Java 8 將Map轉換爲List

將一個Java示例轉換Map爲List

Map<String, String> map = new HashMap<>();

// Convert all Map keys to a List
List<String> result = new ArrayList(map.keySet());

// Convert all Map values to a List
List<String> result2 = new ArrayList(map.values());

// Java 8, Convert all Map keys to a List
List<String> result3 = map.keySet().stream()
	.collect(Collectors.toList());

// Java 8, Convert all Map values  to a List
List<String> 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<String> 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

映射到List

爲了簡單Map的List轉換，只需使用下面的代碼：

ConvertMapToList.java

package com.mkyong;

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) {

        Map<Integer, String> map = 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<Integer> result = new ArrayList(map.keySet());

        result.forEach(System.out::println);

        System.out.println("\n2. Export Map Value to List...");

        List<String> result2 = new ArrayList(map.values());

        result2.forEach(System.out::println);

    }

}

Output

1. Export Map Key to List...
50
20
40
10
30

2. Export Map Value to List...
dragonfruit
orange
watermelon
apple
banana

Java 8 - 映射到List

對於Java 8，您可以將其Map轉換爲流，將其處理並將其返回List

ConvertMapToList.java

package com.mkyong;

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) {

        Map<Integer, String> map = 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<Integer> result = map.keySet().stream()
                .collect(Collectors.toList());

        result.forEach(System.out::println);

        System.out.println("\n2. Export Map Value to List...");

        List<String> 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<String> result3 = map.keySet().stream()
                .filter(x -> !"banana".equalsIgnoreCase(x))
                .collect(Collectors.toList());

        result3.forEach(System.out::println);

    }

}

Output

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

Java 8 - 將Map轉換成List

這個例子有點極端，map.entrySet()用來轉換Map成List

ConvertMapToList.java

package com.mkyong;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

//https://www.mkyong.com/java8/java-8-how-to-sort-a-map/
public class ConvertMapToList {

    public static void main(String[] args) {

        Map<Integer, String> map = 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<Integer> resultSortedKey = new ArrayList<>();
        List<String> resultValues = map.entrySet().stream()
                //sort a Map by key and stored in resultSortedKey
                .sorted(Map.Entry.<Integer, String>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);

    }

}

Output

//resultSortedKey
50
40
30
20
10

//resultValues
dragonfruit
watermelon
orange
apple