很少有Java 8示例向您展示如何将对象List转换为Map以及如何处理重复的键。
Hosting.java
package com.mkyong.java8
public class Hosting {
private int Id;
private String name;
private long websites;
public Hosting(int id, String name, long websites) {
Id = id;
this.name = name;
this.websites = websites;
}
//getters, setters and toString()
}1.列出地图– Collectors.toMap（）
创建Hosting对象的列表，并使用Collectors.toMap将其转换为Map。
TestListMap.java
package com.mkyong.java8
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class TestListMap {
public static void main(String[] args) {
List list = new ArrayList<>();
list.add(new Hosting(1, "liquidweb.com", 80000));
list.add(new Hosting(2, "linode.com", 90000));
list.add(new Hosting(3, "digitalocean.com", 120000));
list.add(new Hosting(4, "aws.amazon.com", 200000));
list.add(new Hosting(5, "mkyong.com", 1));
// key = id, value - websites
Map result1 = list.stream().collect(
Collectors.toMap(Hosting::getId, Hosting::getName));
System.out.println("Result 1 : " + result1);
// key = name, value - websites
Map result2 = list.stream().collect(
Collectors.toMap(Hosting::getName, Hosting::getWebsites));
System.out.println("Result 2 : " + result2);
// Same with result1, just different syntax
// key = id, value = name
Map result3 = list.stream().collect(
Collectors.toMap(x -> x.getId(), x -> x.getName()));
System.out.println("Result 3 : " + result3);
}
} 输出量
Result 1 : {1=liquidweb.com, 2=linode.com, 3=digitalocean.com, 4=aws.amazon.com, 5=mkyong.com}
Result 2 : {liquidweb.com=80000, mkyong.com=1, digitalocean.com=120000, aws.amazon.com=200000, linode.com=90000}
Result 3 : {1=liquidweb.com, 2=linode.com, 3=digitalocean.com, 4=aws.amazon.com, 5=mkyong.com}2.列表到地图-重复的钥匙！
2.1运行下面的代码，将引发重复的键错误！
TestDuplicatedKey.java
package com.mkyong.java8;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class TestDuplicatedKey {
public static void main(String[] args) {
List list = new ArrayList<>();
list.add(new Hosting(1, "liquidweb.com", 80000));
list.add(new Hosting(2, "linode.com", 90000));
list.add(new Hosting(3, "digitalocean.com", 120000));
list.add(new Hosting(4, "aws.amazon.com", 200000));
list.add(new Hosting(5, "mkyong.com", 1));
list.add(new Hosting(6, "linode.com", 100000)); // new line
// key = name, value - websites , but the key 'linode' is duplicated!?
Map result1 = list.stream().collect(
Collectors.toMap(Hosting::getName, Hosting::getWebsites));
System.out.println("Result 1 : " + result1);
}
} 输出–下面的错误消息有点让人误解，它应该显示“ linode”而不是键的值。
Exception in thread "main" java.lang.IllegalStateException: Duplicate key 90000
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.HashMap.merge(HashMap.java:1245)
//...2.2要解决上述重复的键问题，请像下面这样传入第三个mergeFunction参数：
Map result1 = list.stream().collect(
Collectors.toMap(Hosting::getName, Hosting::getWebsites,
(oldValue, newValue) -> oldValue
)
); 输出量
Result 1 : {..., aws.amazon.com=200000, linode.com=90000}注意
(oldValue, newValue) -> oldValue==>如果密钥重复，您是喜欢oldKey还是newKey？
3.3试试newValue
Map result1 = list.stream().collect(
Collectors.toMap(Hosting::getName, Hosting::getWebsites,
(oldValue, newValue) -> newvalue
)
); 输出量
Result 1 : {..., aws.amazon.com=200000, linode.com=100000}3.列表到地图–排序和收集
TestSortCollect.java
package com.mkyong.java8;
import java.util.*;
import java.util.stream.Collectors;
public class TestSortCollect {
public static void main(String[] args) {
List list = new ArrayList<>();
list.add(new Hosting(1, "liquidweb.com", 80000));
list.add(new Hosting(2, "linode.com", 90000));
list.add(new Hosting(3, "digitalocean.com", 120000));
list.add(new Hosting(4, "aws.amazon.com", 200000));
list.add(new Hosting(5, "mkyong.com", 1));
list.add(new Hosting(6, "linode.com", 100000));
//example 1
Map result1 = list.stream()
.sorted(Comparator.comparingLong(Hosting::getWebsites).reversed())
.collect(
Collectors.toMap(
Hosting::getName, Hosting::getWebsites, // key = name, value = websites
(oldValue, newValue) -> oldValue, // if same key, take the old key
LinkedHashMap::new // returns a LinkedHashMap, keep order
));
System.out.println("Result 1 : " + result1);
}
} 输出量
Result 1 : {aws.amazon.com=200000, digitalocean.com=120000, linode.com=100000, liquidweb.com=80000, mkyong.com=1}PS在上面的示例中，流在收集之前进行了排序，因此“ linode.com = 100000”成为了“ oldValue”。
参考文献
- Java 8收集器JavaDoc
- Java 8 –如何对地图排序
- Java 8 Lambda：比较器示例
标签： 收藏家 重复键 Java 8 列表 映射 排序 流
翻译自: https://mkyong.com/java8/java-8-convert-list-to-map/