.sort(list,groupComparator);   //先根据deptid分组

        Comparator<Employee> salaryComparator=new Comparator<Employee>() {
            @Override
            public int compare(Employee o1, Employee o2) {

                return o1.getDeptid()==o2.getDeptid()?(int) (o2.getSalary() - o1.getSalary()):0 ;
            }
        };
        Collections.sort(list,salaryComparator);  //然后根据工资降序

        //设置等级rank
        Employee employee = list.get(0);
        int deptid = employee.getDeptid();
        int k=1;
        employee.setRank(k);
        for (int i = 1; i < list.size(); i++) {
            Employee employee1 = list.get(i);
            int deptid1 = employee1.getDeptid();
            if(deptid==deptid1){
                employee1.setRank(++k);
            }else{
                k=1;
                employee1.setRank(k);
                deptid=employee1.getDeptid();
            }

        }

        //打印Employee
        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }

    }

}

class Employee{
    private int empid;

    public int getEmpid() {
        return empid;
    }

    public void setEmpid(int empid) {
        this.empid = empid;
    }

    public int getDeptid() {
        return deptid;
    }

    public void setDeptid(int deptid) {
        this.deptid = deptid;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    private int deptid;
    private double salary;
    private int rank;
    public Employee(int empid, int deptid, double salary) {
        this.empid = empid;
        this.deptid = deptid;
        this.salary = salary;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "empid=" + empid +
                ", deptid=" + deptid +
                ", salary=" + salary +
                ", rank=" + rank +
                '}';
    }

    public void setRank(int rank) {
        this.rank = rank;
    }
}