Java 8 Lambda : Comparator 例子

客官°小女子只卖身不卖艺 2022-06-15 02:47 144阅读 0赞

在下面例子中，我们将展示怎么用Java 8 Lambda 表达式去写一个对List集合排序的`Comparator` 。

1. Classic `Comparator` example.

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

2. Lambda expression equivalent.

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

## 1. 不用 Lambda 实现排序 ##

示例演示用一个 `` `Developer`  对象实现对其age属性的排序功能。正常的，你会用Collections.sort，通过匿名`Comparator` 比较器实现，如下所示：

TestSorting.java

package com.mkyong.java8; 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; } }

Output

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]

当排序需求改变时，你只能通过一个新的匿名类实现，如下所示：

//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()); } });

它的确可以实现，你去创建一个类仅仅是因为你想去改变某行代码认为它有一点诡异的。

2. 用Lambda实现排序

在java 8 中，接口List是支持直接方法排序，而再也不需要用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 表达式例子 :

TestSorting.java

package com.mkyong.java8; 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：

## 3. 更多 Lambda 例子 ##

3.1 Sort By age

//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 Sort by name

//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 Sort by salary

//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 逆序排序.

3.4.1 Lambda expression to sort a List using their salary.

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

Output

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 Lambda expression to sort a List using their salary, reversed order.

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

Output

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]

## References ##

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

[Start Using Java Lambda Expressions]: http://www.developer.com/java/start-using-java-lambda-expressions.html
[Oracle _ Lambda Expressions]: https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html
[Oracle _ Comparator]: https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html