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。
- 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());
}
};- 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：
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