java8-Lambda中比较器Comparator的使用

一、概念

先来看下典型的比较器示例：

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

java8中引入了lambda表达式，比较器可以改成下面这样：

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

对于数组，Arrays.sort()提供了传入比较器的重载方法，对于列表，Collections.sort()也提供了传入比较器的重载方法。

二、使用实例

2.1 通过new 新建比较器

假如我们要通过Developer 对象的年龄进行排序，通常情况下我们使用Collections.sort，new个匿名Comparator 类，类似下面这种：

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("ricky", 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;

    }

}

输出结果：

Before Sort
Developer [name=ricky, 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=ricky, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]

当比较规则发生变化时，你需要再次new个匿名Comparator 类：

 //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());
        }
    });

这样也可以，不过你会不会觉得这样有点怪，因为其实不同的只有一行代码而已，但是却需要重复写很多代码？

2.2 通过lambda表达式新建比较器

在java8中，List接口直接提供了排序方法， 所以你不需要使用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改写：

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("ricky", 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=ricky, 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=ricky, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]

2.3 更多的例子

根据年龄：

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

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

    //lambda, valid, parameter type is optional
    listDevs.sort((o1, o2)->o1.getAge()-o2.getAge());

根据名字：

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

    //lambda
    listDevs.sort((Developer o1, Developer o2)->o1.getName().compareTo(o2.getName()));

    //lambda
    listDevs.sort((o1, o2)->o1.getName().compareTo(o2.getName()));

根据薪水：

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

    //lambda
    listDevs.sort((Developer o1, Developer o2)->o1.getSalary().compareTo(o2.getSalary()));

    //lambda
    listDevs.sort((o1, o2)->o1.getSalary().compareTo(o2.getSalary()))

从小到大排序：

Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
    listDevs.sort(salaryComparator);

从大到小倒序：

Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
    listDevs.sort(salaryComparator.reversed());

//
Comparator<Developer> salaryComparator = (o1, o2)->o2.getSalary().compareTo(o1.getSalary());

参考：

blog.csdn.net/u014042066/…