Java 8 Lambda：比较器示例

在这个例子中，我们将向展示如何使用Java 8 Lambda表达式编写Comparator来对List进行排序。

1.典型Comparator例子。

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

2. 上面等效于下列Lambda表达式

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

1.不使用Lambda进行排序

下面是用年龄比较Developer对象的例子。 通常使用Collections.sort并传递一个匿名Comparator类，如下所示：

TestSorting.java

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("test", 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=test, 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]

当排序要求发生变化时，您只需传入另一个新的匿名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());
        }
    });                

这种方式用起来很麻烦。

2.用Lambda排序

在Java 8中， List接口直接支持sort方法，不再需要使用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表达示例：

TestSorting.java

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("test", 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=test, 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示例

3.1按年龄分类

    //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按名称排序

    //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按工资排序

    //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倒序。

3.4.1使用他们的薪水对List进行排序的Lambda表达式:

    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按照薪水倒序的Lambda表达式：

    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]