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引言
在这个例子中,我们将向您展示如何使用 java8 lambda
表达式编写一个 Comparator
来对 List
进行排序。
- 经典的比较器示例:
Comparator<Developer> byName = new Comparator<Developer>() {
@Override
public int compare(Developer o1, Developer o2) {return o1.getName().compareTo(o2.getName());
}
};
- 使用 lambda:
Comparator<Developer> byName =
(Developer o1, Developer o2)->o1.getName().compareTo(o2.getName());
1. 没有 Lambda 的排序
先新建一个 Developer 类,然后比较 Developer 对象的年龄,通常我们使用 Collections.sort 并传递匿名 Comparator 类,如下所示:
package com.jimzhang.lambda;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
/**
* 〈一句话功能简述〉<br> 〈排序〉
*
* @author zhangjinmiao
* @create 2019/8/4 10:19
*/
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.toString());
}
//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);
}
}
public static List<Developer> getDevelopers() {List<Developer> developers = new ArrayList<>();
developers.add(new Developer("lisi", new BigDecimal("8000"),23));
developers.add(new Developer("wangwu", new BigDecimal("9000"),24));
developers.add(new Developer("maliu", new BigDecimal("10000"),25));
developers.add(new Developer("zhangsan", new BigDecimal("7000"),22));
return developers;
}
}
当排序要求改变时,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());
}
});
这是可行的,但是,您是否觉得仅仅因为想要更改一行代码就创建一个类有点奇怪?
2. 使用 Lambda 排序
在 Java 8 中,List 接口直接支持排序方法,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();}
});
3. 更多例子
1. 按年龄排序
//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());
2. 按名字排序
//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. 按薪水排序
//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()));
4. 反向排序
- 薪水正序排序
Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
listDevs.sort(salaryComparator);
输出:
Developer{name='zhangsan', salary=7000, age=22}
Developer{name='lisi', salary=8000, age=23}
Developer{name='wangwu', salary=9000, age=24}
Developer{name='maliu', salary=10000, age=25}
2. 反向排序
Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
listDevs.sort(salaryComparator.reversed());
输出:
Developer{name='maliu', salary=10000, age=25}
Developer{name='wangwu', salary=9000, age=24}
Developer{name='lisi', salary=8000, age=23}
Developer{name='zhangsan', salary=7000, age=22}
源码见:java-8-demo
系列文章详见:Java 8 教程
正文完
发表至: java
2019-08-19