Java 8 –マップをリストに変換
Map
をList
に変換するJavaの例はほとんどありません
Mapmap = new HashMap<>(); // Convert all Map keys to a List List result = new ArrayList(map.keySet()); // Convert all Map values to a List List result2 = new ArrayList(map.values()); // Java 8, Convert all Map keys to a List List result3 = map.keySet().stream() .collect(Collectors.toList()); // Java 8, Convert all Map values to a List List result4 = map.values().stream() .collect(Collectors.toList()); // Java 8, seem a bit long, but you can enjoy the Stream features like filter and etc. List result5 = map.values().stream() .filter(x -> !"apple".equalsIgnoreCase(x)) .collect(Collectors.toList()); // Java 8, split a map into 2 List, it works! // refer example 3 below
1. リストにマップ
単純なMap
からList
への変換には、以下のコードを使用します。
ConvertMapToList.java
package com.example; import java.util.ArrayList; import java.util.HashMap; import java.util.List; import java.util.Map; public class ConvertMapToList { public static void main(String[] args) { Mapmap = new HashMap<>(); map.put(10, "apple"); map.put(20, "orange"); map.put(30, "banana"); map.put(40, "watermelon"); map.put(50, "dragonfruit"); System.out.println("\n1. Export Map Key to List..."); List result = new ArrayList(map.keySet()); result.forEach(System.out::println); System.out.println("\n2. Export Map Value to List..."); List result2 = new ArrayList(map.values()); result2.forEach(System.out::println); } }
出力
1. Export Map Key to List... 50 20 40 10 30 2. Export Map Value to List... dragonfruit orange watermelon apple banana
Java 8 –リストへのマップ
Java 8の場合、Map
をストリームに変換して処理し、List
として返すことができます。
ConvertMapToList.java
package com.example; import java.util.HashMap; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class ConvertMapToList { public static void main(String[] args) { Mapmap = new HashMap<>(); map.put(10, "apple"); map.put(20, "orange"); map.put(30, "banana"); map.put(40, "watermelon"); map.put(50, "dragonfruit"); System.out.println("\n1. Export Map Key to List..."); List result = map.keySet().stream() .collect(Collectors.toList()); result.forEach(System.out::println); System.out.println("\n2. Export Map Value to List..."); List result2 = map.values().stream() .collect(Collectors.toList()); result2.forEach(System.out::println); System.out.println("\n3. Export Map Value to List..., say no to banana"); List result3 = map.keySet().stream() .filter(x -> !"banana".equalsIgnoreCase(x)) .collect(Collectors.toList()); result3.forEach(System.out::println); } }
出力
1. Export Map Key to List... 50 20 40 10 30 2. Export Map Value to List... dragonfruit orange watermelon apple banana 3. Export Map Value to List..., say no to banana dragonfruit orange watermelon apple
3. Java 8 –マップを2リストに変換
この例は少し極端で、map.entrySet()
を使用してMap
を2List
に変換します
ConvertMapToList.java
package com.example; import java.util.ArrayList; import java.util.HashMap; import java.util.List; import java.util.Map; import java.util.stream.Collectors; //https://www.example.com/java8/java-8-how-to-sort-a-map/ public class ConvertMapToList { public static void main(String[] args) { Mapmap = new HashMap<>(); map.put(10, "apple"); map.put(20, "orange"); map.put(30, "banana"); map.put(40, "watermelon"); map.put(50, "dragonfruit"); // split a map into 2 List List resultSortedKey = new ArrayList<>(); List resultValues = map.entrySet().stream() //sort a Map by key and stored in resultSortedKey .sorted(Map.Entry. comparingByKey().reversed()) .peek(e -> resultSortedKey.add(e.getKey())) .map(x -> x.getValue()) // filter banana and return it to resultValues .filter(x -> !"banana".equalsIgnoreCase(x)) .collect(Collectors.toList()); resultSortedKey.forEach(System.out::println); resultValues.forEach(System.out::println); } }
出力
//resultSortedKey 50 40 30 20 10 //resultValues dragonfruit watermelon orange apple