Java8 Lambda表达式之比较器

在这个例子中，我将向你展示如何使用Java8的lambda表达式写的比较器排序列表。

1. 经典Comparator例子

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

2. Lambda表达式方式

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

1. 使用Lambda排序

这个例子使用年龄比较Developer对象，通常你使用Collections.sort并且通过一个匿名函数实现Comparator：

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]

当排序的需求要更改时，你需要通过一个新的Comparator比较器：

//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());
    }
});

上面的代码可以工作，但是，你认为创建一个类只是因为你想改变一个单一的代码，不觉得有点奇怪吗？

2. 使用Lambda排序

在Java8中，List支持sort方法，不需要使用Collections.sort了

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

Lambda表达式示例：

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("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]

3. 更多Lambda表达式例子

3.1 根据年龄排序

//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());

3.2 根据姓名排序

//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()));

3.3 根据薪水排序

//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()));

3.4 排序反转 3.4.1 使用lambda表达式对薪水进行排序

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

输出

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]

3.4.2 使用lambda表达式对薪水进行反转排序

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

输出：

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

参考资料：

1. Start Using Java Lambda Expressions
2. Oracle : Lambda Expressions
3. Oracle : Comparator

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