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 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？
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 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]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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