现在就加入见鬼网（FaceGhost.COM），You Know，For Problem

原链接：mkyong

在下面的例子中，我们将向你演示如何使用java8中的lambda表达式使用Comparator对一个List集合进行排序

1. 传统的Comparator例子

Comparator<Developer> byName = new Comparator<Developer>() {
    @Override
    public int compare(Developer o1, Developer o2) {
        return o1.getName().compareTo(o2.getName());
    }
};

2.与之等价的Lambda表达式

Comparator<Developer> byName = (Developer o1, Developer o2)->o1.getName().compareTo(o2.getName());

1.传统的排序

示例中使用对象“Developer ”的 “age”属性进行比较，通常情况下，你可使用Collections.sort 或 通过匿名Comparator类

如下：

package com.mkyong.java8;

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

public class TestSorting {

    public static void main(String[] args) {

        List<Developer> listDevs = getDevelopers();

        System.out.println("Before Sort");
        for (Developer developer : listDevs) {
            System.out.println(developer);
        }

        //sort by age
        Collections.sort(listDevs, new Comparator<Developer>() {
            @Override
            public int compare(Developer o1, Developer o2) {
                return o1.getAge() - o2.getAge();
            }
        });

        System.out.println("After Sort");
        for (Developer developer : listDevs) {
            System.out.println(developer);
        }

    }

    private static List<Developer> getDevelopers() {

        List<Developer> result = new ArrayList<Developer>();

        result.add(new Developer("mkyong", new BigDecimal("70000"), 33));
        result.add(new Developer("alvin", new BigDecimal("80000"), 20));
        result.add(new Developer("jason", new BigDecimal("100000"), 10));
        result.add(new Developer("iris", new BigDecimal("170000"), 55));

        return result;

    }

}

输出：

Before Sort
Developer [name=mkyong, salary=70000, age=33]
Developer [name=alvin, salary=80000, age=20]
Developer [name=jason, salary=100000, age=10]
Developer [name=iris, salary=170000, age=55]

After Sort
Developer [name=jason, salary=100000, age=10]
Developer [name=alvin, salary=80000, age=20]
Developer [name=mkyong, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]

当排序发送变化时，你只需要新建另一个匿名的Comparator 即可

//sort by age
Collections.sort(listDevs, new Comparator<Developer>() {
    @Override
    public int compare(Developer o1, Developer o2) {
        return o1.getAge() - o2.getAge();
    }
});

//sort by name
Collections.sort(listDevs, new Comparator<Developer>() {
    @Override
    public int compare(Developer o1, Developer o2) {
        return o1.getName().compareTo(o2.getName());
    }
});

//sort by salary
Collections.sort(listDevs, new Comparator<Developer>() {
    @Override
    public int compare(Developer o1, Developer o2) {
        return o1.getSalary().compareTo(o2.getSalary());
    }
});

它是可行的，但是，如果你想通过一行的代码就可以实现的话？而通过创建已个类可能会比较繁琐。

2.Lambda表达式下的排序

在java8中，List接口已经支持直接通过sort方法，已经不需要使用Collections.sort

//List.sort() since Java 8
listDevs.sort(new Comparator<Developer>() {
    @Override
    public int compare(Developer o1, Developer o2) {
        return o2.getAge() - o1.getAge();
    }
});

Lambda例子

package com.mkyong.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

public class TestSorting {

    public static void main(String[] args) {

        List<Developer> listDevs = getDevelopers();

        System.out.println("Before Sort");
        for (Developer developer : listDevs) {
            System.out.println(developer);
        }

        System.out.println("After Sort");

        //lambda here!
        listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());

        //java 8 only, lambda also, to print the List
        listDevs.forEach((developer)->System.out.println(developer));
    }

    private static List<Developer> getDevelopers() {

        List<Developer> result = new ArrayList<Developer>();

        result.add(new Developer("mkyong", new BigDecimal("70000"), 33));
        result.add(new Developer("alvin", new BigDecimal("80000"), 20));
        result.add(new Developer("jason", new BigDecimal("100000"), 10));
        result.add(new Developer("iris", new BigDecimal("170000"), 55));

        return result;

    }

}

输出：

Before Sort
Developer [name=mkyong, salary=70000, age=33]
Developer [name=alvin, salary=80000, age=20]
Developer [name=jason, salary=100000, age=10]
Developer [name=iris, salary=170000, age=55]

After Sort
Developer [name=jason, salary=100000, age=10]
Developer [name=alvin, salary=80000, age=20]
Developer [name=mkyong, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]

附 Developer 类

package com.xx.comparator;

import java.math.BigDecimal;

public class Developer {

    private String name;
    private BigDecimal salary ;
    private Integer age;




    public Developer(String name, BigDecimal salary, Integer age) {
        super();
        this.name = name;
        this.salary = salary;
        this.age = age;
    }


    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public BigDecimal getSalary() {
        return salary;
    }
    public void setSalary(BigDecimal salary) {
        this.salary = salary;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }


    @Override
    public String toString() {
        return "Developer [name=" + name + ", salary=" + salary + ", age=" + age + "]";
    }






}