java8-Lambda中比较器Comparator的使用

此教程旨在让读者学会使用lambda并结合Comparator进行集合排序操作

• 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());    }};

• 等价的Lambda的方式

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

不使用Lambda的排序

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 SortDeveloper [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 SortDeveloper [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？

通过lambda进行排序

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 示例

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;    }}

输出结果:

Before SortDeveloper [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 SortDeveloper [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]

更多的Lambda 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()))

• 倒序

  • 正常排序

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

  • 倒序

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

翻译

ricky 20170728

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