如何轉換為 <String, <List<String> &gt;轉換為String [] []？

Question

我不確定應該將數組String [][] ans初始化為什么。

如果我將其設置為treeMap的大小，則存在一個NPE。

String [][] ans = new String[size][];

因此，我將其初始化為大於預期的大小，但是數組中存在空值。

目前，我的treeMap為{C = [E]，M = [F，B]，S = [F，B]，T = [F，B，L]，TD = [E，L]，W = [E， L]，Z = [F]}

我希望它成為

[[C，E]，[M，F，B]，[S，F，B] ....]

import java.util.*;

public class Copy {
    public static String[][] oMI(String[][] m){

        //ans[0][0]= "test";
        Map<String, List<String>>  map  = new HashMap<>();
        int length = m.length;
        for (int i=0; i < m.length;i++){
            for (int j=1; j < m[i].length;j++){
                String key = m[i][j];

                map.computeIfAbsent(key, k-> new ArrayList<String>()).add(m[i][0]);
            }
        }
        Map<String,List<String>> treeMap = new TreeMap<String, List<String>>(map);
        System.out.println(treeMap);

        int size = treeMap.size();

        String [][] ans = new String[20][20];
        int l = 0;
        int s = 0;
        String ing = "";
        for (Map.Entry<String,List<String>> entry: treeMap.entrySet()){
            s = entry.getValue().size();

            if (entry.getKey() !=null)
                ing = entry.getKey();
            else ing ="null";

            ans[l][0] = ing;

            int mn = 0;
            for (String v : entry.getValue()) {
                ans[l][mn] = v;       
                l++;
                mn++;
            }
        }
        return ans;
    }

    public static void main(String[] args){

        String  [][] ma =
                {
                        {"F", "T", "Z", "S", "M"},
                        {"E", "C", "W", "TD"},
                        {"B", "M", "S", "T"},
                        {"L", "W", "T", "TD"}

                };
        oMI(ma);
    }
}

Answer 1

如果您需要不使用流的解決方案，則可以使用以下方法：

    int size = treeMap.size();
    String [][] ans = new String[size][]; //initialize the known dimension only
    int arrayRow = 0;
    for (Map.Entry<String,List<String>> entry: treeMap.entrySet()){

        int rowSize = entry.getValue().size() + 1;
        String[] tempArray = new String[rowSize]; //an temp array to hold key + vlaues
        tempArray[0] = entry.getKey();            //add key to temp array

        int mn = 1;
        for (String s : entry.getValue()) { tempArray[mn++] = s; } //add values to temp array
        ans[arrayRow++] = tempArray; //add temp array to output array
    }

    return ans;