如何将地图列表转换为 CSV

Question

我需要将地图列表转换为 CSV 对象，如下所示

List<Map<String,Object>> maps = new ArrayList<Map<String,Object>>();

地图对象包含以下格式的值

地图1：

(header 1, value1)

(header 2, value2)

(header 3, value3)

(header 4, value4)

地图2：

(header 1, value5)

(header 2, value6)

(header 3, value7)

(header 4, value8)

我正在寻找如下所示的 CSV

header1, header2,header3, header4

value1, value2,value3,value4

value5,value6,value7,value8

我试图读取地图（键、值）并写入 CSV 文件，但写入格式

标题 1，值 1

标头2，值2

标题 3，值 3

标头 4，值 4

标头 1，值 5

像下面这样是我尝试过的代码片段

(Map<String, Object> map : maps) {

 for (Map.Entry<String, Object> entry : map.entrySet()) {

w.append(entry.getKey()).append(",").append(entry.getValue()‌​.toString()).append(‌​"\n"); } } 

Answer 1

下面的代码会帮助你。 它还没有为您的目的做好准备。 您需要更改它，以便它打印到文件而不是返回String 。

private static String toCSV(List<Map<String, Object>> list) {
    List<String> headers = list.stream().flatMap(map -> map.keySet().stream()).distinct().collect(toList());
    final StringBuffer sb = new StringBuffer();
    for (int i = 0; i < headers.size(); i++) {
        sb.append(headers.get(i));
        sb.append(i == headers.size()-1 ? "\n" : ",");
    }
    for (Map<String, Object> map : list) {
        for (int i = 0; i < headers.size(); i++) {
            sb.append(map.get(headers.get(i)));
            sb.append(i == headers.size()-1 ? "\n" : ",");
        }
    }
    return sb.toString();
}

对于以下输入，它给出了以下输出：

public static void main(String[] args) {
    List<Map<String, Object>> list = new ArrayList<>();
    Map<String, Object> map1 = new HashMap<>();
    map1.put("header1", "value1");
    map1.put("header2", "value2");
    map1.put("header3", "value3");
    map1.put("header4", "value4");
    Map<String, Object> map2 = new HashMap<>();
    map2.put("header1", "value5");
    map2.put("header2", "value6");
    map2.put("header3", "value7");
    map2.put("header4", "value8");
    list.add(map1);
    list.add(map2);
    System.out.println(toCSV(list));
}

这种形式是完全可逆的，因此您可以编写自己的方法从这种 CSV 读取到List<Map<String, String>

另一个示例，当其中一个映射中不存在密钥时：

Answer 2

以下代码解决了您的问题：

import org.apache.commons.lang.StringUtils;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.SortedSet;
import java.util.TreeSet;

public class SomeMaps {

    public static void main(String[] args) {
        Map<String, String> firstMap = new HashMap<>();
        Map<String, String> secondMap = new HashMap<>();

        firstMap.put("header 1", "value1");
        firstMap.put("header 2", "value2");
        firstMap.put("header 3", "value3");
        firstMap.put("header 4", "value4");

        secondMap.put("header 1", "value5");
        secondMap.put("header 2", "value6");
        secondMap.put("header 3", "value7");
        secondMap.put("header 4", "value8");

        List<Map<String, String>> maps = new ArrayList<>();
        maps.add(firstMap);
        maps.add(secondMap);

        // extract all headers
        SortedSet<String> keys = new TreeSet<>(firstMap.keySet());
        for(Map<String, String> map : maps){
            keys.addAll(map.keySet());
        }


        String header = StringUtils.join(keys, ",");
        System.out.println(header);

        // generate content of CSV file
        for(Map<String, String> map : maps){
            String line = getLineFromMap(map, keys);
            System.out.println(line);
        }
    }

    private static String getLineFromMap(Map<String, String> someMap, SortedSet<String> keys) {
        List<String> values = new ArrayList<>();
        for (String key : keys) {
            values.add(someMap.get(key) == null ? " " : someMap.get(key));
        }
        return StringUtils.join(values, ",");
    }
}

Answer 3

考虑以下列表

[
{
"header1": "value1",
"header2": "value2",
"header3": "value3",
"header4": "value4"
},
{
"header1": "value1",
"header2": "value2",
"header3": "value3",
"header4": "value4"
},
{
"header1": "value1",
"header2": "value2",
"header3": "value3",
"header4": "value4"
}
]

为了将其转换为 CSV 文件，我添加了一个简单的方法....

public String createCSV(List<LinkedHashMap<String, String>> list) throws IOException{
    List<String> headers = list.stream().flatMap(map -> map.keySet().stream()).distinct().collect(Collectors.toList());

    String path= "dataCSV.csv";

    try(FileWriter writer= new FileWriter(path, true);){
           for (String string : headers) {
                 writer.write(string);
                 writer.write(",");
           }
           writer.write("\r\n");

           for (LinkedHashMap<String, String> lmap : list) {
                 for (Entry<String, String> string2 : lmap.entrySet()) {
                        writer.write(string2.getValue());
                        writer.write(",");
                 }
                 writer.write("\r\n");
           }
    }catch (Exception e) {
        e.printStackTrace();
    }
    return "File created"}

该文件将被创建，它应该包含 List 中的以下条目

       header1  header2 header3 header4
        value1  value1  value1  value1
        value2  value2  value2  value2
        value3  value3  value3  value3
        value4  value4  value4  value4