java8

2023-12-18 02:57| 来源: 网络整理| 查看: 265

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 byName = new Comparator() { @Override public int compare(Developer o1, Developer o2) { return o1.getName().compareTo(o2.getName()); } }; 等价的Lambda的方式 Comparator 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 listDevs = getDevelopers(); System.out.println("Before Sort"); for (Developer developer : listDevs) { System.out.println(developer); } //sort by age Collections.sort(listDevs, new Comparator() { @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 getDevelopers() { List result = new ArrayList(); 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() { @Override public int compare(Developer o1, Developer o2) { return o1.getAge() - o2.getAge(); } }); //sort by name Collections.sort(listDevs, new Comparator() { @Override public int compare(Developer o1, Developer o2) { return o1.getName().compareTo(o2.getName()); } }); //sort by salary Collections.sort(listDevs, new Comparator() { @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() { @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 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 getDevelopers() { List result = new ArrayList(); 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:

根据年龄

//sort by age Collections.sort(listDevs, new Comparator() { @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() { @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() { @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 salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary()); listDevs.sort(salaryComparator);

倒序

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

ricky 20170728

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